Chapter 20. Diversity of Uniramia. Characteristics. Class Chilopoda. Chiloped Characteristics. Terrestrial Mandibulates

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1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 20 Terrestrial Mandibulates Diversity of Uniramia The subphylum Uniramia is a controversial taxon including the millipedes, centipedes and insects. Uniramians are primarily terrestrial with a few found in freshwater habitats. Characteristics The myriapods include the centipedes, millipedes, pauropods and symphylans. The insects have reduced their body tagmata to head, thorax and abdomen. Abdominal appendages are greatly reduced or absent. The common ancestor of insects probably resembled the myriapod body form. Uniramia only have one pair of antennae and appendages are always uniramous. Uniramian gills in aquatic larvae are not homologous with crustacean gills. Uniramians use tracheae to distribute respiratory gases, similar to onychophorans and annelids. Excretion usually involves Malpighian tubules. Class Chilopoda Characteristics Centipedes are terrestrial and have flattened bodies with up to 177 somites. Each somite, except the one behind the head and the last two, bears a pair of jointed legs, the last pair of which serves a sensory function. Appendages of the first body segment form poison claws. The head has one pair of antennae, a pair of mandibles and one or two pairs of maxillae. Eyes on either side of the head consist of groups of ocelli. Salivary glands empty into the anterior end of the straight digestive tract. Chiloped Characteristics Two pairs of Malpighian tubules empty into the hind intestine. The elongated heart has a pair of arteries in each somite; ostia provide return flow of hemolymph. A pair of spiracles in each somite allows air to diffuse through branched air tubes of the tracheae. The arthropod nervous system includes a portion that serves as a visceral nervous system.

2 Reproduction Sexes are separate with unpaired gonads and paired ducts. Some lay eggs and others are viviparous. Young resemble adults. Natural History Centipedes are found under logs, bark and stones. They are carnivorous, eating earthworms, cockroaches and other insects. The house centipede has 15 pairs of long legs and is common in bathrooms and damp cellars. Most are harmless to humans but a few large, tropical centipedes are dangerous. Class Diplopoda Millipedes have 2 pairs of legs per somite, probably from fusion of 2 segments. Their cylindrical bodies have from 25 to 100 somites. The head has two clusters of simple eyes and a pair each of antennae, mandibles and maxillae. Each abdominal somite has two pairs of spiracles opening into air chambers and tracheal air tubes. The two genital apertures are toward the anterior end. Reproduction The appendages of the seventh somite are specialized for copulatory organs. After copulation, the female lays eggs in a nest and guards them. Larvae have only one pair of legs to each somite. Millipedes are less active than centipedes; they walk with a graceful rather than wriggling motion. Most eat decayed plants but a few eat living plant tissue. Most are slow moving and roll into a coil for defense. Some secrete toxic or repellant fluids from special repugnatorial glands on the side of the body. Natural History Class Insecta Diversity Insecta are the most diverse and abundant of all arthropods. The number of known species is estimated at one million. There is continued evolution among modern insects; the fossil record indicates they are a stable group. Insects play major medical and economic roles with humans, and are critical to animal ecology.

3 Characteristics Insects have three pair of legs and often two pair of wings on the thoracic region of the body. Insects range from less than 1 mm to 20 cm in length; the larger insects are tropical. Distribution Insects are found in nearly all habitats except the sea. Insects are common in freshwater, brackish water and salt marshes. Insects are abundant in soils, forest canopies, and can be found in deserts and wastelands. Most animals and plants have insects as parasites externally and internally. Adaptive Traits that Promote Wide Distribution Flight and small size. Their well-protected eggs withstand rigorous conditions and are readily dispersed. A wide variety of structural and behavioral adaptations gains them access to every possible niche. Adaptability Most structural modifications are in wings, legs, antennae, mouthparts and alimentary canal. Specialization for eating only one part of a host plant allows many insect species to coexist on a plant. The hard and protective exoskeleton is well-adapted to life in desert regions. The exoskeleton holds in water which, along with metabolism that saves water, allows for desert survival. The exoskeleton is made of complex plates, or sclerites, connected by hinge joints. Muscles attaching sclerites allow precise movement. The rigidity is due to scleroproteins and not mineral matter; this lightness allows flight. External Form and Function Fig. 20.4a The insect is more homogenous in tagmatization than the variable crustaceans. The cuticle of a somite is composed of a dorsal notum, a ventral sternum and a pair of lateral pleura.

4 Usually there is a pair of large compound eyes. One pair of antennae varies greatly in form; they can feel, taste and hear. Mouthparts consist of a labrum, a pair of mandibles and maxillae, a labium and a hypopharynx Head Thorax The thorax consists of the prothorax, mesothorax and metathorax; each has a pair of legs. Wings If two pairs of wings are present, they are on the mesothorax and metathorax. Wings consist of a double membrane. Veins serve to strengthen the wing; the vein pattern is used to identify insect taxa. Legs Walking legs end in terminal pads and claws. Hindlegs of grasshoppers and crickets are enlarged for jumping. Mole crickets have front legs adapted for burrowing in the ground. Forelegs of the praying mantis allow it to grasp prey. Honeybees have leg adaptations for collecting pollen. Abdomen The insect abdomen has from nine to 11 segments; the last is reduced to a pair of cerci. Larval and nymphal forms may have abdominal appendages lacking in adults. The external genitalia are usually at the end of the abdomen. Locomotion: Walking Power of Flight Insects walk using the first and last leg on one side and the middle leg on the opposite side in alteration with the reverse; this provides stability. A water strider has nonwetting footpads that do not break the surface water tension. /ditch/water_strider.htm Insect wings are not homologous with bird and flying mammal wings. Insect wings are outgrowths of cuticle from the mesothoracic and metathoracic segments. Most flying insects have two pairs of wings; the Diptera (true flies) have one pair. Halteres are reduced wings that provide the fly with balance during flight. Non-reproductive ants and termites are wingless; lice and fleas have also lost their wings.

5 Modifications of Wings Flight Muscles of Insects Wings for flight are thin and membranous. The thick and horny front wings of beetles are protective. Butterflies have wings covered with scales; caddisflies have wings covered with hairs. Direct flight muscles attach to a wing directly. Indirect flight muscles alter the shape of the thorax to cause wing movement. The wing is hinged on a pleural process that forms a fulcrum; all insects cause the upstroke with indirect muscles that pull the tergum downward. Dragonflies and cockroaches contract direct muscles to pull the wing downward. Bees, wasps and flies arch the tergum to cause the downstroke indirectly. Beetles and grasshoppers use a combination of direct and indirect muscles to move wings. Flight Muscle Contraction Synchronous muscle control uses a single volley of nerve impulses to stimulate a wing stroke. Asynchronous muscles stretch the antagonistic muscle and cause it to contract in response. Asynchronous muscles only need occasional nervous stimulation. Potential energy can be stored in resilient tissues. Wing beats may vary from a slow 4/second in butterflies to over 1000/second in midges. Direct flight muscles also alter the angle of wings to twist the leading edge to provide thrust. This figure-8 movement moves the insect forward. Fast flight requires long, narrow wings and a strong tilt, as in dragonflies and horse flies. Wing Thrust Nutrition-Digestive System The foregut consists of the mouth with salivary glands, esophagus, crop and gizzard. Some digestion, but no absorption, occurs in the crop as salivary enzymes mix with food. The gizzard grinds food before it enters the midgut, the main site of digestion and absorption. The ceca may increase the digestive and absorptive area. The hindgut is primarily a site for water absorption. Overview of Digestive Processes Ingestion of foods and their reduction by digestion only begins the steps in nutrition. Foods reduced by digestion are absorbed into the circulatory system. Foods are transported to the tissues of the body. They are assimilated into the structure of cells. Oxygen is also transported to tissues where food products are oxidized to yield energy and heat. Food not immediately used is stored for future use. Wastes produced by oxidation must be excreted. Food products unsuitable for digestion are egested in the form of feces.

6 Overview of Digestion Digestion mechanically and chemically breaks food into small units for absorption. Food solids contain carbohydrates, proteins and fats that must be reduced to simpler molecules. An animal must then reassemble the digested and absorbed units into the animal s own compounds. Intracellular digestion is limited in the size of food particle that can be utilized. Digestion in sponges and protozoa is entirely intracellular Extracellular Digestion The invention of the alimentary system allowed extracellular digestion to take place. This allowed cells lining the lumen of the alimentary canal to specialize for digestion or absorption. Development of mouth-to-anus flow-through systems allowed regional specialization of digestion. Action of Digestive Enzymes Digestive enzymes are hydrolytic enzymes or hydrolases; molecules are split by adding water. Proteins must be split into hundreds or thousands of small amino acid molecules. Carbohydrates must be reduced to simple sugars. Fats are reduced to glycerol and fatty acids although some are absorbed without being hydrolyzed. Specific enzymes form an enzyme chain so one may complete what another has started. Motility in the Alimentary Canal Food moves through the digestive tract by cilia, specialized musculature or both. Acoelomate and pseudocoelomate animals lack mesodermally derived gut musculature and use cilia. Most molluscs also use cilia; the coelom is weakly developed. In coelomic animals, the gut is lined with circular and longitudinal layers of smooth muscle. Gut movements cause segmentation; this mixes food but does not move it through the gut (See Fig 32-8). Peristalsis moves food down the gut. Regional Function of the Alimentary Canal Receiving Region Mouthparts may include mandibles, jaws, teeth, radula or bills. The buccal cavity and pharynx are inner chambers. Most metazoans, other than suspension feeders, have salivary glands to produce lubricating secretions. Receiving Region continued Salivary Glands Specialized saliva may contain toxins to quiet struggling prey. Leech saliva contains an anaesthetic and enzymes to prevent blood coagulation and increase flow. Salivary amylase is found in herbivorous molluscs, insects and primate mammals. Salivary amylase breaks starch into two-glucose fragments of maltose.

7 Conduction and Storage The esophagus of vertebrates and many invertebrates moves food to the digestive system. In annelids, insects and octopods, the esophagus is expanded into a crop, a food storage area. Among vertebrates, only birds have a crop; it softens grain and allows mild fermentation. Grinding and Early Digestion The stomach is a region for initial digestion and storage of food in vertebrates and some invertebrates. Herbivorous animals often continue the grinding and crushing of plants in the stomach. Swallowed stones and grit assist the muscular gizzard of oligochaete worms and birds. The insect proventriculus has chitinous teeth, and crustaceans have a gastric mill. Digestive diverticula are blind tubules or pouches that supplement the stomach and secrete enzymes and/or absorb nutrients. The Problem with Cellulose The woody cellulose that encloses plant cells is a very abundant molecule. Only the enzyme cellulase can break down the cellulose molecule. No metazoan animal can produce cellulase for direct digestion of cellulose. Many herbivorous animals harbor bacteria and protozoa in their gut that do produce cellulase. These microorganisms ferment cellulose under anaerobic conditions of the gut, producing fatty acids and sugars. Terminal Digestion and Absorption: The Intestine In invertebrates with digestive diverticula, the intestine may serve only to carry wastes away. In invertebrates with simple stomachs and in vertebrates, intestines digest and absorb nutrients. One method to increase digestive surface is to increase the length of the intestine. A coiled intestine is rare in invertebrates but may be eight times body length in some mammals. Invertebrates may use infolding to increase surface area as in the typhlosole in oligochaetes. Absorption Little food is absorbed in the stomach; digestion is not complete and absorptive surface is limited. Most digested food is absorbed by the villi of the small intestine. Region of Water Absorption and Concentration of Solids The large intestine consolidates the undigested material as semisolid feces. Reabsorption of water is the main function and is critical in desert species. Some animals have specialized rectal glands to absorb water and ions, leaving nearly dry fecal pellets.

8 Various Feeding Strategies Feeding strategy Definition Carnivore Feeding on animals Folivore Feeding on leaves Frugivore Feeding on fruits Graminivore Feeding on grasses Granivore Feeding on seeds Insectivore Feeding on insects Nectarivore Feeding on nectar Omnivore Feeding on both animals and vegetation Osteovore Feeding on bones and marrow Piscivore Feeding on fish Sanguinivore Feeding on blood Insect Nutrition Most insects feed on plant tissues or juices and are herbivorous or phytophagous. Many caterpillars are specialized to eat only certain species of plants (ex. Monarchs). Certain ants and termites cultivate fungus gardens for food. Many beetles and other insect larvae are saprophagous. Some insects are predaceous on other insects or other animals. Fig Parasitism in Insects Many species are parasitic as adults and/or larvae. Many parasitic insects, in turn, have parasites, which is a condition called hyperparasitism. Parasitoids live inside a host until they eventually kill the host; they are important in pest control. Sucking mouthparts form a tube to pierce tissues of animals or plants. Houseflies and blowflies have sponging mouthparts; the soft lobes at the tip absorb food. Biting mouthparts can seize and crush food. Mouthparts Circulation A tubular heart in the pericardial cavity moves hemolymph forward through the dorsal aorta. The heartbeat is a peristaltic wave. Accessory pulsatile organs help move the hemolymph into wings and legs. Hemolymph has plasma and amebocytes but does not function with oxygen transport.

9 Gas Exchange Terrestrial animals are faced with the dilemma of exchanging gases but preventing water loss. The tracheal system is a network of thin-walled tubes that branch throughout the insect body. Spiracles open to the tracheal trunks; there are two on the thorax and 7-8 on the abdomen. A valve on the spiracle often cuts down on water loss; the spiracle may also serve as a dust filter. Tracheae Composed of a single layer of cells lined with cuticle that is shed at each molt. The tracheae branch out into fluid-filled tubules called tracheoles that reach individual body cells. This system provides gas transport without use of oxygen-carrying pigments. Excretion and Water Balance Both insects and spiders utilize Malpighian tubules in conjunction with rectal glands. Malpighian tubules vary in number but join between the midgut and hindgut. The blind ends of the tubules float freely in the hemocoel bathed in hemolymph. Potassium is actively secreted into the tubules; other solutes follow the gradient. The main waste product is uric acid; it flows across at the upper end that is mildly alkaline. In the lower end of the tubule, potassium combines with carbon dioxide and is reabsorbed. Rectal glands then reabsorb chloride, sodium and water; the wastes pass on out. Nervous System Insect nervous systems resemble that of larger crustaceans, with fusion of ganglia. Some have a giant fiber system. A stomadeal system corresponds to the autonomic system of vertebrates. Neurosecretory cells in the brain function to control molting and metamorphosis. Sense Organs Many insects have keen sensory perception. Most sense organs are microscopic and located in the body wall. Different organs respond to mechanical, auditory, chemical, visual and other stimuli.

10 Visual Reception Insects have two types of eyes: simple and compound. Ommatidia structure is similar to that of crustaceans. Insects can see simultaneously in almost all directions; the image is myopic and fuzzy. Flying insects have a higher flicker-fusion rate; they distinguish flashes per second. Reproduction Sexes are separate in insects and fertilization is usually internal. Parthenogenesis is common in Homoptera and Hymenoptera. Sexual Attraction Female moths secrete a powerful pheromone to attract males from a great distance. Fireflies use flashes of light to detect mates. Some insects use sounds, color signals and other courtship behaviors. Reproduction continued Many insects deposit sperm in the vagina during copulation. In some orders, spermatophores are transferred or deposited on substrate. During evolution from aquatic to terrestrial life, spermatophores were used first. The female may only mate once and store the sperm to fertilize eggs throughout her life. Females may lay a few eggs and provide care of young, or lay huge numbers. Metamorphosis and Growth Various forms of metamorphosis produce degrees of change among different insect groups. Most insects change form after hatching from an egg. Each stage between molts is called an instar. Insects develop wings during the last stage where they are useful in reproduction. Holometabolous Metamorphosis Fig About 88% of insects undergo this complete metamorphosis. This separates the physiology of larval growth, pupal differentiation and adult reproduction. Larvae and adults often live in completely different environments and therefore do not compete. After several larval instars, a larval moth or butterfly becomes a pupa inside a cocoon or chrysalis. Pupae often pass the winter in this stage; the final molt occurs and the adult emerges in spring. Stages are egg-larva-pupa-adult.

11 Hemimetabolous Metamorphosis Fig Some insects undergo a gradual metamorphosis. Grasshoppers, cicadas, mantids, true bugs, mayflies and dragonflies exhibit this metamorphosis. Young are called nymphs. Bud-like growths in early instars show where the adult wings will eventually develop. Stages are egg-nymph-adult. Direct Development Silverfish and springtails have young similar to adults except in size and sexual maturation. Stages are egg-juveniles-adult. These are primitively wingless insects. Physiology of Metamorphosis Hormones regulate insect metamorphosis. Regulation is via negative feedback control of release of various hormones Ecdysone starts the molting process. Molting continues as long as juvenile hormone (neotenine) is sufficiently present. Cessation of juvenile hormone production in the pupa leads to an adult at the last molt. Diapause Diapause is a period of dormancy in the annual life cycle that is independent of conditions. Winter dormancy is called hibernation; summer dormancy is called estivation. Any stage (eggs, larvae, pupae or adults) may remain dormant to survive adverse conditions. This allows them to synchronize with the environment. Diapause is genetically determined but it may be triggered by environmental cues such as day length. Diapause always occurs at the end of an active growth stage; the insect is then ready for another molt. Therefore, many larvae do not develop beyond this point until spring in spite of mild temperatures. Defense Protective coloration, warning coloration and mimicry are protective adaptations. Stink bugs and others have repulsive odors and tastes. Some insects are aggressive (e.g. bees and ants). The monarch caterpillar incorporates a poisonous substance from its food plant, milkweed. The bombardier beetle can spray an attacking enemy with irritating chemicals.

12 Behavior and Communication Due to very sensitive perception, many insects respond to many environmental stimuli. Responses are governed by both the physiological state of the animal and its nerve pathways. Many insect behaviors are complex sequences of responses. Most insect behavior is innate but some involve simple learning. Pheromones These chemicals are secreted by one individual to affect the behavior of another individual. Pheromones attract the opposite sex, trigger aggregation, fend off aggression and mark trails. Bees, wasps and ants can recognize nestmates and signal an alarm if strangers enter the nest. Pheromones can be used to trap insects to monitor populations. Sound Production and Reception Sounds are used as warning devices, advertisement of territory, and courtship songs. Crickets chirp for courtship and aggression. The male cicada vibrates paired membranes on it abdomen to attract females. Tactile Communication Tactile communication involves tapping, stroking, grasping and antennae touching. Some beetles, flies and springtails use bioluminescence. Some female fireflies mimic another species flash pattern and attract males and then eat them. Social Behavior Some social communities are temporary and uncoordinated. Other social groups are highly organized and depend on chemical and tactile communication. Caste differentiation is common in the most organized social groups. Groups exhibiting social behavior include bees, wasps, termites, and ants. See your textbook for detailed descriptions. Insects and Human Welfare Beneficial Insects Insects produce honey, beeswax, silk and shellac. Of more economic importance, bees pollinate $10 billion worth of food crops in the U.S. annually. Pollinating insects and flowering plants are tightly co-evolved. Predaceous and parasitoid insects are vital in controlling many pest insect populations. Dead animals are rapidly consumed by fly maggots. Insects are critical components of most food chains and a central food for many fish and birds.

13 Insects and Human Welfare Harmful Insects Harmful insects eat and destroy our plants and fruits. Nearly every cultivated crop has several insect pests; this requires substantial $ for insect control. Bark beetles, spruce budworms, the gypsy moth and others are serious forest pests. Insects also destroy food, clothing and property. Medically important insects include vectors for disease agents. Control of Insects Broad-spectrum insecticides damage beneficial insect populations along with the targeted pest persist in the environment and accumulate strains of insects have evolved a resistance Biological control B.t. toxin Some viruses and fungi may be economical pesticides. Some natural predators or parasites of insect pests Integrated pest management combined use of all possible, practical techniques