Lecture 2: Introduction to Insect Physiology PDF

LECTURE 2 INTRODUCTION TO INSECT PHYSIOLOGY: Insect growth, development and reproduction, their Metabolic systems , coordination and integration Marife S. Sayat Faculty, Crop Protection Department BS Agriculture Progra...

LECTURE 2 INTRODUCTION TO INSECT PHYSIOLOGY: Insect growth, development and reproduction, their Metabolic systems , coordination and integration Marife S. Sayat Faculty, Crop Protection Department BS Agriculture Program Course: AGRIC MC 312: General Physiology and Toxicology  A. Introduction to Insect Growth, Development, and Reproduction Molting An insect’s skeleton is on the outside of its body and is called an exoskeleton. It serves as a support for muscles and internal organs as well as a covering. As the insect’s rigid exoskeleton cannot expand much, it must be shed and replaced with a larger one as the insect grows. This process is called molting. The life stage between each molt is called an instar. Course: AGRIC MC 312: General Physiology and Toxicology  Kinds of Insect Growth Molting may occur up to three or four times or, in some insects, fifty times or more during its life. The two kinds of insect growth are directly related to molting patterns. Those that show a determinate pattern of growth have a fixed number of molts, whereas those that have indeterminate growth continue to molt indefinitely Course: AGRIC MC 312: General Physiology and Toxicology  The Stages of Molting Apolysis Sclerotisation After emergence the new cuticle is soft and this a Molting hormones are released into the particularly vulnerable time for the insect as its hard haemolymph and the old cuticle separates protective coating is missing. After an hour or two the from the underlying epidermal cells. exocuticle hardens and darkens. The wings expand by the force of haemolymph into the wing veins. 1 2 3 Ecdysis Shedding of the remnants of the old cuticle, and this begins with the splitting of the old cuticle, usually starting in the midline of the thorax’s dorsal side. Course: AGRIC MC 312: General Physiology and Toxicology  Life Cycle An insect’s life-cycle can be divided into three types:  Ametabolous o No metamorphosis These insects are primitively wingless where the only difference between adult and nymph is size. Example: o Order:Thysanura (Silverfish). Course: AGRIC MC 312: General Physiology and Toxicology  Hemimetabolous  Incomplete metamorphosis o The terrestrial young are called nymphs and aquatic young are called naiads. Insect young are usually similar to the adult. Wings appear as buds on the nymphs or early instars. When the last moult is completed the wings expand to the full adult size. o Example: o Order: Odonata (Dragonflies) Course: AGRIC MC 312: General Physiology and Toxicology  Holometabolus o Complete metamorphosis o These insects have a different form in their immature and adult stages, have different behaviors and live in different habitats. The immature form is called larvae and remains similar in form but increases in size. They usually have chewing mouthparts even if the adult form mouth parts suck. At the last larval instar phase the insect forms into a pupa, it doesn’t feed and is inactive, and here wing development is initiated, and the adult emerges. o Example: o Order: Lepidoptera (Butterflies and Moths). Course: AGRIC MC 312: General Physiology and Toxicology  Metamorphosis and Reproductive Cycles Stage Complete Metamorphosis Incomplete Metamorphosis Egg Present Present Larva Present Absent Nymph Absent Present Pupa Present Absent Adult Present Present Course: AGRIC MC 312: General Physiology and Toxicology  Insect Reproduction and Development Life Cycle Egg Larva Pupa Adult Stages Description Fertilized The The The final, egg laid immature, transitional sexually by the worm-like stage between mature female stage of the larva and stage of the insect, insect, adult, where insect's life which will which the insect cycle, hatch into undergoes undergoes a capable of a larva. a series of dramatic reproducing molts to transformation. and starting grow and the cycle develop. anew. Course: AGRIC MC 312: General Physiology and Toxicology Course: AGRIC MC 312: General Physiology and Toxicology Course: AGRIC MC 312: General Physiology and Toxicology  Sexual and Asexual Reproduction Reproduction A biological process by which new individual organisms (off- springs) are produced from their parents. Sexual or asexual process by which organisms generate new individuals of the same kind. A fundamental feature of all known life. Course: AGRIC MC 312: General Physiology and Toxicology  Importance of Insect Reproduction 1 Ecosystem Balance 2 Agricultural Impact Insects play crucial roles in Many crops depend on insect pollination and food chains. pollination. Understanding Their reproduction maintains insect reproduction is vital for biodiversity and ecological sustainable agriculture. stability. 3 Pest Control 4 Scientific Research Knowledge of insect Studying insect reproduction reproduction helps develop provides insights into effective pest management evolution and genetic strategies. This protects mechanisms. It contributes to crops and human health. broader biological understanding. Course: AGRIC MC 312: General Physiology and Toxicology  Sexual and Asexual Reproduction Types of Reproduction in Insects Insects have different & opposite sexes and reproduce by sexual and asexual reproduction. These are the following; 1. Oviparity  Most common mode of reproduction in insects.  Insects lay the fertilized eggs, which hatch outside the body of the female.  Such insects are said to oviparous  Butterflies, moths, flies, beetles, bugs, locusts, grasshoppers, dragonflies, mayflies, etc. Course: AGRIC MC 312: General Physiology  Sexual and Asexual Reproduction Types of Reproduction in Insects 2. Viviparity  Eggs complete their embryonic development and hatch within the body of the female.  Hence the female gives birth to the young ones, instead of laying eggs.  Such insects are said to viviparous  Aphids, certain flies, stylopids, etc. Course: AGRIC MC 312: General Physiology  Sexual and Asexual Reproduction Types of Reproduction in Insects 3. Parthenogenesis  Eggs undergo full development without having been fertilized.  Females lay the unfertilized eggs  Eggs have both the haploid (half) or diploid (double) number of chromosomes.  These eggs undergo full development and give rise to males, females or both sexes.  Aphids, bees, wasps, some whiteflies, thrips, etc. Course: AGRIC MC 312: General Physiology  Sexual and Asexual Reproduction Types of Reproduction in Insects 4. Paedogenesis  Immature stages (larvae & pupae) produce young  Have functional ovaries in which eggs are developed parthenogenetically  E.g. In some cecidomyids the larvae gives birth to other larvae, which become adult under favorable conditions.  In certain midges (Hernia & Tanytarsus) the pupa produces larvae, some which become normal adults.  A few beetles (Micromathus) also show this phenomenon, Course: AGRIC MC 312: General Physiology  Sexual and Asexual Reproduction Types of Reproduction in Insects 6. Hermaphroditism 5. Polyembryony  It is an extremely rare phenomenon  It is a rare phenomenon  A single individual has both the male and female reproductive organs  A single egg produces two or more larvae  E.g. in the scale insects (Icerya purchasi) the outer cells of the gonads produces eggs, while the inner cells produces sperms,  E.g. several parasitic wasps (Hymenoptera), some cecidomyids & a few stylopids.  Thus the eggs are fertilized in the gonads by sperms in the same individual.  In the Phorid flies of the genus Termitostroma each individual has a pair of ovaries & testis to release the eggs & sperms. Course: AGRIC MC 312: General Physiology B. Metabolic Systems of Insects Insect External Anatomy The insect is made up of three main body regions (tagmata), the head, thorax and abdomen.  The head comprises six fused segments with compound eyes, ocelli, antennae and mouthparts, which differ according to the insect’s particular diet, e.g. grinding, sucking, lapping and chewing.  The thorax is made up of three segments: the pro, meso and meta thorax, each supporting a pair of legs which may also differ, depending on function, e.g. jumping, digging, swimming and running. Usually the middle and the last segment of the thorax have paired wings.  The abdomen generally comprises eleven segments and contains the digestive and reproductive organs. External Anatomy of Ants External Anatomy of Cockroach  Insect Respiratory System Tracheal System Spiracles and Air Sacs Insects do not have lungs like The tracheal system is vertebrates. Instead, they rely on a connected to the outside network of tubes called the tracheal environment through small system to distribute oxygen openings called spiracles, throughout their bodies. This system which allow air to enter and consists of branching tubes that carry exit the insect's body. Some air directly to the insect's cells, insects also have air sacs, bypassing the need for a circulatory which act as storage and system. distribution hubs for the oxygen-rich air. Respiratory Regulation Insects can regulate their respiratory system by controlling the opening and closing of their spiracles, as well as the contraction of their abdominal muscles. This allows them to adjust their oxygen intake and carbon dioxide release to meet the demands of their activities and environment.  There are many different patterns of gas exchange demonstrated by different groups of insects. Gas exchange patterns in insects can range from continuous, diffusive ventilation, to discontinuous gas exchange. Terrestrial and a large proportion of aquatic insects perform gaseous exchange as previously mentioned under an open system. Other smaller numbers of aquatic insects have a closed tracheal system, for example, Odonata, Tricoptera, Ephemeroptera, which have tracheal gills and no functional spiracles. Endoparasitic larvae are without spiracles and also operate under a closed system. Here the tracheae separate peripherally, covering the general body surface which results in a cutaneous form of gaseous exchange. This peripheral tracheal division may also lie within the tracheal gills where gaseous exchange may also take place Insect Circulatory System 1 Open Circulatory System 2 Dorsal Vessel and Hemolymph Unlike the closed circulatory systems The insect's circulatory system is of vertebrates, insects have an open centered around a tube-like structure circulatory system. This means that called the dorsal vessel, which acts their blood, called hemolymph, flows as a simple heart. The hemolymph is freely through the body cavity and pumped through the dorsal vessel does not travel through a complex and then flows freely through the network of vessels. body cavity, delivering nutrients and oxygen to the insect's tissues. 3 Hemocytes and Immune Response Insect hemolymph contains specialized cells called hemocytes, which play a crucial role in the insect's immune response. These cells help to defend the insect's body against pathogens and other threats, ensuring its overall health and survival.  The main function of insect blood, hemolymph, is that of transport and it bathes the insect’s body organs. Making up usually less than 25% of an insect’s body weight, it transports hormones, nutrients and wastes and has a role in osmoregulation, temperature control, immunity, storage (water, carbohydrates and fats) and skeletal function. It also plays an Pumping of the hemolymph occurs by waves of peristaltic contraction, originating at the body's posterior end, pumping forwards into the essential part in the molting process. dorsal vessel, out via the aorta and then into the head where it flows An additional role of the hemolymph in out into the haemocoel. some orders, can be that of predatory defense. It can contain unpalatable Pumping rate accelerates due to periods of increased activity. Movement of hemolymph is particularly important for thermoregulation and malodourous chemicals that will in orders such as Odonata, Lepidoptera, Hymenoptera and Diptera. act as a deterrent to predators. Insect Digestive System Specialized Feeding Adaptations Insects have evolved a wide range of specialized feeding adaptations, from the chewing mouthparts of beetles to the long, slender proboscis of butterflies. These adaptations allow insects to exploit a diverse array of food sources, from plant matter to animal tissues. Gut Microbiome Efficient Nutrient Utilization Many insects also rely on a complex Despite their small size, insects are gut microbiome to aid in the digestion remarkably efficient at extracting and of their food. These symbiotic utilizing the nutrients from their food. relationships between insects and This ability, combined with their their gut bacteria play a crucial role in diverse feeding adaptations, allows the insect's overall health and insects to thrive in a wide range of nutritional status. environments and ecological niches. Digestive Tract Structure The insect digestive system typically consists of a foregut, midgut, and hindgut, each with its own specialized functions. The foregut is responsible for storing and breaking down food, the midgut for further digestion and absorption, and the hindgut for waste elimination. The foregut (stomadaeum) is differentiated into five parts: Buccal chamber, pharynx, oesophagus, crop and gizzard. Midgut (mesenteron or ventriculus) is short, tubular lined with glandular endoderm. Hindgut (proctodaeum) comprises ileum, colon and rectum. Insect Nervous System Central Nervous System The insect's nervous system is centered around a brain-like structure called the ganglia, located near the head. This central nervous system coordinates the insect's sensory input and motor functions, allowing it to respond to its environment and execute complex behaviors. Peripheral Nerves Extending from the central nervous system are numerous peripheral nerves that carry information to and from the insect's various body parts. These nerves allow the insect to perceive its surroundings and coordinate its movements with precision. Sympathetic or Stomatogastric Visceral Nervous System The insect nervous system integrates a wide range of sensory inputs, including vision, olfaction, gustation, and mechanoreception. This sensory integration enables insects to navigate their environments, locate food and mates, and respond to potential threats. Insect Sensory Organs Olfaction Vision Insects rely heavily on their sense of Insects possess a wide range of visual smell, using specialized olfactory organs capabilities, from simple light detection to like antennae to detect pheromones, complex, compound eyes that can food sources, and other environmental perceive color and motion. Their visual cues. This sense of smell is essential for systems play a crucial role in tasks like insect behavior, from finding mates to navigation, prey/predator detection, and avoiding predators. communication. Gustation Mechanoreception Insects use specialized gustatory Insects have a variety of specialized organs, often located on their mouthparts sensory structures, such as bristles and or feet, to detect and analyze the campaniform sensilla, that allow them to chemical composition of their food and detect mechanical stimuli like touch, environment. This sense of taste is vibration, and wind. This sense of touch critical for insect feeding behavior and helps insects navigate their environment helps them identify suitable food and respond to potential threats. sources. Auditory receptors for hearing, present on the anal cerci respond to air or earth borne vibrations. Insect Excretory System Excretion is performed by Malpighian tubules.  Each tubule is lined by glandular and ciliated cells. They absorb nitrogenous waste products and convert them into uric acid which is excreted out through the hindgut. Therefore, this insect is called uricotelic. Insect Reproductive Systems Male reproductive systems  Contains a pair of testes, usually located near the back of the abdomen.  Each testes is subdivided into functional units (called follicles) where sperm are actually produced.  Near the distal end of each follicle, there are a group of germ cells (spermatogonia) that divide by mitosis and increase in size to form spermatocytes. Mature sperm pass out of the testes through short ducts (vasa effentia) and collect in storage chambers (seminal vesicles) that are usually little more than enlarged sections of the vasa.  Similar ducts (vasa deferentia) lead away from the seminal vesicles, join one another near the midline of the body, and form a single ejaculatory duct leads out of the body through the male’s copulatory organ (called an aedeagus). Female reproductive systems  The female’s reproductive system contains a pair of ovaries.  When the insect is actively reproducing, these organs swell with developing eggs and may nearly fill the abdomen.  Each ovary is subdivided into functional units (called ovarioles) where the eggs are actually produced.  A typical ovary may contain dozens of ovaries, generally aligned parallel to one another.  Near the midline of the body, these lateral oviducts join to form a common oviduct which opens into a genital chamber called the bursa copulatrix.  Female accessory glands (one or more pairs) supply lubricants for the reproductive system and secrete a protein- high egg shell (chorion) that surrounds the entire egg.  These glands are usually connected by small ducts to the common oviduct or the bursa copulatrix. C. Insect Coordination and Integration Insect Navigation and Spatial Awareness Navigation Method Example Species Key Features Sun Compass Honey Bees Uses polarized light patterns Magnetic Sense Monarch Butterflies Detects Earth's magnetic field Landmark Desert Ants Remembers visual Recognition cues Odor Plumes Moths Follows chemical gradients Symbiotic Relationships and Interspecies Coordination Mutualism Ecosystem Engineering Pollination Leaf-cutter ants cultivate fungus gardens. Termites build complex mounds. These Bees coordinate with plants for The ants provide nutrients, while the structures modify local environments, pollination. This relationship shapes plant fungus serves as a food source. benefiting other species. evolution and ecosystem diversity.