Animal Biology Lecture Notes (PHA209) on Arthropods and Mollusks - 30 Jan 2025 PDF

Animal Biology (PHA209) ARTHROPODS (Insects & Arachnids) Instructor: Dr. Abong 1 ARTHROPODS 2 ARTHROPODS Arthropods are invertebrate (no backbone) animals and form the largest group of species in...

Animal Biology (PHA209) ARTHROPODS (Insects & Arachnids) Instructor: Dr. Abong 1 ARTHROPODS 2 ARTHROPODS Arthropods are invertebrate (no backbone) animals and form the largest group of species in the animal kingdom and contains an enormous diversity of members, with the number of species exceeding that of all other phyla combined. They share several characteristics that distinguish them from all other animal groups, although some of these features are absent in a particular species or group at some period of development. Nevertheless, there are traits specific to each class in the phylum to help identify them. Among the morphologic characteristics are bilateral symmetry, a hard exoskeleton (which helps enclose and protect the muscles and other organs: it is strong and relatively impermeable to water), a segmented body, and paired, jointed appendages Examples of arthropods include insects, ticks, spiders, horseshoe crabs, millipedes, chiggers, centipedes, shrimp, sowbugs and scorpions 3  An open circulatory system, with or without a dorsally situated heart pumps the blood (hemolymph) via arteries to the various organs and body tissues. Blood is returned to the heart through body spaces known as hemocoels. In addition, respiratory, excretory, and nervous systems are present. Arthropods affect the health of humans by being either direct agents for disease or agents for disease transmission. The arthropods of medical importance are found in Classes Insecta, Arachnida, and Crustacia, which have their own distinguishing features. For instance, in class insecta, the body is divided into head, thorax and abdomen with one pair of antennae. Diseases like malaria, yellow fever, onchocerciasis, and trypanosomiasis are primarily transmitted by insects. Arthropods directly influence humans’ well-being, not only because they are hosts of parasitic organisms and vectors of a wide variety of pathogens, but also by causing tissue damage and disease. 4  They also affect human health by reducing the availability of food. Insects destroy an estimated 20% of all food crops, and this destruction continues despite the increasing use of pest management tactics such as insecticides, genetically modified organisms and physical control methods in fields and storage areas. Livestock are also affected by arthropod-borne infections. Vast areas of Africa are short of protein foods because cattle suffer a number of vector-borne diseases, including trypanosomiasis transmitted by tsetse flies and a variety of tick-transmitted diseases. Although the pathogenic effects of arthropods are most pronounced in the tropics, they are by no means negligible in other parts of the World including the United States. Even though arthropods cause problems for humans and livestock, they are also beneficial as pollinators, producers of honey, natural regulators of harmful insects, and essential members of food chains Growth by metamorphosis is another characteristic of the arthropods. In some groups, growth is gradual; each change from one stage to the next is known as a molt and gives rise to a stage somewhat larger but morphologically similar to its predecessor (incomplete metamorphosis). Another developmental strategy involves egg, larva, pupa and adult. In this case, each stage is morphologically distinct (complete metamorphosis). Examples include the flies and the fleas. 5 CLASSIFICATION 6 CLASS INSECTA An insect is an arthropod having three body regions (head, thorax and abdomen), three pair of jointed legs/appendages, one pair of antennae and generally one or two pair of wings in the adult stage. Head with antennae (for touch and smell), eyes (for vision) and mouth. Three segmented thorax with legs and wings (if present). Abdomen composed of varying number of segments depending on the species. Internally contains most of the digestive system and the reproductive system Diverse 80% of all described animal species are insects. One-half of the described species of insects are beetles. Numerous It has been estimated that the total weight of all insects on the planet exceed the combined weight of all people on earth by a factor of 200 times. Oviparous – Most insects are oviparous, meaning they lay eggs, and their offspring hatches from these eggs. Eggs are produced and laid from the abdomen region of an insect They are mostly terrestrial 7 INSECT DEVELOPMENT The insects have two distinct types of development. 1). The more primitive insect orders pass through a series of stages (3) by incomplete metamorphosis (Fig. 38.1). E.g. Aphids, grasshoppers, cockroaches, true bugs A typical life cycle involves the egg, a (usually) fixed number of immature nymph stages, and the mature adult stage (Egg > Nymph > Adult). The insect molts between stages, sheds its old exoskeleton, and reveals a new skin within. Nymphs are similar to the adult, but lack wings and are sexually immature. 2). In contrast, complete metamorphosis is characteristic of some of the more advanced insect orders, including the flies (Diptera) (Fig. 38.2) and the fleas (Siphonaptera) (Fig 38.3). E.g. Moths, butterflies, beetles and flies The life cycle of an insect exhibiting complete metamorphosis includes 4 stages: the egg, larval stages, a pupa stage, and the adult stage (Egg > Larva > Pupa > Adult). 8  Incomplete or Hemimetabolous Metamorphosis Insects have an exoskeleton which is a hard outer covering that supports and protects an insect’s body. Part of the exoskeleton is composed of a substance called chitin. (Chitin is a tough, fibrous carbohydrate.) Exoskeletons cannot grow with the insect. Therefore, insects must shed their exoskeletons to increase their body size. This shedding process is called molting Complete or Holometabolous Metamorphosis 9 Insects as pests Beneficial insects Usually, an insect is considered to Pollinators (apples, be a pest if it; watermelons, squash) Transmits diseases ( e.g. mosquitoes), Products (honey, silk, dyes) Feeds on food (e.g. flour moth), Scavengers crops (e.g. corn earworm) or structures ( e.g. termite), Predators of some parasites Is a nuisance (e.g. cockroaches Food for fish and birds and lovebugs). Scientific study Art, religion, relaxation World-wide only 8% of insects are considered pests. 10 Some of the systems which make insects unique (Digestive System, Circulatory System, Nervous System, and Respiratory System) The digestive system is basically a tube that runs through the middle of the body. It consists of a foregut, midgut (stomach) and hindgut. The excretory system is made up of malpighian tubules (which function like our kidneys) which remove wastes from the insect body through the hindgut. The circulatory system is an open system that is not confined to arteries and veins. Haemolymph (insect blood) is circulated through the body cavity by a dorsal “heart” and a short aorta. The blood is picked up in the abdomen and carried forward to the head where it flows back into the body cavity The nervous system consists of a small brain in the head that is connected by a double nerve cord to a series of ventral ganglia or nerve centers located in several segments of the body Insects do not have lungs. They breathe passively (diffusion of oxygen in and carbon dioxide out) through a system of tubes called tracheae. The tracheae open to the outside through spiracles. Inside the body, the tracheae branch into finer tubes called tracheoles which provide oxygen to cells. 11 Table 38.1 lists some arthropods of importance to human health, the pathogens they harbor and transmit, and the diseases they cause. The methods by which the arthropod vectors transmit pathogens vary. Some pathogens, unchanged by any interaction with the vector, are transmitted mechanically from one host to another on contaminated legs or mouthparts of the arthropod or in its feces. Other pathogens require passage through the arthropod as part of their life cycle. In such cases, the pathogens undergo specific developmental changes, which usually include multiplication, within the arthropod. Arthropods can also be pathogens themselves. They can infest the host, migrating through the body or developing in situ while feeding on host tissue. Other arthropods cause mechanical injury through bites, chemical injury through injection of venom, or allergic reactions to the materials they transmit via the bite or sting. 12  The salivary secretions of arthropods in general, and insects in particular, have proven to be extraordinarily complex. These secretions serve as potent immunogens and stimulate the bothersome allergic reactions to the insect’s bite. They also serve, in many cases, to carry the viral, bacterial, protozoal or nematode pathogens for which so many arthropods serve as vectors. These salivary secretions evolved not to cause allergic responses or convey pathogens, but for a much more basic reason. They facilitate the capacity of the arthropod to take blood from a host whose physiology and defense mechanisms are designed to prevent the loss of blood. In almost all blood-sucking arthropods studied to date, the saliva of each species has at least one anticlotting, one vasodilator and one antiplatelet compound. The molecular diversity of these compounds is great, even among closely related genera of blood feeders. 13 Classification of insects into orders The 30-35 Orders of Insects are separated based on: Wings Structure, number, etc. Mouthparts Chewing Piercing-sucking (mosquitoes, bugs, etc.) Siphoning (butterflies, moths) Metamorphosis Simple (or Incomplete) Complete 14 CLASS ARACHNIDA 15 ARACHNIDS The arachnids comprise a class of arthropods that includes the ticks, mites, scorpions, and spiders. The characteristics of the Arachnida clearly differentiate it from the class insecta. CHARACTERISTICS All arachnids are wingless, have six pairs of appendages (chelicerae: used to hold prey, pedipalps: used in to feed, and four pairs of walking legs as adults), They usually show only two distinct body regions: a cephalothorax and an abdomen. Arachnids are mostly terrestrial. Metamorphosis among the arachnids is of the incomplete type. The immature, non-reproductive stages are smaller but morphologically like the adults. In many groups, arachnids in the first, or larval, stage may have only three pairs of legs. 16  Arachnida is comprised of 11 extant orders, 3 of which are Acari, Araneae, and Scorpiones. The order Acari includes mites and ticks. Ticks are exclusively hematophagous, whereas mites feed on a variety of substances, including insect eggs, cells and blood. The spiders (order Araneae) are mainly insectivorous, feeding on body fluids of captured insects. Some larger tarantulas may feed on small mammals or birds. Scorpions (order Scorpiones) feed on arthropods or small animals that they have immobilized with their stinging apparatus, which is located at the tip of the abdomen. Most members of these three orders do not affect human health directly. Each order includes some members of medical importance. Ticks and mites injure their victims by their feeding habits and serve as vectors for a number of important diseases (Table 39.1). Spiders inject toxins that can cause severe systemic or tissue reactions, and the toxins injected by the stings of certain species of scorpions can cause severe reactions in affected individuals, rarely death may ensue. 17 Study questions List 8 characteristics of Arthropods Distinguish between the two classes of arthropods (Insecta and Arachnida). 18 MOLLUSKS Instructor: Dr. Abong 19 20 Characteristic Features of Mollusks 21 General Characteristics Mollusks are invertebrates with soft, unsegmented bodies often protected by hard outer shells They are triploblastic, bilaterally symmetrical coelomates Body enclosed by a blanket-like mantle that secretes a shell made of calcium or some other stiff structure A mantle cavity between the mantle and internal organs; the anus, reproductive, and excretory ducts open into the mantle cavity used for breathing, excretion, and reproduction They do not have repeated body parts like segmented worms. Their internal organs are located in one area. A ventral, muscular “foot” that is highly modified among the various groups of mollusks. Used for movement and attachment, and/or capture of food. Head with sensory organs and a mouth Aquartic mollusks have gills. Terrestrial mollusks have lungs. Herbivorous or carnivorous. All of them with a complex digestive system 22  Most of them are hermaphrodites In separate sexes individuals, there is no sexual dimorphism Most of them lay eggs (oviparous) from which larvae emerges. It will go under metamorphosis Have a true heart Simple to complex nervous system. With exception of Cephalopods, one advancement that mollusks have is an open circulatory system. The blood is not always inside of veins Bodies separated into head-foot and visceral mass But for the Bivalves, all mollusks have a radula. The radula is the tooth chitinous ribbon in the mouth of most mollusks. It is quite similar to a tongue. It is used for cutting and chewing food before it enters the esophagus. They have a nervous system which is quite unique. It only consist of nerve cords and ganglia (masses of nerve cell bodies). Many have a free swimming trochophore larva Over 50.000 species have been reported 23 Body Plan 24 Habitat and niche Some mollusks/molluscs habitats include: Marine (mostly) Freshwater (a few) Land (a few) Mollusks niches (life-style): Most live on the ocean bottom and are mostly sedentary (little to no physical activity) Some are free-swimming (the cephalopods) 25 Reproduction Sexual Most are dioecious Some hermaphrodites (gastropods) Internal or external fertilization Fertilized egg develops into trochophore larvae (free swimming, planktonic larvae that feeds on tiny plants) Asexual Some regenerate 26 Ecological Roles Mollusks fill a wide variety of ecological niches Predators Filter feeders Food source Break down decaying plant matter Filter feeding bivalves can be an indicator of water quality Shipworms (not worms) burrow through wood boats and docks Terrestrial mollusks are agricultural pests 27 Classes of Mollusks The Mollusks Phylum has three main classes 1. Gastropods (Ex. Snails) 2. Bivalves (Ex. Clams) 3. Cephalopods (Ex. Squid) Biologists classify mollusks into groups based on physical characteristics such as the presence of a shell, the type of shell, the type of foot, and the complexity of the nervous system. 28 Class Gastropoda i. Most popular class of mollusks ii. They have adapted to life on land. iii. Consists of snails and snail-like creatures iv. Specific characteristics v. Most have a dorsally located/protective shell (often coiled) vi. They have a well-developed radula (rough tongue) that is used to eat by scrapping off food. vii. Many have eyes viii. 4 tentacles (2 optic and 2 sensory) ix. Visceral mass (organs) almost always located inside the shell. Their visceral mass is rotated 180 degrees during development. x. They are herbivores or predators (carnivorous) xi. Habitats include ocean, lake, river bottoms, coastal shores, and land. 29 30 31  Class Bivalvia i. Examples are clams, mussels and oysters ii. They have no head iii. They have two (hard, protective and hinged) shells held together by powerful muscles iv. They have a ventrally located foot that sticks out between the two valves v. They tend to burrow into soft mud or sand or attach to rocks or other shells vi. Habitats include marine and freshwater vii. Large gills are used for respiration and filter feeding viii. Water enters and exits through siphons 32 Class Cephalopoda Cephalopods contain the largest mollusks. Examples are Octopi and Squids. Specific characteristics include; i. They have a closed circulatory system (blood always within heart or veins). ii. They have a well-developed nervous system with eyes. iii. May lack a shell (like an octopus) iv. Shell may be reduced to a stiffening rod (like the squid or nautilus) v. The foot is highly modified to form a group of tentacles around the mouth vi. They use their tentacles to capture food. vii. They are found in deep and shallow waters along many coasts. viii. Squids and Nautilus are free-swimming and move very quickly ix. Octopuses are found among rocks or crawling on the bottom of the ocean and swim by jet propulsion. x. Complex nervous system with a large brain, two lateral/large eyes, excellent eyesight/vision. 33 34 Review questions 1. What are the main characteristics of mollusks? 2. What are the major groups of mollusks? 35