Medical Entomology Lecture Notes PDF

LECTURE NOTES For Environmental Health Students MEDICAL ENTOMOLOGY Zinabu Anamo Negga Baraki Haramaya University In collaboration with the Ethiopia...

LECTURE NOTES For Environmental Health Students MEDICAL ENTOMOLOGY Zinabu Anamo Negga Baraki Haramaya University In collaboration with the Ethiopia Public Health Training Initiative, The Carter Center, the Ethiopia Ministry of Health, and the Ethiopia Ministry of Education March 2008  Funded under USAID Cooperative Agreement No. 663-A-00-00-0358-00. Produced in collaboration with the Ethiopia Public Health Training Initiative, The Carter Center, the Ethiopia Ministry of Health, and the Ethiopia Ministry of Education. Important Guidelines for Printing and Photocopying Limited permission is granted free of charge to print or photocopy all pages of this publication for educational, not-for-profit use by health care workers, students or faculty. All copies must retain all author credits and copyright notices included in the original document. Under no circumstances is it permissible to sell or distribute on a commercial basis, or to claim authorship of, copies of material reproduced from this publication. ©2008 by Zinabu Anamo, B.Sc, M.Sc and Negga Baraki, B.Sc, MPH All rights reserved. Except as expressly provided above, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission of the author or authors. This material is intended for educational use only by practicing health care workers or students and faculty in a health care field.  PREFACE The importance of an organized material for the study of a subject is very important and especially where reference materials and text books relevant to the subject matter are in restricted access. The course of medical entomology offered to environmental science students is a pre-requisite to the course vector control. This is aimed as an introductory course to vector control and insects of public health importance, most common in Ethiopia. The aim is to give a general insight to the biology of arthropods, their habits and habitats before embarking on a detailed study of insect vectors. This lecture note is prepared in line with the curriculum of the current training of Environmental Health Science Students at the different sister universities networked through the Ethiopia Public Health Training Initiative (EPHTI). The contents of the lecture-note are as agreed with the relevant departments in the sister universities who offer similar training. The relevant information for the course is organized from various books of entomology and medical entomology. The aim is to save time for the instructor to concentrate on interactive discussion and minimize the usual oral lecturing and facilitate practical training. iii The contents of this lecture note are organized in a logical flow of six chapters. The first chapter is introduction to the subject matter and public health problems of arthropods. The second chapter focuses on identification of arthropods, Chapter three is about insects, Chapter four is about Insect morphology, Chapter five is on Developments and life histories of insects and Chapters six is on Insect behavior and activity. It is noteworthy here to indicate to the learner that this material is not full in itself to give all the knowledge and experience required, but is a good road map to influence the reader to get initiated for searching more and valid information from various sources. This material, therefore, does not replace any standard text book. In conclusion, the authors are open and thankful for any comments from readers that help to improve the material. iv  ACKNOWLEDGMENT Our sincere thanks and appreciation goes to The Carter Center /EPHTI/ for financial and material support of this lecture note. Many thanks to Ato Bizatu Mengiste and Ato Zeyede Kebede for their valuable comments forwarded during intra workshop review for improving the lecture note to the maximum benefit of the students. We would like to extend our thanks to reviewers from sister institutions: Dr. Natsanet Worku (Gonder University), Professor K.N. Panicker (Addis Ababa University), Ato Alemayahu Woldecherkos (Hawassa University), Ato Mohamud Adulkader (Mekele University), Ato Ahemed Mohammed (Jimma University) and W/o Atsedu Yashwalul (Defense University College) for their practical comments, which we have taken the utmost care to include in the lecture note. We are also grateful to Ato Gebre-Emanuel Teka (Senior Environmental Health professional now on retirement) and Dr. Messay Fettene (Akililu Lemma Institute of pathobiology, Addis Ababa University) for their immense contributions to the betterment of this teaching material. v Finally, we are grateful to all those who contributed directly or indirectly to the development of this lecture note. vi  TABLE OF CONTENTS Contents Pages Preface …………………………………………………. i Acknowledgement …………………………………… iii Table of contents …………………………………….. v List of figures …………………………………………. ix CHAPTER ONE: INTRODUCTION TO MEDICAL ENTOMOLOGY...………………… 1 1.1 Learning objectives ……………………………….. 1 1.2. Introduction ……………………………………….. 1 1.3 Definition of terms …………………………………. 3 1.4 Existing public health problems ………………….. 6 CHAPTER TWO: ARTHROPODS AND THEIR IDENTIFICATION …………………. 9 2.1 Learning Objectives ……………………………….. 9 2.2. Introduction ………………………………………... 9 2.3 Common identification Characteristics of Arthropods: ………………………………………… 10 2.4 Taxonomy (Scientific Classification) of Arthropods ………………………………………… 11 2.5 Arthropod Habitats ………………………………… 19 vii 2.6. Arthropod Species Abundance ………………….. 20 2.7. Advantages and disadvantages of arthropods... 22 2.8. General control methods of arthropods ………… 26 CHAPTER THREE: THE INSECTS …………………. 31 3.1 Learning Objectives ………………………………. 31 3.2. Introduction ……………………………………….. 31 3.3. The Orders of Insecta ……………………………. 32 3.4. Uses of insects ……………………………………. 62 3.4.1 Insects as human food (entomophagy): ……… 62 3.4.2 Insects as feed for domesticated animals: …… 64 3.4.3. Other benefits of insects ………………………. 65 3.5. Insect diversity ……………………………………. 67 3.5.1 Why insect are so successful in their species diversity? ……………………………………………….. 67 3.6 Insect collecting technique ……………………….. 70 CHAPTER FOUR: THE MORPHOLOGY OF INSECTS ………………………… 79 4.1 Learning Objectives ……………………………….. 79 4.2 Introduction ………………………………………… 79 4.3. The External Morphology ……………………….. 80 4.3.1. Exoskeleton …………………………………….. 83 4.3.2. Body Divisions of Insects ……………………… 86 4.4. Internal Morphology of Insects ………………….. 105 4.4.1. Digestive system: ………………………………. 106 viii 4.4.2 Excretory system............................................. 108 4.4.3. Circulatory System ……………………………... 110 4.4.4. Respiratory System …………………………….. 113 4.4.5 Muscular System ………………………………... 118 4.4.6 Nervous System ………………………………… 122 4.4.7. Reproductive System ………………………….. 129 CHAPTER FIVE: INSECT DEVELOPMENT AND LIFE HISTORIES ………………… 137 5.1 Learning Objectives ……………………………….. 137 5.2. Growth ……………………………………………... 137 5.3. Life history patterns and phases ………………... 139 5.3.1. Developmental Stages of Insects …………….. 140 5.3.2. Phases in insect ontogeny ……………………. 144 5.4. Effects of Environmental factors on Insect development ……………………………………………. 145 5.4.1. Temperature ……………………………………. 146 5.4.2. Photoperiod …………………………………….. 146 5.4.3. Humidity …………………………………………. 146 5.4.4. Mutagens and toxins …………………………… 147 5.4.5. Biotic effects ……………………………………. 147 CHAPTER SIX: INSECT BEHAVIOR AND ACTIVITY …………………………… 149 6.1 Learning Objectives ……………………………….. 149 6.2. Introduction ………………………………………... 149 ix 6.3. Insect behavior ……………………………………. 150 6.4. Insect Senses ……………………………………... 163 6.4.1. Sense of touch …………………………………. 163 6.4.2. Sense of smell and taste ………………………. 164 6.4.3. Sense of hearing ……………………………….. 164 6.4.4. Sense of sight …………………………………... 164 6.5. Defenses in Insects ………………………………. 165 References: ……………………………………………. 175 Annex …………………………………………………… 177 Glossary ……………………………………………….. 182 x  LIST OF FIGURES Page Figure 2.1. common classification tree of phylum Arthropoda ………………………………... 15 Figure 3.1. Examples of diptera …………………….. 38 Figure 3.2. Lice of human …………………………….. 40 Figure 3.3. Adult chewing louse. …………………….. 42 Figure 3.4. The bedbug………………………………… 45 Figure 3.5. An adult flea ………………………………. 48 Figure 3.6. Examples of Hymenoptera ………………. 50 Figure 3.7. Examples of lipdoptera …………………... 52 Figure 3.8. Examples of orthoptera …………………. 53 Figure 3.9. An adult Cockroach ………………………. 56 Figure 3.10. Examples of Coleoptera ………………... 59 Figure 3.11. Examples of isopteran Termites……….. 61 Figure 3.12 Forceps to pick up insects ………………. 71 Figure 3.13 Film canisters to hold small insects…….. 72 Figure 3.14 Insect killing jar …………………………… 73 Figure 3.15 Insect collecting net ……………………… 74 Figure 3.16 Pheromone trap …………………………. 77 Figure 4.1. The major body axes and the relationship xi of parts of the appendages to the body………………. 82 Figure 4.2. The general structure of insect cuticle …. 85 Figure 4.3. Generalized adult-winged insect........ 86 Figure 4.4. Lateral view of the head of a generalized pterygote insect…………………………… 88 Figure 4.5. Pre-oral and anterior foregut morphology in insects…………………………………… 90 Figure 4.6. Frontal view of the head and dissected mouth parts of an adult earwig……………. 91 Figure 4.7. Insect mouth parts……………………….... 95 Figure 4.8. Types of insect antennae…………………. 98 Figure 4.9. Generalized wing illustrating venation…... 100 Figure 4.10. The hind leg of a cockroach…………….. 101 Figure 4.11. The insect leg…………………………….. 103 Figure 4.12. Dissection of a male black field cricket… 105 Figure 4.13. Intestinal tract of a generalized insect…. 108 Figure 4.14. Circulatory system structure……………. 113 Figure 4.15. Cross section through the abdomen illustrating some of the tracheation…….. 116 Figure 4.16. Some variations on the tracheal system of insects ……………………………….... 118 Figure 4.17. Musculature of a grasshopper………….. 122 Figure 4.18. The nervous system of generalized insect……………………………………… 127 xii Figure 4.19. Diagramatic representation of the development of the central nervous system. …………………………………… 128 Figure 4.20. Comparison of generalized reproductive systems of insect………………………… 132 Figure 5.1. Developmental stages of different insects from egg to adult…………………………… 141 Figure 6.1. Honey bees at a feeding station ………… 154 xiii Medical Entomology CHAPTER ONE INTRODUCTION TO MEDICAL ENTOMOLOGY 1.1 Learning objectives By the end of this chapter the learner will be able to: Identify existing public health problems with regard to arthropods Define important terms with regard to the subject matter 1.2. Introduction Arthropods are one of the most remarkable creatures on the earth, and they merit study for at least two major reasons. First, arthropods have unsurpassed diversity and niches; because of their extensive variation. These animals can provide an in-depth understanding of nature and the many ways that biological problems have been met. Arthropods fly, jump, hide, they see ultraviolet light, they produce and molt an extraordinary exoskeleton, and they posses’ magnificent colors and shapes. Few habitats exclude arthropods. In withstanding harsh environments, they are unparalleled. Some live in the arid deserts, some in hot springs up to 80 Oc, others 1 Medical Entomology on mountain picks as high as 6096 meters, some in tropical rain forest, and there are arthropods that live in Arctic O temperatures that reach below – 20 c. A second major reason is that knowledge of arthropods is essential as we manipulate eco-systems for increased food production and better health. Back in the early 1900s, many entomologists were concerned about the competition for food between human kind and arthropods, and some entomologists believed that arthropod control was imperative for survival of human race. Although such a position may seem some what extreme, arthropods do consume or spoil sufficient crops and products to feed many millions of people who starve each year. And arthropod/ insect –transmitted diseases, to humans, animals and crops, remain a threat to health and civilizations. As a result, it is good to understand “what arthropods are to the layman”. Fore example: To some people arthropods are unpleasant creatures: crabs, spiders, beetles, caterpillars. To some people arthropods are fliers: mosquitoes, moth, bees, and wasps. 2 Medical Entomology To some others, arthropods are biters and stingers; bees, wasps, ants, termites, scorpions. To some arthropods are jumpers: fleas, crickets, grasshoppers, To some people arthropods are singers: crickets, cicadas… Arthropods to some others are sign of good luck: fly, June beetle, To others, they are sign of evil eyes: black beetle. To other people, arthropods are source of food; larvae, ants, grasshoppers, etc To some people, arthropods are medical agents: bees, beetles, scorpions, spiders. The above are some of the thoughts of people who do not know the characteristics of arthropods and who had no chance to study entomology. Therefore it is worth to teach people the proper characteristics of arthropods by understanding entomology. 1.3 Definition of terms Entomology: It is a science that deals with the study of arthropods in general, and incorporates sciences like zoology, biology, parasitology and micro-biology. 3 Medical Entomology Arthropods: “Arthro” means jointed and “Poda” means legs. Arthropods are invertebrate animals with jointed-legs and identified by their peculiar characteristics. This will be described in detail in chapter two. Medical Entomology: This is a branch of entomology which deals with arthropods which affect the health and well-being of man and vertebrate animals. In other words, medical entomology is the medical science directly concerned with vectors that affect human and animal health. There are also other branches of entomology. For example: Industrial Entomology/Economical Entomology: deals with industrially or economically important arthropods (industrial pests). Agricultural Entomology: - Agricultural pest science dealing with arthropods that affect plants and animals. Mechanical disease transmission: disease agents are carried from one host to another by arthropods simply mechanically carried by the body parts (example wings, hairs, feces, vomitus, etc). In this type of disease transmission no change takes place in the number, form or developmental stages of the organism, but simply deposited in the body, food or drink of the host. Biological disease transmission: the agent will exhibit changes in form and or number of developmental stages in the arthropod before entry to the host. This includes hereditary 4 Medical Entomology (transovarian) and transital transmissions: Propagative, cyclodevelopmental and cyclopropagative. Propagative: In propagative type of disease transmission only the number of pathogens increases and the developmental stage remain constant. The diseases plague and typhus are good examples of propagative type of disease transmission. Cyclo-developmental: In this type of disease transmission, only the developmental stage (form) of the disease pathogen is changed (small to big, immature to matured stage, etc.), while the number of the pathogenic organism remains constant. Example Filariasis Cyclo-propagative: This type of disease transmission is a combination of both propagative and cyclo-developmental; whereby the disease pathogen undertakes a change both in number and developmental form (stage). Example Malaria. Trans ovarian/ Trans-stadial transmission: It is a type of disease transmission, whereas the causative agent is transmitted to the immature stage (usually to the egg) from the adult insects and / or other arthropods which carry disease pathogens. 5 Medical Entomology When the infected egg completes its developmental stage; it becomes infective or can transmit the disease to man and other animals. Ticks and sand flies are very good examples of arthropods that exhibit hereditary disease transmission. 1.4 Existing public health problems In tropical countries, the largest groups of illnesses are probably insect-borne. It is therefore, important to know the habits of the insect vectors and how they transmit diseases. It is difficult to implement control measures of insects, without some knowledge of entomology and specifically medical entomology. To this effect, this course is concerned with the study of arthropods (especially of insects) that are of public health importance. Due to their occurrence in large numbers in domestic situations, the problems of arthropods include spoiling food and other materials by their feeding habits, causing nuisance and perhaps being involved in the transmission of infectious organisms. Others feed on domestic fabrics and structure of buildings, rendering them unusable or unsound. Moreover, a wide array of arthropods cause toxic reactions in vertebrates. The cause of intoxication may be direct (bites, stings, 6 Medical Entomology defensive secretions) or indirect because of hypersensitivity (allergy). 7 Medical Entomology Review questions 1. Define entomology. 2. Define medical entomology. 3. Discuss existing public health problems associated with arthropods. 4. Explain the different disease transmission mechanisms by arthropods. 8 Medical Entomology CHAPTER TWO ARTHROPODS AND THEIR IDENTIFICATION 2.1 Learning Objectives By the end of this chapter, the learner will be able to: List common characteristics for the identification of arthropods Explain briefly taxonomy of arthropods Describe biological functions of arthropods Identify the arthropod habitat Explain importance of arthropods Discuss about the war against arthropods 2.2. Introduction Arthropods are by far the most successful phylum of animals, both in diversity of distribution and in number of species and individuals. They have adapted successfully to life in water, on land and in the air. About 80% of all known animal species belong to the Arthropoda - about 800,000 species have been described, 9 Medical Entomology and recent estimates put the total number of species in the phylum at about 6 million. Arthropods are found in a great variety of habitats than any other animal group; on top of mountains, at great depths in the ocean and in the icy wilderness of Antarctica. They can survive great extremes of temperature, toxicity, acidity and salinity. 2.3. Common identification Characteristics of Arthropods: Arthropods are grouped under the animal kingdom. They are invertebrate animals. Despite the enormous diversity found among them, they all share the following common characteristics: 1. Bilaterally symmetrical body sub-divided into segments. 2. Body covered with exoskeleton which is made up of a tough and rigid substance known as chitin. 3. Jointed appendages are present on some body segments. 4. Body cavity between the alimentary canal and the body wall. 5. Open circulatory system that works by diffusion unlike the arteries and veins in higher animals like humans which are the closed type. 10 Medical Entomology 6. Have ventral ladder type of nervous system: These are called ganglia and are situated at different places in the body of the arthropod with a ladder type linkage: message passes from one ganglia to the other and finally to the big ganglia at the head trough nerves. 7. Growth by molting, which is controlled by hormones 2.4. Taxonomy (Scientific Classification) of Arthropods The formal naming of arthropods/insects follows the rules of nomenclature developed for all animals. Formal scientific names are required for an unambiguous communication between scientists, no matter what their native language amongst the thousands used worldwide. Vernacular (common) names cannot fulfill this need: the same insects may have different vernacular names even amongst peoples that speak the same language. For instance, the British refer to ladybirds whereas the same coccinellid beetles are known to Americans as ladybugs. Many insects have no vernacular names, or one common name is given to many species as if only one is involved. These difficulties are addressed by the Linnaean system, in which every species described is given two names. The first is the generic name, used for a usually broader grouping than the second name, which is species 11 Medical Entomology name. Species is defined as a group of similar individuals which are able to produce fertile offspring. The complete scientific name (Bi-nomenclature) of an animal consists of the name of the genus (Generic name); which begins by a capital letter; and the species (Tribal name); which begins by a small letter. For example, the scientific (specific bi- nomenclature) name of:- Man is Homo sapiens and Malarial mosquito is Anopheles gambiae, Anopheles dancalicus etc. These Latinized names are always used together and are italicized. The combination of the generic and specific names provides a unique name for every organism. Thus, the name Aedes aegypti is recognized by any medical entomologist, anywhere, whatever the local name (and there are many) for this disease-transmitting mosquito is. In scientific publications, the species name is often followed by the name of the original describer of the species and year of registration. Various groups, also called taxa, are recognized amongst the insects. As for all other organisms, the basic biological taxon, lying above the individual and population, is the species, which is both the fundamental nomenclature unit in taxonomy and arguably, a unit of evolution. Multi-species studies allow recognition of genera, which are more or less discrete higher 12 Medical Entomology groups. In a similar manner, genera can be grouped into tribes, tribes into subfamilies, and subfamilies into families. The families of insects are placed in relatively large, but easily recognized groups called orders. This hierarchy of rank (or categories) thus extends from the species level through a series of ‘higher’ levels of greater and greater inclusivity until all true insects are included in one class, Insecta. The animals identified with the characteristic listed above belong to the phylum arthropoda. The phylum arthopoda, like those of other phyla, have certain subgroups. The largest and most inclusive group is the kingdom, and the smallest is the species. For example, the classification of man and malarial mosquitoes from the kingdom down to the species is shown below for illustration. Classification Man Malarial mosquito Kingdom Animal Animal Phylum Chordata Arthropoda Class Mammalia Insecta Order Primates Diptera Family Hominidae Culicidae Genus Homo Anopheles Species sapiens gambiae, nilli, kingi, dancalicus, fenstus, smithi 13 Medical Entomology etc. Arthropods can commonly be classified into different sub groups as shown in the classification tree (figure 2.1) below. The phylum arthopoda is the largest of the animal phyla. There are numerous classes under it, but about five of these classes are medically important. 14 Medical Entomology Arthropoda Insecta Arachinida Diplopoda Chilopoda Crustacea Apterygota Pterygota Dictyoptera Hemiptera Anoplura Diptera Siphonaptera Cockroach bed bugs lice Fleas triatominaebugs Nematocera Brachycera Cyclorrhapha Culicidae Psychodidae Simulidae Tabanidae Ostridae Muscidae Glossinidae Phelebotominae horse fly myce fly house fly tse tse fly 15 Medical Entomology Anophelinae Culicinae Black fly Anopheles Culex,Aedes Figure 2.1: common classification tree of phylum arthropoda (adapted from public health pests) 16 Medical Entomology The medically important classes are the following: 1. Class Insecta/Hexapoda (the six legers)-The insects. The insects (class insecta) are the most abundant species. In fact, about ¾ million species are known, i.e. about 75% of all arthropods are insects. They are the greatest pest animals as well and the greatest animals of medical importance (lots of diseases are transmitted through insects). The body parts of insects are grouped into three: head, thorax and abdomen. The head contains eyes, one pair of antennae and three pairs of appendages developed as mouth parts (details are discussed in chapter four). The thorax has three pairs of legs, and one or two pairs of wings in most insects (some insects have no wings). The abdomen is segmented with the end (posterior) part serving to show the sex of the insect. A “V” shaped abdomen helps for egg disposing, carrying the genital and excretory organs. The abdomen also contains diffusion tubes called spiracles for air exchange (respiratory organs). 2. Class Chilopoda - The centipedes (they have one pair of legs per segment) 3. Class Diplopoda - The millipedes (they have two pairs of legs per segment). 17 Medical Entomology The centipedes and the millipedes jointly are known as the Myriopods. They are very similar to the arthropods in their superficial appearance, but they have distinct heads bearing antennae and mouth parts. They bear two body divisions; head and trunk. The biting of myriopods is said to be allergic to some people, while some are also venomous. The millipedes secret chemicals for defense purposes (bad smell). 4. Class Crustacea - Cyclops, the sea- food group such as lobsters, crabs, cry fish, etc The crustacean has evolved a two fold division of the body into a cephalothrax means prosoma (head and chest) and opisthosoma means abdomen. The former bears sensory organs and mouth parts to form the head region and also five pairs of enlarged appendages for walking (in the higher forms). The crustaceans have two pairs of antennae. The prosoma carries the main sense organs (internally and externally) that is the antennae, the eyes and the feeding parts. Opisthosoma consist the spiracles (respiratory organs) and the sex organs 5. Class Arachnida(the eight legers) - Spiders, mites, ticks, scorpions etc. The class Arachnida has four pairs of legs. The head and the thorax are fused forming a cephalo-thorax. The appendages 18 Medical Entomology (legs) are located on the cephalothorax. The head has no antennae, but pedipalps and different mouth parts from that of insects. 2.5 Arthropod Habitats Depending upon species, arthropods live in various habitats. The following are some of the factors that control habitats of arthropods: food, disease, breeding media, climate, competition, natural enemies and etc. The habitats of arthropod include soil, water, ambient air, man, animal and plants. 1. The soil: Arthropods may be found on the surface of the soil or under ground (in pebbles, in bolder, in caves, in the sand, in lime stone formations, etc). Examples: ants, termites, beetles, spiders, wasps, mites, scorpions, flies, crickets, cockroaches, moths, fleas, cicadas, etc. 2. Water Arthropods may live in fresh waters (natural or man made), salty waters (Oceans, seas) or hot springs. Examples of water dwellers are: backswimmers, crabs, lobsters, crayfish, etc. 19 Medical Entomology 3. In the ambient air (temporary fliers) The ambient air although can not be a permanent habitat, some arthropods specially the fliers can be found temporarily. The fliers are fast spreaders of contamination and pollutions. Flying is high civilization in the culture of arthropods (as well as man). Speed is one of the factors for survival of the fittest. Therefore, it is hard to control the fliers. Examples of fliers are: Bees, beetles, mosquitoes, flies, grasshoppers, wasps, butterflies, moths, dragonflies, ants and termites (initially), etc. 4. On man: Ectoparasites- these are dangerous groups to health since they feed on human blood. These parasitic arthropods could be obligatory ectoparasites (example louse) or intermittent (on and off: example ticks). 5. On animals: Examples lice, ticks, mites, fleas, mosquitoes, ox-warble fly, etc. 6. On plants: Examples Beetles, aphids, spiders, gall insects, scale insects, manna insects, lacs etc. 2.6. Arthropod Species Abundance There are more species of insects than all other animals combined. Percentage of insect species in comparison with other animals: 20 Medical Entomology Category Examples Percentages 1. Insects (all orders of insecta) Fleas, flies, lice, 70% Grasshoppers, wasps, beetles, Butterflies, etc 2. Other arthropods (arachnida, Ticks, mites, spiders, 8% chilopoda, diplopoda, crustacea, scorpions, centipedes, and others) lobsters, crabs, etc 3. Mollusca Snails, oysters, clams, 9% Octopus, etc. 4. Chordate (mammals, reptiles, Man, bird, fish, elephant, 6% Snakes, etc. 5. Other animals Microorganisms 7% 21 Medical Entomology 2.7. Advantages and disadvantages of arthropods The effect of arthropods may be seen in relation to health and their benefit. A. Health Effects: Arthropods affect the health and comfort of man in many different ways. The common fear of insects (entomophobia) possessed by many people is perhaps the least serious. Proper knowledge of the appearance of harmful, beneficial and harmless arthropods can do much to remedy these conditions. The Following are some examples of the health effects attributed to arthropods: Arthropods attack man, domestic and wild animals. They bite and suck blood. They pass infective organisms and may inject toxin to man and animals (mechanically or biologically). They cause myiasis (infestation by larva of diptera) on man and animals. Annoy and irritate man and animals. They cause envenomization by their bite, sting, spines or by their secretions. Envenomization may cause 22 Medical Entomology swelling, pain, redness, rash, fever, allergic reactions, blood poisoning, or death in some cases. Arthropods parasitize man, animals and plants: for example louse, and ticks on animals, and aphids on plants. Cause accidental injury to sense organs: they enter the eyes, ears, mouth or nostrils. They cause allergic/asthmatic reactions by their odor, secretions, and by their dead body fragments. Crop adulteration is another effect of arthropods due to their droppings of fecula, dead body, egg shells, urine or microorganisms. Arthropods cause Entomophobia (fear of insects): nervous disorder, hysterics, hallucination etc. The following are also other examples of some arthropods that may affect human comfort and health: 1. Chigger – causes intense itching; dermatitis 2. Rat mite – causes intense itching; dermatitis 3. Grain itch mite – causes dermatitis and fever 4. Scabies mite – burrows in skin causing dermatitis (scabies). 5. Hard ticks – painful bite, tick paralysis, usually fatal if ticks not removed: 6. Soft ticks – some species are very venomous. 23 Medical Entomology 7. Black widow spider – its bite causes local swelling, intense pain and occasionally death. 8. Scorpions – painful sting, sometimes death. 9. Centipedes – painful bite. 10. Lice- intense irritation, reddish papules. 11. Bedbug – blood suckers (irritating to some). 12. True bug – painful bite, local inflammation. 13. Beetles – severe blisters on skin from crushed beetles. 14. Caterpillar –rush on contact with the hairs or spines. 15. Bees, wasps, ants – painful sting, local swelling. 16. Flies – painful bite, swelling, bleeding puncture, myiasis. 17. Mosquitoes – irritation. B. The Beneficial Effects of Arthropods Fortunately, for mankind not all arthropods are harmful. Arthropods contribute to a lot of benefit on commercial products, agriculture and health. It is therefore, necessary to distinguish between “beneficial” and “harmful” arthropods. The following are some understood benefits of arthropods: 1. One of the greatest benefits man receives through arthropods (insects) is the pollonization of plants. Approximately 50 seed and fruit crops depend on honeybees for pollinization. Clover, onions, apples and others would not yield without insect pollinators, butterflies, ants, flies and bees 24 Medical Entomology 2. Silk is produced by insects. The caterpillar of the silk producing moth (Bombyx mori) spins a cocoon prior to changing to the adult stage. The filament forming the cocoon is continuous and ranges in length from 800 – 1200 yards. An ounce of silkworm eggs will produce 30,000 - 35,000 caterpillars which will yield 100 - 200 pounds of cocoons. These cocoons will produce 10 -12 pounds of raw silk. 3. Honey and wax is the other product of insects (honey bees must collect 37,000 loads of nectar from plants in order to make one pound of honey). 4. Arthropods are very helpful in improving the soil. The burrowing of ants, beetles and other insects enables air to penetrate the soil. 5. Lac insect (Kerria lacca) is a source of a commercial varnish. 6. Arthropods that prey upon and destroy other animals are called predators. They help to reduce the number of insects. E.g. spiders, ants, dragon flies. 7. Some arthropods lay their eggs on the larva of other arthropods, the eggs hatch and the young larvae feed up on the body juice of the host as a parasite. 8. Insects are valuable as food for humans and animals. Chickens, turkeys, hogs and fish utilize many insects as an important source of their food. Some of the primitive 25 Medical Entomology races of man use insects for food. E.g. grasshopper, cricket, beetle, caterpillar, termite and ant. 9. More than other things insects have served man as sources for scientific knowledge and technological innovations: Man has learned science of flight (planes, space crafts) from insects. Insects have thought man flight techniques since they had this civilization long before man. Learned carpentry wood - work such as tunneling from beetles. The science of navigation and communication, utilization of air conditioning, use of photogenic light, use of chemicals as warfare, paper production, pottery, engineering work, farming, etc.So many technologies seem to have been copied from arthropod works. 2.8. General control methods of arthropods The war (control and eradication) on arthropods by man has been going on for long period of time. The war will probably continue because man has never eradicated a single insect species except in certain limited localities. The control of medically important arthropods employs many principles used against pests of agricultural importance, but there may be 26 Medical Entomology quite different reasons for control. The basic purpose in controlling medically important arthropods is to preserve the health and well-being of man, where as control of arthropods affecting crops and livestock is fundamentally guided by economic principles. The protection of human lives and promotion of human comfort can not be measured by monitory considerations alone because man views his own well fare as priceless. Complex ethical and emotional considerations arise when control practices affect a whole region, and in order to be effective, the application of pesticides may involve public and private property and agricultural, urban and wild lands. Controlling arthropods includes the following methods: 1. Personal protection: Physical barriers between a vertebrate and arthropods, chemical barriers that repel arthropods from actual biting; and arthropod toxicants that are applied directly to or with in a vertebrate. E.g. Insecticide treated bed nets are widely used in Ethiopia and sub-Saharan Africa and subtropical countries worldwide for the control of malaria and leishmaniasis. 2. Environmental manipulation: modification of the specific breeding habitat of an arthropod can provide effective control. For example, drainage of marshy areas, destruction of burrow pits and hoof prints for controlling malaria 27 Medical Entomology 3. Barrier zones and quarantines: an area free from certain vectors, either naturally or as a consequence of control programs, may need protection from invasion. 4. Biological control (Bio control): All animal populations, including arthropods affecting man and animals, are reduced in numbers by certain other forms of life. For arthropods, these control agents are categorized as predators (both vertebrate and invertebrate), parasites (generally meaning metazoan arthropods or nematodes), or pathogens (viruses, rickettsiae, bacteria, fungi, protozoa etc). Insects can also be controlled genetically. 5. Local control methods: Even though it is difficult to take control action against arthropods without the fundamental knowledge of entomology and other related sciences, it is obvious that through trial and error, man has established many local ways of fighting and controlling arthropods. The following are some of these local control methods practiced in our (Ethiopian) communities: 1. Hot ash to kill or drive away insects like ants. 2. Hot water against ants, bedbugs, lice, fleas, etc 3. Certain odorous and sticky leaves like white eucalyptus, mimosa, etc are used as insect repellants. 28 Medical Entomology 6. Chemical control methods: Chemical insecticides act in two ways: - As stomach poisons: These are taken up by the insect in the form of bait, or may be applied to surfaces over which the insect (arthropod) will walk, taking up material on its legs and body. This will then be taken into the alimentary canal when the insect cleans itself. - As contact poison: These may be applied to the atmosphere through which the insect is flying, or to surfaces over which it will walk. The chemical penetrates the cuticle or enters the spiracles and, depending on the active ingredient, will act on the nervous system by disrupting nerve impulses, causing uncoordinated behavior followed by paralysis and death of the insect. For example indoor residual house spraying. 29 Medical Entomology Review questions 1. List common identification characteristics for arthropods 2. Explain taxonomy of arthropods 3. Discuss health effects and benefits of arthropods 4. Enumerate the arthropod habitats 5. Discuss the general methods of control of arthropods. 30 Medical Entomology CHAPTER THREE THE INSECTS 3.1 Learning Objectives By the end of the chapter the learner will be able to: Explain insect diversity Discuss uses of insects Identify the different orders of insecta Recognize the reasons behind the success of the diversity of insects Understand the different types of insect collecting techniques 3.2. Introduction Insects are extremely successful animals and they affect many aspects of our lives, despite their small size. All kinds of natural and modified ecosystems, both terrestrial and aquatic, support communities of insects that present a bewildering variety of life styles, forms and functions. Ecologies of insects are highly diverse and often they dominate food chains and food webs in biomass and species richness. They may be aquatic or terrestrial throughout, or during part of their lives. Their life styles encompass solitary, gregarious, sub social 31 Medical Entomology and highly social modes. They may be conspicuous or concealed and active by day or night. Insect life cycles are adapted to a variety of abiotic conditions, including seasonal extremes of heat and cold, wet and dry, and notably to unpredictable climate. Therefore, insects should be studied for many reasons. 3.3. The Orders of Insecta The insecta (hexapoda) constitute the largest class in numbers of species in the phylum arthropoda, which in turn comprises of a greater number of species than all other phyla of the animal kingdom combined. Various estimates of described species of insects in the world range from 625,000 to 1500000, and the number ultimately known will probably be much greater. The following (table 3.1) includes those insects that are of some known public health importance. 32 Medical Entomology Table 3.1 The number of described species of important orders of insects Order Common names Estimated No in the world 1. Diptera Flies, gnats, mosquitoes… 85,000 species 2. Anoplura sucking lice 250 species 3. Mallophaga Chewing lice 2,675species 4. Hemiptera True bugs 55,000 species 5. Siphonaptera Fleas 1,100species 6. Hymenoptera Ants, bees, wasps 103,000species 7. Lepidoptera Butterflies, moths 112,000species 8. Orthroptera Grasshoppers, Crickets 22,500 species 9. Coleoptera Beetles, weevil’s 277,000 species 10. Dictyoptera Cockroaches 4000 species 11. Isoptera Termites 60 species 33 Medical Entomology According to the classification system used, some 26-29 orders of insects may be recognized. Differences arise principally since there are no hard- and-fast rules for deciding the taxonomic ranks. Brief descriptions of some of the orders of insecta which are of public health importance are presented below. Order Diptera Members of the order diptera are a diverse group in both structure and development. These include all the flies, gnats and mosquitoes. Beyond their having a single pair of wings (the hind are reduced to balancing organs called halters) and all being homometabolic, the suborder have quite different patterns of development and structures. There are over 85000 species of dipterans in 140 families. The insects grouped into the order diptera are the two winged (di = two; ptera = wings). The wing could be used as a classifying factor. As insects, they are with three body division (head, thorax, and abdomen). All these insects are characterized by having only one pair of wings; the hind pair has degenerated, therefore, all that remains is a pair of drumstick-like organs, the halters, used for balance in flight. 34 Medical Entomology Dipterans are important to humans for a variety of reasons, many flies are pests. In addition, many serve as either mechanical or biological vectors of infectious agents. Tse tse fly transmits the agent causing African sleeping sickness; mosquito transmits malaria, lymphatic filariasis, and hundreds of viruses; biting midges transmit filarioid nematodes and viruses such as blue tongue virus; tabanids transmit tularemia. Since these flies are blood-suckers, they can be serious pests regardless of whether they are vectors of infectious agents. Many flies are parasitic as larvae; they can be serious medical and economic problems. Diptera are only able to take fluid food, which in the case of bloodsucking flies is obtained by injecting the piercing mouthparts (proboscis) into living tissue. In other flies, food is liquidized externally by puddling it with spongy mouth parts in digestive fluid regurgitated from the foregut (crop). All Diptera go through a complete metamorphosis in their life cycle, developing from the egg through a number of larval stages to the pupa from which the adult emerges. The larva, which is the feeding and growing stage, is typically found in a completely different environment from the adult, although the adult will be associated with the larval environment when mating and laying eggs. 35 Medical Entomology A large group within the diptera, sometimes known as the calypterate flies (because the halters are shielded from above by saucer-like processes known as calypters), includes houseflies (musca species), bluebottle (calliphora species), green bottle (lucilia species), lesser houseflies (fannia species) and grey flesh flies (sarcophaga and wohlfahrtia species). These species are closely associated with human and have adapted to the human domestic environment (synanthropic). They are small to moderate, wings restricted to mesothorax, and metathorax. Mouth parts vary from non-functional to biting and sucking. Immature stages (larvae, maggots) variable, without jointed legs, with sclerotized head capsule or variably reduced ultimately to remnant mouth hooks. Mouth parts of diptera vary into two aspects: Those having spongy (non-biting) mouth parts and not able to penetrate into the skin. Example Male mosquitoes. Some feed on plant flower nectars, hence not risk to health. But some others can feed on solid substances (by dissolving) or fluids by sucking. So they are dangerous for transmission and contamination of food and utensils with disease agents; example the common housefly 36 Medical Entomology Groups with biting mouth part/piercing and sucking type. These are with sharp mouth parts (proboscis) for piercing the skin and blood sucking; so important biological vectors; example. Tse tse fly, female mosquitoes, etc The mode of development (life cycle) of all the diptera group is complete (complex) metamorphosis. The presence of a pair of halters (i.e. two halters) at the base of the thorax is another factor for identification of diptera. In the laboratory, diptera are the most used as experimental subjects for various research works: example the drosophila groups are used to study population explosion modeling. 37 Medical Entomology (A) (B) Figure 3.1. Examples of Diptera: A) Musca domestica, the house flies B) Typical mosquitoes (adapted from public health pests. A guide to investigation, biology and control 1990) 38 Medical Entomology Order Anoplura. ‘a’ means without, ‘oplas’ means sting, and ‘oura’ means tall. These are the sucking lice. They are minute to small (from 0.4 to 6.5 mm) and may be characterized by their narrow than long head, two to five segmented antennae, piercing-sucking mouthparts that are retracted into head, greatly reduced eyes, absence of wings and cerci, and dorsoventrally flattened body. The legs are short, and the single tarsus and claw are modified into a grasping organ. Sucking lice feed on blood, and their entire life cycle is spent on mammalian hosts. Metamorphosis is incomplete (gradual). Eggs are glued to the hair of the host. A high degree of host specificity and preference for specific regions on the host are recognizable. The human louse, Pediculus humanus, infests humans, and whether it feeds on the head or body region has direct influence on its morphology and behavior (these two varieties, head (Pediculus humanus capitis) and body lice (Pediculus humanus corporis), sometimes treated as two separate species). They are very similar in appearance, but biologically they are very different; the head louse is found only on the hair of the head, sucking blood from scalp, where as the body louse lives on underclothing and feeds on the body. Adults appear about nine days after hatching from the egg. The crab louse, Pthirus pubis, another species found in 39 Medical Entomology man is found mainly in the pubic and perianal region of humans. The pubic louse doesn’t transmit disease. However, an infestation known as phthiriasis or ‘crabs’ may cause considerable discomfort and sometimes embarrassment, since it is typically acquired by close contact, usually sexual intercourse, with an infested person. Prevalence of louse in the human population is a sign of poverty and unhygienic life. Figure 3.2. Lice of humans. A) The body and head louse B) the pubic louse (Adapted from fundamentals of Entomology Third edition, 1987) 40 Medical Entomology Order Mallophaga These are the chewing lice. They are small (from 2 to 6 mm) and have a head usually broader than long, modified chewing and piercing mouthparts, reduced compound eyes, two to five segmented tarsi, no cerci, and lack wings. The body is flattened dorosoventrally. Eggs are fastened to feathers or hair of the host. Metamorphosis is incomplete (gradual). Both nymphs and adults ingest dead skin, feather, hair, or scabs. Under high population pressures, the dermal skin layer also may be attacked, particularly around wounds. There are 2,675 species and these are divided into six families. Most chewing lice infest birds, although a few utilize mammals as a host. Host specificity is marked, transferred to one host to another normally occurs only between two birds of the same species as the birds mate or nest. If a host dies, the louse fauna usually perishes. This order is of economic importance when domestic animals become infested; over 40 species are known to parasitize poultry. Loss of weight and lowered egg production, in the case of birds are two common results of infestations. The chewing lice spend their entire lives on animal hosts like sheep, goat, horses, cattle and antelopes. Man comes in between when caring for these animals. The chewing lice feed on blood by sucking. Their behavior of continual host contact 41 Medical Entomology and their blood sucking habits make them potentially dangerous vectors. Figure 3.3. An adult Chewing louse (Adapted from fundamentals of Entomology Third edition, 1987) Order Hemiptera. ‘Hemi’ means half and ‘ptera’ means wing. Some groups of this order are winged and some others non- 42 Medical Entomology winged. They are sometimes called the true bugs. Examples: - Bedbug, assassin bug (killer bug), kissing bug. The true bugs vary in length up to 100mm, compound eyes are usually large, antennae are from four to five segmented and often longer than the head. Mouthparts are piercing- sucking with the segmented beak arising from the anterior of the head, tarsi are one-three segmented, and cerci are absent. In most species, wings are present and positioned flat over the abdomen when at rest, separated by an enlarged scutellum; the front pair of wings is usually thickened at the base and membranous apically to form a hemelytron. The hind wings are membranous and slightly shorter than the hemelytra. In some like bedbugs, poultry bugs and bat bugs, the wings are reduced to inconspicuous pads. Great variation in legs exists. Metamorphosis is gradual (incomplete). Eggs are deposited in the habitat in which development occurs; many nymphs and adults are terrestrial, but a significant number are aquatic. Food is liquid (either sap or blood) and varies from the common herbivores to carnivorous. A number of true bugs are of economic importance. Some species of assassin bugs are naturally infected with Chagas’ disease; most of these bugs belonging to the genus Triatoma. The infection may be transmitted to humans by rubbing the 43 Medical Entomology protozoan organism in Triatoma feces through the skin by scratching. Groups of this order may serve as vectors on man and animals (e.g. chagas disease or trypanosomiasis is transmitted by the bite of kissing bug through armadillos). They have a life cycle of gradual metamorphosis. They may be detected in various habitats which include: Water habitat: Examples of some of the bugs inhabiting on water bodies are: The water-striders: walk on water The back- swimmers: swim on their backs The water-boatman: row on the water. Habitat on the land: Examples are the bed-bug and the kissing bugs. The bed-bug gives irritation while biting to suck blood and is annoying and a nuisance insect. Nevertheless, no disease is known that is transmitted by bed-bugs. Facultative: Habitat both on water and on land Example: The winged bug known as the giant-water bug. It is big in size and is known to posses some sort of electrical charge (shock). 44 Medical Entomology Figure 3.4. The bed bug (Adapted from fundamentals of Entomology Third edition, 1987) Order Siphonaptera: ‘Siphon’ means tube, ‘a’ means without, and ‘ptera’ means wing. These are the fleas. Fleas are wingless. They are all blood-sucking, temporary ectoparasites of warm-blooded animals, mainly mammals, but a few will feed on birds. Only a small proportion will attack humans. Fleas, like bedbugs, are comparatively host-specific, but will often feed readily on other animals if their preferred host is not available. 45 Medical Entomology Fleas are minute to small (from 0.8 mm to 5 mm) and have the following characteristics: compound eyes are absent or each is represented by a single ommatidium. Most fleas have a pair of small simple eyes (ocelli), although some are blind, usually those which live on hosts with underground burrows. Antennae are short and can be folded into grooves in the head, mouth parts are piercing-sucking, coxae are long and tarsi are five-segmented, cerci are small and one segmented, and wings are absent. Fleas are flattened from side to side (laterally, as opposed to dorsoventrally in most insects); this is a useful adaptation to enable them to move easily through the hairs or feathers of their host. Their length ranges between 1-6 mm; they are oval in shape and light to dark brown in color. The small head has a proboscis that projects down wards and small antennae recessed into grooves. Fleas have powerful legs adapted for jumping and can leap 10-15 cm. The abdomen is the bulkiest part of the body and is conspicuously segmented. The ending is rounded in the female, whereas in male the genitalia are apparent. Metamorphosis is complete. Eggs are oviposited on the host or more often in the host’s nest; in the former case, eggs fall off prior to hatching. The legless larva feeds upon such 46 Medical Entomology organic matter as may be available including fecal material from adult fleas that contains blood residues. Pupation is in silken cocoons. Adults feed on blood from either birds or mammals, the latter being more common. Some species predominantly live on the host, but if the host has a nest, many species of fleas leave the host during non-feeding periods. Beyond irritation, fleas are of medical importance to humans through disease transmission. Fleas are vectors of plague (bubonic form) and endemic or murine typhus. Several species of tape worm can, but not commonly, infect humans after utilizing the flea as an intermediate host. In the tropics, the chigger flea attaches itself to humans and can initiate severe lesion. Fleas can also become pests to such domesticated animals as dogs and cats. They are vectors of disease. They are associated with mammals including man. All mammals have fleas of their own (dogs, cats, etc). Diseases from these animals could be transmitted to one another and to human beings. The bubonic plague is an epidemic between rats, flea and man. Fleas are also causes for chigger on man and other animals. Fleas are annoying and irritating. 47 Medical Entomology Figure 3.5. An adult Flea (Adapted from fundamentals of Entomology Third edition, 1987) Order Hymenoptera These are insects having wings which is membrane like. They include the ants, bees, and wasps. Their sizes range from 0.21 to 65mm in length, excluding the appendicular ovipositor. Characteristics include filiform antennae, chewing or chewing- lapping mouthparts, large compound eyes except for ants, long legs with five segmented tarsi, cerci minute or absent, and wings absent or two pairs that are long and narrow with 48 Medical Entomology fused venation. Metamorphosis is complete. They are described as socially organized groups with labor division and cast system. Through instinct they behave like civilized. The queen is the organizer and the mother. The workers are sterile females; collect nectar, fight enemies, clean the home, remove dead body, etc. Hymenoptera, all posses’ two pairs (four) of wings; are fliers (ants though initially have wings loose them because they are not firmly attached to the body). The groups of this order have mouth parts of the chewing type (chewing mandibles). The mouth of bees has saw like structure and is also adapted to sucking. Biological use: Bees, wasps: cross pollination of plants. Ants: serve as biological control means by feeding on larvae of others destroying unnecessary pests as in orchards and gardens; certain ants kill cockroaches in dwellings. Help clean the environment-feed on some wastes from homes and kitchens such as bones, flesh, orange peel, etc. Vector ness: Not of significant role due to their habitat, but rarely may serve in mechanical contamination 49 Medical Entomology since certain species of ants readily enter houses and are attracted to human food, they are capable of contaminating such foods with viable pathogens on their bodies or in their digestive tracts or mouth parts. Economic advantage: Bee honey production Other health problems: Venomization, annoyance. (C) 50 Medical Entomology Figure 3.6. Examples of hymenoptera. A) Wasp B) An ant C) A bee (Adapted from Parasitology and vector biology second edition, 2000) Order Lepidoptera: “Lipid” means scale, and “ptera” means wings) The order Lepidoptera comprises of the moths and butterflies. These are the most beautiful of all insects; so are frequented as collectors’ items. Color is the result of not only of pigments in the hair and scales, but also from structural ridges and layers that reflect light differently to cause iridescence. Antennae vary greatly and are useful in identification. Groups of this order have two pairs (four) wings, but also absent in some (rarely). The adults have sucking type mouth parts. They are of advantage in plant cross-pollination and some are silk producers (cocoon of the bombidae family = silk worm); nevertheless their larvae are the greatest economical destructors. Their being a vector is not of significance. All Lepidoptera go through a complete metamorphosis, eggs being laid on the food, plant or other material on which the caterpillar-like larval stages feed. In addition to true legs on the thorax, these larvae have several pairs of stumpy false legs (pseudopods) on the abdomen (in comparison with the beetle larvae which do not have pseudopods). The pupal stages are in the form of a chrysalis, often in a web or cocoon. 51 Medical Entomology Examples of larvae of Lepidoptera which are known for their economical destruction include: the army worm, the cloth moth, plant worms (A) (B) Figure 3.7. Examples of lipdoptera: A) Buttery fly B) A moth (Adapted from fundamentals of Entomology Third edition, 1987) Order orthoptera Orthroptera are insects having straight wings. These Include such insects as grass hoppers, preying mantids, katydids, crickets, walking stick, etc. All posses chewing mouth parts, long legs with 1-5 segmented tarsi, and large compound eyes. Wings are usually present and have many veins and are modified with the fore wings often narrowed and thickened into a tegmen, where as the hind wings are broad, membranous, and folded fanwise under the mesothoraxic pair. Flight is mainly through action of the hind wings. Stridulation or sound 52 Medical Entomology production by scraping is a means of attracting mates. An appidicular ovipositor is common and often measures as long as the abdomen. Cerci are often short. Antennae commonly are elongated and multi segmented. Size ranges from 12mm to over 250 mm in length. Egg laying is variable some eggs deposited in the soil (short-horned grasshoppers), but others are deposited in or on vegetation (long-horned grasshoppers). Metamorphosis is incomplete. Most orthoptera are herbivorous, but some are carnivores (mantids). Some species are of economic importance e.g. grasshoppers have been pests of crops through out recorded history, especially in the temperate and arid regions of the world. Field crickets may damage seedlings in truck crops. 53 Medical Entomology Figure 3.8. Examples of orthoptera: Upper, Left to right: Katydids, crickets. Lower grasshopper. (Adapted from the insects. An outline of Entomology1994) Dictyoptera (cockroaches) Cockroaches are an ancient group, extending back to the Silurian and showing little change in general structure since the Devonian, some 320 million years ago. Though pest species are for the most part cursorial (running) insects and nocturnal, many others are active diurnal fliers, inhabiting tropical forests. Others live in the ground or under stones, boards, or various types of rubbish; some are commensal or suspectedly so in nests of ants, termites, or wasps; some inhibit rodent burrows or live in caves in association with bats; some are even aquatic or bore into decayed wood. Cockroaches are usually flattened dorsoventrally with a smooth (sometimes pilose) integument, varying in color from chestnut brown to black in the more pestiferous house- invading species, but are frequently green, orange, or other colors, specially in the tropical species. The prominent antennae are filiform and many- segmented. The mouth parts are of the generalized biting-chewing type (orthopteran). There are two pairs of wings in most species; in some, the wings are vestigial; in others, for example, Blatta orientalis, they are well developed in the male and short in the female. 54 Medical Entomology The outer pair of wings (tegmina) is narrow, thick and leathery; the inner pair is membranous and folds fanlike. The cockroaches are mechanical disease transmitters because of their dirty living and feeding habits, as well destroy property in homes: (they can destroy book bindings and practically eat every human food including human waste). Their body parts (if inhaled as dust contact) produce allergy to some people. The so called domiciliary, domestic, or synanthropic species are becoming adapted to living in close association with man in homes, restaurants, hotel kitchens grocery stores, rest homes, dump basements where food is available, sewer systems connected with any of the above or other man made structures that provide sufficient moisture, food, and hiding places; they carry contaminants to human food, pollute air with their allergens, produce their characteristics disagreeable odors, and degrade the environment aesthetically Cockroaches. Cockroaches favor environments where both human pathogens and human food are found and they pass readily from one to the other. They may carry pathogens in and on their bodies, and these may remain viable on the cuticle and in 55 Medical Entomology the digestive tract and faces to the extent that the insects may even be chronic carriers. Figure 3.9. An adult Cockroach (adapted from public health pests. A guide to investigation, biology and control 1990) Order Coleoptera: 56 Medical Entomology “Coleas” means sheath or cover and “ptera” means wings. This group is identified by having two pairs (four) of wings and mandibulate (biting-chewing mouth parts). They vary from small to large, often sturdy and compact, heavily sclerotized or armored, with fore wings modified as rigid elytra covering folded hind wings at rest, legs variously modified, often with claws and adhesives structures. Immature stages (larvae) are terrestrial or aquatic with sclerotized head capsule and opposable mandibles. The coleopteran pass through complete (complex) metamorphosis. They are the largest in the number of species (277,000) compared to other animal groups. The order of coleoptera is the beetles and the weevils. Examples of beetles are tiger beetle, whirligig beetle ground beetle, and diving beetle. Examples of some weevils are boll weevil, bean wevil and root worm. Some coleopterans are scavengers (dead plant eaters= Phytophagous and dead animal eaters = Saprophagous) and some others are predators; hence aid as environmental cleaners. Examples of scavenger/predator beetles: Scarab beetle: removes human and animal wastes (excreta) Carrion beetle: feeds on dead bodies (carcass). Rove beetle: snail eater 57 Medical Entomology However, most are well known as economically destructive groups; example: Weevils: spoil cotton, grains Beetles: destroy potatoes, wood, skin and hides (e.g. Torgidae and dermastidae are two species of beetles which are eaters of skin and hides) The beetles form the largest of the insect orders. They are extremely varied in size, shape and habitats. Only a few species are of public health significance, feeding on stored products, clothing, furnishings and wood. Beetles go through a complete metamorphosis in their life cycle. The larval stage often the most destructive, but many adults are also of economic importance. Beetle larvae have a conspicuous head capsule and six legs on the thorax. They do not have the stumpy false legs (pseudopods) which moth and butterfly caterpillars have on the abdomen. Beetles may be found in land (soil), plants, or in water bodies. Some groups of coleopteran are vectors or may release harmful chemicals. Examples: - Mechanically scatter microorganisms (contamination): scarb beetle-works on human excreta. 58 Medical Entomology Chemically skin blistering: Meloidae groups. Some serve as intermediate hosts of helminthic parasites. Accidental invasion of natural body openings by beetles is also common. (A) (B) (C) Figure 3.10. Examples of coleopteran: A) Ground beetle (Carbidae) B) Scarab beetle (Scarabeidae) C) Long-horned beetle (Cerambycidae) (Adapted from fundamentals of Entomology Third edition, 1987) Order Isoptera The termites (white ants) are grouped in this order. They have two pairs (four) wings although temporarily used. Termites 59 Medical Entomology vary from 2mm to 12mm in length, except for physogastric queens. Termites are characterized by a prognathic head, moniliform antennae with from 9 to 30 segments, chewing mouth parts, short and stout legs with four-segmented tarsi normal, 1 to 8 segmented short cerci, and an absence of wings except for the reproductive caste. Wings, when present, are longer than the body and are membranous. Fore and hind wings are similar in shape and size. White termites are of great biological and economic importance. In the tropics and in forests their feeding recycles nutrients and aids in soil development. In other instances, however, their eating is in direct conflict with humans. Since Isoptera feed upon paper, wood, and other similar cellulose goods, they cause considerable damage. The life cycle of termites is a gradual (incomplete) metamorphosis. All termites are social. Being socially organized, they have labor divisions: queen (mother), soldiers, workers. Termites are destructors in economic sense as they are able to ruin, destroy or spoil house, plants and the soil. The termatica is the house of subterranean termites; spoils crop, forest, grassland and the soil. 60 Medical Entomology Figure 3.11. Examples of Isopteran (Termites). (Adapted from fundamentals of Entomology Third edition, 1987) 61 Medical Entomology 3.4. Uses of insects Insects are valuable to man,though, we think of them in a negative context. Insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit horrible diseases. But without them, we could not exist. They are a fundamental part of our ecosystem. A brief and incomplete list of their positive roles would include the pollination of many, perhaps most higher plants; the decomposition of organic materials, facilitating the recycling of carbon, nitrogen, and other essential nutrients; the control of populations of harmful invertebrate species (including other insects); the direct production of certain foods (honey, for example); and the manufacture of useful products such as silk and shellac. 3.4.1 Insects as human food (entomophagy): About 500 species of insects in more than 260 genera and 70 families are used for food somewhere in the world, especially in central and southern Africa, Asia, Australia and Latin America. Insects are high in protein, energy and various vitamins and minerals: they can form 5-10% of the annual animal protein consumed by some indigenous peoples. Termites, crickets, grasshoppers, locusts, beetles and moth larvae are the most frequently consumed insects. Other 62 Medical Entomology invertebrates such as certain crustaceans and mollusks are favored culinary items. Objections to eating insects cannot be justified on the grounds of taste or food flavor. Many are reported to have a nutty flavor and studies report favorably on the nutritional content of insects, although their amino-acid composition is not ideal and needs to be balanced with suitable plant protein. In central Africa, southern Zairian people (currently democratic republic of Congo) eat caterpillars belonging to a few dozen species. The calorific value of these caterpillars is high; their protein content ranges from 45-80% and they are a rich source of iron. Where there is chronic or seasonal shortage of vertebrate protein reserves elsewhere in sub-Saharan Africa, insect alternatives are often used or even preferred. For instance, caterpillars are the most important source of animal protein in some areas of the Northern Province of Zambia. The edible caterpillars of an emperor moth (Saturniidae), locally called mumpa, are much prized as food. People travel hundreds of kilometers to pick mumpa, which provides a highly lucrative market. The caterpillars contain 60-70% protein on a dry-matter basis and offset malnutrition caused by protein deficiency. Mumpa are either fried fresh or they are boiled and sun-dried prior to storage. Further south in Africa, the Pedi people of northern Transvaal much prefer mopanie 63 Medical Entomology worms, the larvae of the saturniid Gonimbrasia belina, to beef. Insects are also valuable foods in other parts of the world such as the Philippines, Australia, USA, etc. 3.4.2 Insects as feed for domesticated animals: The nutritive value of insects as feed for fish, poultry, pigs and farm-grown mink certainly is recognized in china, where feeding trails have shown that insect-derived diets can be cost-effective alternatives to more conventional fish-meal diets. The insects involved are primarily the larvae and pupae of house flies (Musca domestica), the pupae of silkworms (Tenebrio molitor). The same or related insects are being used or investigated elsewhere, particularly as poultry or fish feedstock. Silkworm pupae, a by-product of the silk industry, can be used as a high-protein supplement for chickens. In India, poultry are fed the meal that remains after the oil has been extracted from the pupae. Fly larvae fed to chicken can recycle animal manure and the development of a range of insect recycling systems for converting organic wastes into feed supplements is inevitable, given that most organic substances are fed on by one or more insect species. Clearly, insects can form part of the nutritional base for people and their domesticated animals. 64 Medical Entomology Insects greatly benefit human society, either by providing with food directly or by contributing to materials that human use or the food that human beings eat. For instance, bees provide with honey, but honey bees also are valuable agricultural pollinators. Furthermore, the services of predatory beetles and parasitic wasps that control pests are recognizable. 3.4.3. Other benefits of insects Nutrient recycling via leaf-litter and wood degradation, carrion and dung disposal, and soil turnover. Plant pollination and sometimes seed dispersal Maintenance of plant community composition and structure via phytophagy, including seed feeding. Supporting insectivorous animals, such as many birds, mammals, reptiles and fish. Each insect species is part of a wider community and, if lost, the complexities and abundance of other lives will be affected. Insects also contain a vast array of chemical compounds, some of which can be collected, extracted or synthesized and used for different purposes. Silk from the cocoons of silkworm moths, Bombyx mori, has been used for fabric for centuries. The red dye, cochineal, is obtained commercially from scale insects of Dactylopius coccus cultured on Opuntia cacti. Another scale insect, the lac insect Kerria lacca, is a source of 65 Medical Entomology a commercial varnish called shellac. Chitin, a component of insect cuticle, or a derivative of chitin, can act as an anticoagulant or a haemostatic agent for tissue repair in humans, enhancing the healing of wounds and burns, reducing serum cholesterol, serving as a non-allergenic drug carrier, providing a biodegradable plastic of high-tensile strength, and enhancing the removal of pollutants from waste water, to mention just a few of its possible applications. Benefits from insects are more than economic or environmental. Characteristics of certain insects make them useful models for understanding biological processes in general. For example, the now wide spread vinegar fly, Drosophila melanogaster, has a short generation time, high fecundity and ease of laboratory rearing and manipulation, making it ideal for genetic and cytological research. Aesthetically, the enormous variety of structure and color in insects is worthy of admiration, by collection or depiction in drawings or photographs. Lastly and perhaps most importantly, the sheer number of insects means that their impact upon the environment, and hence our lives, is highly significant. Insects are the major component of biodiversity and, only for this reason, we should try to understand them better. 66 Medical Entomology 3.5. Insect diversity Estimates of species richness of insects vary from less than five million to as many as 80 million species. Insects constitute around half of global species diversity. If we consider life on land only, insects comprise of an ever greater proportion of living species, since the divesity of insects is a predominantly terrestrial phenomenon. 3.5.1 Why insect are so successful in their species diversity? Insects have such enormous success in their struggle for survival for several reasons. 1. They can adapt to even harshest living conditions. The young of some insects live in pools of crude oil. Others live in embalming solution. Some live in streams where the temperature falls to 32oF (0oc), the freezing point of water. Others live in hot springs where the temperature rises to 120oF (49oc).Although most insects feed on plant life, many have adapted themselves to eating almost anything. Various kinds of insects eat fabrics, opium, mustard plaster, cork, tobacco, face powder, paste, or pepper. 67 Medical Entomology 2. Their small size. Insects can live in places that are too small to other animals, and where they can also find food and protection from enemies. Since insects are small, they need little food. 3. The skeleton of insects protects them against injury and loss of moisture. 4. Most insects have wings. Flying makes it easier for insects to search for food, to escape from enemies and to find mates. 5. Much of the success of insects results from their powers of reproduction. Most insects have short lives. They quickly become adults and reproduce. Most insects lay many eggs. Many kinds produce several generations during a season. Because insects can reproduce so rapidly and in such great numbers, they can change to meet changes in their surroundings that could otherwise wipe them out. Insects also have special methods of reproduction. The females of some species can reproduce without mating. A queen honey bee, after one mating period can lay eggs for the rest of her life. Moreover, insects’ high species diversity has been attributed to several factors. The small size of insects, a limitation imposed by their method of gas exchange via trachea, is one 68 Medical Entomology important determinant of species richness. There are many more niches in any given environment for small organism than for large organisms. Insects have more highly organized sensory and neuro-motor systems than most other invertebrates. Insects normally respond to or cope with altered conditions (e.g. the application of insecticide to their host plant) by genetic change (e.g. leading to insecticide resistance). High genetic heterogeneity or elasticity within insect species allows persistence in the face of environmental change. Interactions with other organisms, such as plants in the case of herbivores insects or hosts for parasitic insects, are thought to promote genetic diversification of eater and eaten. These interactions are often called co-evolution. Co-evolution is defined as reciprocal interactions over evolutionary time between phytophagous insects and their food plants, or pollinating insects and the plants they pollinate. Specific or pair-wise co-evolution refers to the evolution of a threat of one species (such as an insect’s ability to detoxify a poison) in response to a threat of another species (such as the elaboration of the poison by the plant), which in turn originally evolved in response to the threat of the first species (that is, the insect’s food preference for the plant). 69 Medical Entomology 3.6 Insect collecting technique Insects are everywhere!! Insects are often encountered, at least with a little searching, in homes, yards, around building foundations, basements, crawl spaces, flower or vegetable gardens that are not heavily sprayed with pesticides, around lights at night, near streams and lakes, abandoned fields, parks, and forests. Some insects are very sedentary and are easy to catch with a pair of tweezers. Others fly, some pretty slowly and others (like dragonflies) are fast. Catching insects takes some practice. Collectors may want to keep an observation notebook to help them keep track of their expeditions. It is a good idea to make labels for insects that include collection date, location and habitat, as well as the collector's name. Insect collecting tools Different tools may be required to collect insects depending on their characteristics or behaviors. Following are some of the tools used to collect insects: 70 Medical Entomology Tweezers or forceps, to pick up insects Fig 3.12. Forceps to pick up insects (adapted from http://www.uky.edu/Ag/Entomology/ythfacts/bugfun/collecti.htm accessed on June o3, 2008.) 35mm film canisters, to hold small insects 71 Medical Entomology Fig 3.13 Film canisters to hold small insects (adapted from http://www.uky.edu/Ag/Entomology/ythfacts/bugfun/collecti.htm accessed on June o3, 2008.) Killing jars, made from peanut butter jars with nail polish remover or alcohol on an absorbent material such as cotton balls or newspaper. Place a crumpled piece of tissue paper in the jar, to give insects a place to 'hide' so they don't beat themselves up trying to escape. 72 Medical Entomology Fig 3.14 Insect killing jar(adapted from http://www.uky.edu/Ag/Entomology/ythfacts/bugfun/collecti.htm accessed on June o3, 2008.) Collanders can be used for aquatic collection, to 'scoop' out insects at the water's surface or underwater at the edge of a stream or lake. Aquatic insects include water striders, whirlygig beetles, backswimmers, diving beetles, immature mosquitoes, immature dragonflies, and giant water bugs (many of these bugs can bite!). Sweep nets are used for sweeping the grass of meadows and abandoned fields, as well as catching insects in bushes and up in trees. Butterfly nets are best for catching flying insects. 73 Medical Entomology Fig 3.15 Insect collecting net (adapted from http://www.uky.edu/Ag/Entomology/ythfacts/bugfun/collecti.htm accessed on June o3, 2008.) Beat sheets are used to collect slow moving and small insects which have been jarred from plants. An inverted umbrella, white pan or sheet of paper is placed under plants. Shake or jar the insects off of plants onto the beat sheet, then grab them with tweezers or shoo them into jars. Berlese funnels are useful in collecting small insects from soil, leaf litter, or compost. Place a wire screen over a funnel, with the tip of the funnel resting in a jar 74 Medical Entomology above at least 2 inch of alcohol (ethanol is the best type but rubbing alcohol will work). Scoop a bit of soil or debris onto the screen, and then place an electric light directly above the funnel. The heat from the lamp forces insects down the funnel, into the alcohol. Leave undisturbed for 2-5 days, or until soil is dry. If the material is very fine, place a paper towel between the screen and the soil, so fine particles won't get into the alcohol. Light traps are used at night to catch insects. "Black lights" or ultraviolet lights may be more successful than regular outdoor lighting, but even normal outdoor lights attract lots of insects. A white sheet placed behind the light may help with collecting since it gives the flying insects a place to land and fewer escape routes. Bait traps attract insects with food. Rotten meat attracts carrion feeders, while other insects like overripe fruits, fermented foods, sugary foods, or oils (peanut butter). Some insects are even attracted to dung. "Sugaring" is a method of painting tree trunks, etc. with a fermented mixture of fruits, sugar, and an alcoholic beverage such as rum or beer, and is a good method to catch certain types of nocturnal insects. Pitfall traps are useful for catching ground dwelling insects and can also be baited. Soup cans are an 75 Medical Entomology excellent size for pitfall traps. Punch small drainage holes in the bottom, and shield the trap from debris and rain. The top of the can should be level with the ground surface, so an insect will fall right in. Either check traps often or preserve the insects with a mixture of saltwater or soapy water in a can without drainage holes. Pheromone traps use synthetic female hormones to attract male insects to its source. Pheromones for several pest insects are available commercially. 76 Medical Entomology 77 Medical Entomology Fig 3.16 Pheromone trap (adapted from http://www.uky.edu/Ag/Entomology/ythfacts/bugfun/collecti.htm accessed on June o3, 2008.) Review questions 1. Discuss why insects are diverse in the world. 2. Describe use of insect. 3. List orders of class insecta. 4. Discuss in brief about public health importance of diptera and anoplura and citing examples. 5. Discuss about different insect collecting techniques 78 Medical Entomology CHAPTER FOUR THE MORPHOLOGY OF INSECTS 4.1 Learning Objectives By the end of this chapter, the learner will be able to: Describe external morphology Explain internal morphology Explain the functions of the different parts of insects 4.2 Introduction The study of insect structure and form is called morphology (“morpho” means structure, and “logy” means study). All arthropods have several characteristics in common, in particular a skeleton on the out side of the body (exoskeleton) which is segmented, with a pair of jointed appendages (legs, antennae etc.) on most segments. Males and females are distinct. Many anatomical features for the appendages, especially of the mouth parts, legs and abdominal apex, are important in recognizing the higher groups with in the hexapods, including insect orders, families and genera. Differences between species are frequently indicated by anatomical differences. 79 Medical Entomology Terms that describe anatomical positions of body of arthropods Anterior: towards the front Posterior: tail or rear area opposite of the anterior Dorsal: back side-from the anterior to the posterior Ventral: underside portion (area) Aboral: side opposite to the mouth Buccal: mouth area Caudal: tail region Cephalic: head area Cervical: neck region Thoracic: part of an insects body between the head and abdomen Lateral: side area 4.3. The External Morphology The external morphology of an insect is the structure and form of the visible outside parts of the body. The supporting frame work or skeleton of an insect is on the outside, and is so called an exoskeleton. The exoskeleton is a complex structure; exquisite control is required in the formation of a new one at molting. Higher animals (chordate) such as the vertebrates, 80 Medical Entomology have the skeleton inside the body and it is called an endoskeleton. All the new external structures must be formed below the old exoskeleton and produced in such a way that the next stage can be larger. The molt must be controlled so that it is coordinated and occurs relatively quickly. During the molt, the animal struggles to shed the old exoskeleton; because the new exoskeleton is soft for a short time, the animal is more susceptible to predation than at other times. Hormones control molting and also differentiation of the body as it matures sexually. Before discussing the external morphology in more detail, some indications of orientation is required. The bilaterally symmetrical body may be described according to three axes: Longitudinal, or anterior to posterior, also termed cephalic (head) to caudal (tail). Dorsoventrally, or dorsal (upper) to ventral (lower).