Arthropoda I. Exoskeleton, Segmentation and Appendages

Transcription

1 Arthropoda I The Arthropoda is the last protostome phylum we will cover, is the most important protostome phylum, and is the iconic group that represents the Ecdysozoa. It is hard to overstate the importance of the Phylum Arthropoda. The Arthropoda is easily the most diverse and abundant group of animals on Earth. 80% of all named species are arthropods. The Arthropoda contains a million named species (mostly insects), and there might be 50 million more arthropod species that have not yet been named. For every human being on earth, there are 200 million arthropods alive at any one time. In the warm weather, it has been estimated that just the air over one square kilometer of temperate zone land contains 20 million arthropods. Factors which have contributed to the overwhelming success of arthropods include their exoskeletons, specialization of segments and appendages, highly developed respiratory systems, and efficient nervous and sensory systems. Exoskeleton, Segmentation and Appendages The first thing we notice about an arthropod is its exoskeleton, a hard but flexible outer shell composed of a waxy outer layer, lipoprotein, protein and chitin. The exoskeleton is thick where protection is needed, but thin joints allow easy bending. Just as muscles move our endoskeletons by attaching on the outside, muscles move an exoskeleton by attaching on the inside. These muscles are sometimes very big and strong. A lobster's claw bulges with muscle, and the claw is so strong that it can crush a thick oyster shell...or a person's finger. The exoskeleton of arthropods was probably also important in the move from water to land. While the exoskeleton of the ancestral arthropods began merely as a place to anchor muscles, it was also helpful in structural support and in retarding water loss in the first terrestrial arthropods, about 440 million years ago. Just like a nematode, the arthropod occasionally outgrows its exoskeleton and must molt it. Molting is also called "ecdysis," and you may remember that the Arthropoda and Nematodes (that also molted) are ecdysozoan protostomes. The molted arthropod exoskeleton is a perfect replica of the animal, right down to the facets of the compound eyes. The next screen shows a hermit crab and its molted exoskeleton. Often, the newly molted arthropod eats its exoskeleton to regain the minerals it contains. It also must seek refuge, because it is vulnerable to predators and cannot move very well until its new exoskeleton hardens. The growth stages between molts are called instars. Segmentation and appendages are other obvious feature of arthropods. While an annelid seemed to be a long string of identical segments, an arthropod has specialized segments with specialized appendages. For example, in a lobster, the body can be divided into a head, a thorax, and an abdomen. The head has antennae plus a variety of mouthparts, the thorax has the large claws and walking legs, and the abdomen has a series of small appendages that function in reproduction and swimming. The basic design of a segmented body with specialized appendages is impressively adaptable and flexible. Finally, less obviously, arthropods are highly cephalized, bilaterally symmetrical, coelomate protostomes with complete digestive systems. We must put an asterisk on the "coelomate" part, though. As in the mollusks, the coelom of arthropods is very reduced, and most of the body cavity is filled with a series of sinuses called the hemocoel, which collects blood from the open circulatory system. On the diagram of the crayfish below, note that the arteries simply end in the hemocoel. No arthropod reproduces asexually, but a few can reproduce parthenogenetically (using unfertilized, haploid eggs). The great majority use dioecious sexual reproduction, like humans.

2 Diversity There are so many classes of arthropods that they are divided into subphyla. We will use the arthropod classification below: Subphylum Trilobitomorpha (extinct trilobites) Subphylum Chelicerata Class Merostomata (horseshoe crabs) Class Arachnida (spiders, scorpions, ticks, mites) Subphylum Crustacea (lobsters, shrimp, barnacles, many others) Subphylum Myriapoda Class Chilopoda (centipedes) Class Diplopoda (millipedes) Subphylum Hexapoda Class Insecta (insects) This lesson deals with the Trilobitomorpha, Chelicerata, and Crustacea. The next module will deal with the Myriapoda and Hexapoda. The Trilobitomorpha. As far as we know, the trilobites are the earliest arthropods. They all died out 250 million years ago in the great Permian mass extinction, yet we know a good deal about them because we have fossils of nearly 4,000 species, all marine. The picture shows an excellent cast of a trilobite that Dr. K. keeps on his desk as a conversation piece. Unfortunately, no one has ever asked about it! You can even see the compound eyes. Trilobites ranged in size from 0.5 mm to 70 centimeters in length. They were flattened, oval creatures divided into a number of segments with appendages. They had well-developed heads which bore antennae and compound eyes. However, trilobites showed little specialization of their segments. Increasing specialization and fusing of segments will be two of the trends we will be watching in the groups that follow. Most trilobites foraged along the sediment surface, but the smallest ones were planktonic, like the smallest crustaceans of today. Specialized structures indicate feeding strategies that include deposit feeding, scavenging, carnivorous predation, and filter feeding. The Chelicerata. The 80,000 species of chelicerates include the horseshoe crabs (5 living species), sea spiders (500 species), spiders (35,000 species), scorpions (1,200 species), ticks and mites (30,000 species) and a number of other less familiar groups. All chelicerates have bodies divided into two main regions. The seemingly fused front of the body is the cephalothorax, and the more obviously segmented rear is the abdomen. There are no antennae, but there are six pairs of appendages: the chelicerae, a pair of pincers or fangs; the pedipalps (which function in feeding); and four pairs are walking legs. Most chelicerates are predatory carnivores. Spiders are unique among animals in using silken webs for this purpose. Some mites are the only herbivorous chelicerates, and can be serious agricultural pests. Most chelicerates feed either on prey blood and juices like the ticks, chiggers and mites, or, like spiders, inject venom that liquefies the internal organs of the prey. Scorpions and horseshoe crabs are somewhat atypical chelicerates in that they tear their food to bits and swallow it in small pieces.

3 Some chelicerates are unique among animals in possessing respiratory structures called book lungs. Book lungs open to the outside via spiracles and provide a greatly expanded surface area for blood to exchange oxygen and carbon dioxide. Horseshoe crabs have unique structures called book gills. The Crustacea. There are probably over 100,000 species of crustaceans. They range in size from tiny planktonic creatures less than a mm in length (like the copepods, the largest group of crustaceans and one of the commonest animals of the oceans) to giant crabs that span four feet. Crustaceans have jawlike mandibles instead of chelicerae, a body composed of a 5-segmented head (parts of which may be fused in some species), and a trunk that often has a thorax and abdomen. Two pairs of antennae are present. Appendages are usually many-jointed and may be highly specialized. Most large crustaceans like lobsters have gills, sometimes hidden under the exoskeleton of the cepahlothorax. Special structures pump water across these hidden gills. Terrestrial crustaceans like isopods have gills enclosed in chambers that protect them from drying out. Some species use thin membranes in the lining of the carapace to achieve gas exchange. Many very small forms (like copepods) lack gills and rely instead on simple diffusion. Crustaceans use every feeding strategy imaginable. There are filter feeders, detritivores, carnivores, herbivores, scavengers, ectoparasites, and even endoparasites (there is one species that is parasitic in the lungs of snakes!). Members of this subphylum can be found at all depths, in every freshwater, saltwater, and brackish water environment known. Sow bugs and pill bugs (isopods) have been very successful in moist terrestrial environments. Human Impact Lobsters, crabs, shrimp and crayfish are deliciously edible, and South Carolina has a large shrimp fishery. Spiders do an immense amount of good in keeping down the numbers of harmful insects. On the other hand, spiders and scorpions are among the most venomous of organisms. Ticks are vectors of such devastating diseases as Rocky Mountain spotted fever and Lyme disease. Mites are serious agricultural pests, on animals (chickens and turkeys particularly) as well as plants. Evolution There is controversy about the origin of the arthropods, but the traditional view says that they probably arose from segmented annelid ancestors, perhaps through a transitional form like an onychophoran. However, now molecular data are making us think that the lophotrochozoan protostomes (annelids) and the ecdysozoan protostomes (arthropods) arose from different ancestors. The new theory is that the phylum Tartigrada (the water bears) is most closely related to the arthropods. Another question is: are the arthropods monophyletic? We assumed they were for many years because they all share features like jointed appendages, a chitin exoskeleton, and compound eyes. Other taxonomists assert that the arthropods are not a monophyletic group. While we don't know about the origin of the arthropods, we're surer about their history. The trilobites came first, about 600 million years ago, and marine chelicerates like the horseshoe crabs evolved from them. The first spiders probably invaded land from fresh water about 360 million years ago. The origin of the crustaceans is unclear, but we have crustacean fossils dating back 505 million years. The next module continues with the Myriapoda and Hexapoda, including the enormously important insects.

4 Arthropoda II In the previous module, we looked at the general characteristics of arthropods and discussed three of the four arthropod subphyla: the Trilobitomorpha, the Chelicerata and the Crustacea. The last arthropod subphyla, the Myriapoda and the Hexapoda, are so large that they warrant a module of their own. The Myriapoda The Myriapoda ( many feet ) includes the wormlike Chilopoda (centipedes) and Diplopoda (millipedes). They and the insects have mandibles as their main mouthparts, but it is thought that this represents convergent evolution rather than shared ancestry. Centipedes and millipedes look somewhat alike, but centipedes have one pair of legs per segment and are fast-running predators with poison claws. Despite the fact that their name implies they have 100 legs, centipedes commonly have about thirty. Millipedes have two pairs of legs per segment and are slow-moving, inoffensive eaters of detritus (decaying organic matter). Millipedes may be slow, but they're still almost half a billion years ahead of us the first evidence of terrestrial arthropods is a millipede burrow from 450 million years ago. The Hexapoda (Insects) Insects are certainly the most important invertebrates on land. 75% of all named species of animals are insects, and the weight of the insects of the world probably exceeds the weight of all other kinds of animals, including humans. One biologist has observed, "We don't have to worry about the day when insects rule the world, because they already do. We should realize that we have to make peace with our landlord." The insect body is divided into three parts: head, thorax and abdomen. Three pairs of walking legs (hence the name Hexapoda ) are confined to the thorax, and there are no jointed appendages on the abdomen. The fruit fly below is a typical example. Like the Crustacea, the insect head has jawlike mandibles (often modified to suit feeding habits) plus other elaborate mouthparts, but only one pair of antennae and prominent compound eyes. The grasshopper and fly head below show unmodified and modified mouthparts. Flight Ability to fly gives any terrestrial animal tremendous advantages. Although some spiders and mites can drift on web parachutes, insects are the only invertebrates capable of directed flight. For 100 million years, insects were the only flying animals. Insect wings are rigid outgrowths of the exoskeleton, not jointed appendages. The pattern of venation in the wing is important for identification, and has been extensively studied. Insects like the dragonfly, that have two similar pairs of wings, are thought to be primitive. A more advanced arrangement is seen in the case of beetles, which have converted the front pair of wings into a hard, protective shell called the elytra. Flies have reduced the second pair of wings to a pair of small balancing organs, the halteres, and this is also considered to be an advanced modification. Flight entails an impressive amount of exertion. A fruit fly may beat its wings 100 times/second, and can keep up this pace for about 2 hours before exhaustion. This is almost one million beats. If you decided to flap your arms to imitate a fruit fly, you would only be able to complete about 2 flaps/ second, and to complete 1 million cycles you would have to keep flapping for six days without rest. And of course, all that flapping wouldn't get you anywhere. The fly would be long gone.

5 Feeding Insects employ every kind of feeding strategy imaginable. Generally speaking, insects can be characterized as biters, suckers, or spongers. Biters, like the grasshoppers or the wasp below, have the least specialized mouthparts. Mandibles bite and then chew with a side-to-side motion. Suckers have mouthparts that may also be modified for piercing, like those of the mosquitoes, fleas, or aphids (below). Some suckers do not have piercing mouthparts, but have long tube-like structures for sipping nectar. Bees and butterflies are examples. Spongers, like flies, have mouthparts for soaking up and feeding on blood, nectar, or other cellular fluids. As in the annelids, the gut shows much regional specialization. A typical arrangement might be a foregut, midgut and hindgut. The foregut would include a muscular pharynx, a distensible crop that would bulge enormously if the insect rapidly ate a large amount of food, and a proventriculus, or grinding gizzard. The midgut is called the stomach, and is the site of most enzyme production and absorption. The hindgut, or intestine, is concerned with formation of feces. An interesting feature of insect digestion is that the gut contains no mucus glands to help lubricate the passage of food, so the foregut encapsulates each fecal pellet in a thin, chitinous envelope, the peritrophic membrane. This membrane protects the gut lining and lets enzymes in and the products of digestion out. In our discussion of the protistans, we mentioned the symbiotic gut flagellates of termites. Termites have the hindgut modified for the maintenance of a "culture" of protozoans and bacteria that aid in the digestion of cellulose. Gas Exchange and Excretion A few of the very small insects exchange gases by direct diffusion across the body surface. In larger insects, specialized openings in the integument called spiracles allow direct access of tissues to environmental oxygen. The spiracles open into a system of tracheae that divide repeatedly. The finest tubes, the tracheoles, can be less than 1 micrometer in diameter, and may supply individual cells, especially muscle cells. Movement of gases into and out of the tracheae is accomplished by simple diffusion, but movements of the insect help in ventilation too. A "downside" of tracheal tubes is that once the insect gets larger than a certain size, diffusion can't supply oxygen fast enough to meet its needs. But for small insects, a tracheal system has a substantial "upside." The wing muscles of insects like gnats contract about 1000 times per second, and have the highest oxygen consumption rates known. This is possible because the insect does not depend on a circulatory system to deliver oxygen to them. Oxygen delivery is direct from the atmosphere through the tracheae. This allows insects to increase their metabolism remarkably with exercise. A human athlete can increase his oxygen consumption to about 7 times the resting rate, and some vertebrate animals can increase their maximum oxygen consumption rate to about 15 times the resting rate. However, a fly has no trouble increasing its oxygen consumption to 100 times the resting value. Excretion in insects occurs through a system of threadlike Malpighian tubules that absorb wastes from the hemocoel and then move them into the gut, where they are mixed with feces and water is resorbed from them. Even the excretion products of insects show how well adapted they are to the terrestrial environment. Aquatic arthropods excrete ammonia, which is highly toxic and requires large amounts of water for its elimination. But insects excrete uric acid, which is insoluble and non-toxic, and which is excreted as a thick, white paste with very little loss of water.

6 Reproduction All insects and myriapods are dioecious, many groups have parthenogenetic members (in which an unfertilized egg can develop), and most species lay eggs. Myriapods, like many of the chelicerates, rely on indirect fertilization (in which a sperm packet is "handed" from male to female). Insects use copulation, or direct fertilization, to achieve internal fertilization. Myriapods and primitive insects have incomplete metamorphosis, in which the young insect looks like a smaller version of the parent. These immature stages are called nymphs, and each nymph is a different instar. Advanced insects like the flies, beetles, wasps and butterflies undergo complete metamorphosis, with radically different larval, pupal, and adult forms. Because the larvae and the adult often feed on different resources (and the pupae usually don't feed at all), the different life stages do not compete with one another. The sequence below shows a fruit fly egg, and then the first, second and third instar larvae, the pupa, and a newly emerged adult. The two filaments on the egg are gas exchange organs. Note the easily visible tracheal tubes in "Larva 2" and "Larva 3." One of the most important influences on the rate of these life cycle changes is temperature. Fruit flies kept at 18 C will take days to pass through all the stages shown; fruit flies kept at 25 will take only 8-10 days. Nervous System Insects are probably the most cephalized invertebrates. Like the annelids, they have a pair of ventral nerve cords with a ganglion in each segment. The head contains a brain with connections to all the sensory organs concentrated on the head. The complex behavior of insects, especially the social insects like the bees and ants, is well known. Earlier we speculated about a human who was flapping his arms to imitate a fruit fly. The point was the human would get tired a lot sooner than the fly. Why do you think this would happen? a) The human circulatory system could not deliver oxygen fast enough to keep muscles aerobic. b) We cannot get as much energy from food as flies can. c) Our neural synapses fatigue. Flies don't have synapses. d) Flies excrete uric acid, an abundant source of energy. We excrete urea, which has no energy value. Yes, when muscles get tired in sudden, rapid exertion, most of the time it's because of a failure of the circulatory system to deliver enough oxygen, and lactic acid builds up. Insects do have synapses, they get no more energy from food than we do, and uric acid has no more energy content than urea. Insect Diversity There may be as many as 50 million species of insects, far too many for us to describe in this series of lessons. Instead, we will focus on the largest 6 of the 32 orders of insects, here listed in order of size: Coleoptera Diptera Lepidoptera Hymenoptera Hemiptera Orthoptera beetles flies and mosquitoes moths and butterflies ants, wasps and bees true bugs, leafhoppers, aphids, cicadas grasshoppers, crickets

7 Coleoptera. Beetles make up almost a third of all described species of animals! One family of beetles, the weevils, has more species (65,000) than any other phylum, and far more than our own Phylum Chordata (39,000). This should indicate to you that the beetles are an enormously successful group of arthropods. Beetles range in size from microscopic to about a foot in length and inhabit all environments except the open sea. The first pair of beetle wings has been transformed into a hard elytra. Beetles have all the feeding strategies mentioned elsewhere in this lesson. One group, the dung beetles, even exploits the rich and partially digested source of nutrition found in feces. Beetles even communicate-- consider the (misnamed) "fireflies." Diptera. The flies and mosquitoes, or Diptera, are found almost everywhere, and are one of the few insect orders with one pair of wings. Their larvae, called maggots, are important scavengers. Some flies, like the infamous "medfly" of California, can cause severe economic losses to farmers. Tsetse flies transmit African sleeping sickness, which we have already mentioned in the protist module. Perhaps the most important dipteran to biologists is the common fruit fly, Drosophila. Since the 1920s, Drosophila has been one of the most important animals in basic genetics. This has inspired much research on its ecology and evolution. Finally, in recent years, its developmental biology and molecular genetics have been extensively investigated. A detailed illustration of Drosophila is shown on the next screen. In nature, Drosophila sponges up the yeast and other fluids that accumulate on sugary food sources. These food sources could be ripe fruits, sap exuded from trees, decaying mushrooms, or flowers. In the laboratory, it has been found that the kinds of yeasts present (which change as the food source decays) have a great influence on the fly's health. Mosquitoes are the dipterans that have the greatest impact on most of us, especially in South Carolina. Mosquitoes are mostly an annoyance in the US, but abroad they transmit malria, yellow fever, dengue fever, encephalitis and some parasitic nematodes. In the early 1900s, a French company attempted to build a Panama Canal and had to drop the project after the death of 50,000 workers, mostly from malaria and yellow fever. The American building effort that finally opened the Panama Canal in 1914 was accompanied by strenuous efforts to drain swamps and control mosquitoes. Lepidoptera. The Lepidoptera includes the moths and butterflies. Their broad wings (colorful because of scales) and their long mouthparts specialized for sucking nectar from flowers make identification easy. Lepidopteran adults (like the painted lady butterfly below) tend to be colorful and inoffensive, but some larvae (called caterpillars) cause great harm. The gypsy moth and tent caterpillar damage trees, the corn borer and armyworm damage corn, the bollworm damages cotton, and the list goes on. However, the list must also include the silkworm, which gives us some of our finest fabrics. Hymenoptera. Hymenopterans (the bees, wasps and ants) are particularly interesting, not just because they have stingers, but also because many species form large social communities. These societies display altruism, division of labor, "planning" for the future, and cooperation. These characteristics have been praised as models for human behavior since ancient times. Today, although the characteristics are just as praiseworthy in humans, we realize that the insects have evolved these behaviors because less social hymenopterans apparently could not compete with those who cooperated with each other. Some hymenoperterans are aggressive carnivores (fire ants and yellow jackets come to mind), and others are herbivores. Mouthparts of this group are often modified for sucking nectar from flowers as well as for biting and chewing. They have two pairs of membranous wings. Honeybees provide us with honey and pollinate our crops. Although beneficial, they can turn very aggressive, as in the 1980s "Killer Bees" video below. This video anticipated that the killer bees would cross the Rio Grande by This did happen, and in 2010 a man in southern Georgia was stung to death by the bees. They may spread as far north as Chesapeake Bay. Some parasitic wasps lay their eggs on insect pests like caterpillars and aphids. When the eggs hatch, the larvae burrow into the pests and eat them from the inside.

8 Hemiptera. The Hemiptera includes the true bugs, and a large group that includes leafhoppers, aphids and cicadas. So although it is common for most people to call any insect a "bug," bugs are only a part of one order of insects. Hemipterans tend to be flat, shield-shaped insects, and all have piercingsucking mouthparts. Most use these beaks to feed on plant juices, but there are several carnivorous species, such as bed bugs and assassin bugs. Orthoptera. Grasshopers, crickets, locusts, mantids, and katydids are common and abundant insects. Most orthopterans are herbivores (remember the biblical "plagues of locusts"?), but there are a few predatory species. Stridulation--the sound of summer nights--is a common behavior of male orthopterans. It was once thought that this sound was created when the organism rubbed its back legs together, but now it is known that the sound comes from a rubbing of the wings against the body. You find a dead insect on your window sill. It has two pairs of membranous wings without scales, chewing mouthparts, and it seems that a stinger is extending from the rear of its abdomen. It is probably a member of the order a) Lepidoptera. b) Hymenoptera. c) Diptera. d) Coleoptera. Because it has two pairs of wings it can't be Diptera, and the lack of scales excludes Lepidoptera. The fact that both pairs of wings are membranous excludes the Coleoptera, who have the first pair modified into an elytra. The stinger clinches it for the Hymenoptera. It may be a wasp. Human Impact The impact of insects on humans is enormous. We tend to think first of their negative effects. Pest insects cause billions of dollars in damage to crops, and termites damage wooden buildings. Venemous insects like bees and fire ants cause us injury and sometimes death. Many insects (think of mosquitoes, lice, fleas, roaches, and house flies) are vectors of disease, especially in tropical regions.!! On the other hand, insects pollinate flowering plants, including crops. Predatory insects like dragonflies and ladybug beetles control pest insects. The enormous biomass of insects makes them and important food source in nature, and even "disgusting" maggots play an important role in cleaning up dead carcasses. Silkworm larvae give us beautiful fabric. Evolution While we used to think that the insects were monophyletic, now we think that a group of tiny, wingless insects called the collembolans seems to have evolved independently. Probably most insects evolved from a primitive crustacean called a fairy shrimp about 430 million years ago, about the time that land plants first appeared. Fossils of wingless insects date back 350 million years. Cockroaches, mayflies, dragonflies, and cicadas have not changed much since the origin of winged insects about 285 million years ago. On the other hand, flies are considered to be more evolved insects because of their highly modified wings and mouthparts. Insect Mysteries To end on a thoughtful note, two insect mysteries have always intrigued me, and may show the chance element in evolution. First, why are insects the only invertebrates that can fly? Flight is such a tremendous advantage that you'd think it would have evolved numerous times in the invertebrates. But apparently it only evolved once, in the early insects. If it only evolved once, it was very close to never evolving at all. Think

9 how different our world would be without flying insects! The implications for flowering plant alone (mostly pollinated by flying insects) are staggering. Second, 75% of all animal species are insects and 70% of the world's surface is covered by ocean. Why haven't the insects colonized the oceans? There are many freshwater insects, but only a tiny number of marine forms. Could colonization of the oceans still happen? Will our distant descendants read about how submarine ants are swarming over the Great Barrier Reef? Probably not, but who knows? The things that didn't happen are sometimes just as interesting as the things that did happen.

10 Biology 1110 Arthropoda 1. Why is it said that the Arthropoda is such a big and important phylum? 2. What is an exoskeleton? How does it help an arthropod move? How does it contribute to arthropod success? 3. What is molting, and why does molting nearly incapacitate an arthropod? Why do we say that an arthropod is a ecdysozoan protostome? 4. Why do we think that the arthropod exoskeleton helped the arthropods invade dry land? 5. The arthropods show segmentation with specialization, and the trend is towards less obvious segmentation and more specialization. Explain. 6. What is an open circulatory system? What is a hemocoel? What function does a hemocoel perform for an arthropod? 7. Describe the typical reproductive strategies of arthropods. 8. What are the four subphyla of living arthropods? Tell to which subphylum each of the following belongs: a flea, a shrimp, a centipede, a scorpion, a tick, a horseshoe crab, a butterfly. 9. To which subphylum does each of the following classes belong: Insecta, Chilopoda, Merostomata, Arachnida, Diplopoda?

11 10. Describe the typical arthropod features found in the trilobites. What aspect of trilobite structure is thought to be primitive? 11. Summarize the features of a chelicerate arthropod. Name some common types of chelicerates. How do typical chelicerates feed? 12. Describe the features of a typical crustacean. How could you tell a crustacean from a chelicerate? 13. Name some common types of crustaceans. 14. Contrast the gas exchange mechanisms of small and large crustaceans. How do they compare with the gas exchange mechanisms of chelicerates? 15. How do crustaceans feed? 16. According to the traditional view mentioned in the module, which group of arthropods evolved first, and from what ancestor? Does molecular data agree with this view? If not, how is it different? 17. What groups of arthropods are included in the Myriapoda? What does Myriapoda mean? 18. How would you distinguish between a centipede and a millipede? 19. Describe the basic anatomy of an insect. Why are insects called hexapods? 20. Are insects the only invertebrates that can fly? Distinguish between a primitive wing arrangement and a highly evolved wing arrangement.

12 21. Give an example of an insect with each of the following kinds of mouthparts: biting, sucking, sponging. 22. What is the function of each of the following segments of the insect gut: pharynx, crop, proventriculus, stomach, intestine. 23. Describe the operation of a tracheal tube gas exchange system. Why does a tracheal system offer substantial oxygen delivery advantages for small insects? Why wouldn t it work for a very large insect? 24. Why is excretion of uric acid an advantage for terrestrial insects? 25. Distinguish between incomplete and complete metamorphosis. What may be advantageous about complete metamorphosis? How does temperature influence rate of development? 26. Fill in the common names of the insects in the following orders: Hemiptera Hymenoptera Lepidoptera Coleoptera Orthoptera Diptera Which order is the largest? The second largest? 27. What were the two insect mysteries cited at the end of the module?

13 Biology 1110 Arthropoda 1. The Arthropoda contains... of all known animal species % 2. 55% 3. 60% 4. 85% 2. Arthropod exoskeletons 1. must be periodically molted. 2. are moved by muscles on the inside. 3. has joints composed of thin membranes. 4. All of these. 3. The exoskeleton of arthropods made them pre-adapted for the move to a terrestrial environment mainly because it 1. retarded water loss. 2. served as a reservoir of minerals. 3. served as a disposal site for wastes. 4. carried sensory receptors that worked in air. 4. Annelids and arthropods differ the most in their type of 1. gut. 2. symmetry. 3. coelom. 4. level of organization. 5. We used to think that arthropods arose from the annelids, but now we believe that the phylum most closely related to the arthropods is the 1. Hirudinea. 2. Porifera. 3. Tardigrada. 4. Echinodermata. 6. Which of the following phyla is most closely related to the Arthropoda? 1. Platyhelminthes 2. Nematoda 3. Mollusca. 4. Chordata 7. When we compare the trilobites with the modern arthropods, one difference we see is that the trilobites had 1. no appendages on most of their segments. 2. little specialization of their segments. 3. no concentration of sensory organs on the head. 4. much more specialized feeding habits. 8. The chelicerates differ from the crustaceans and most insects because instead of having... as their main mouthparts, the chelicerates have 1. mandibles... pincers and fangs. 2. pedipalps... maxillipeds. 3. pedipalps... cercae. 4. chelicerae... pedipalps. 9. Other distinguishing features of the Chelicerata are 1. biramous antennae. 2. four pairs of walking legs and a body with two main regions. 3. a tracheal system and wings in some species. 4. a mainly filter feeding lifestyle. 10. Most large crustaceans have... as their main gas exchange organ. 1. gills. 2. book lungs. 3. parapodia. 4. spiracles is a feeding style not found in the Crustacea. 1. Eating detritus 2. Filter feeding 3. Eating meat 4. All of these feeding habits are found in the Crustacea.

14 12. It would be hard to confuse an insect with either a millipede or a centipede, but how could you distinguish between a millipede and a centipede? 1. A millipede has chelicerae and a centipede has mandibles. 2. A millipede has a thousand legs and a centipede has 100 legs. 3. A millipede eats detritus and a centipede is a predator. 4. A millipede reproduces exclusively asexually. 13. A typical feature of the insects is 1. three pairs of walking legs. 2. a lack of abdominal appendages. 3. wings. 4. All of these. 14. A primitive insect would tend to have 1. eyes on every segment and identical walking legs. 2. two similar pairs of wings and chewing mouthparts. 3. tubelike mouthparts and excretion of ammonia. 4. simple rather than compound eyes and chelicerae instead of mandibles. 15. One of the advantages of using a tracheal system for gas exchange is that 1. the efficiency of the Krebs Cycle is increased by 40%. 2. tracheae keep blood vessels from collapsing under low pressures. 3. direct delivery of oxygen to the tissues is faster than relying on a circulatory system. 4. no water is lost when carbon dioxide leaves the body. 16. Incomplete metamorphosis is an insect life cycle in which the young pass through a series of stages called 1. zooea, gastrea and gonea. 2. nymphs. 3. larva, pupa and adult. 4. planula, veliger and trochophore. 17. The shrimp, crayfish and pill bugs are all 1. myriapods. 2. crustaceans. 3. gastropods. 4. chelicerates. 18. Which of the following is not a chelicerate? 1. mites. 2. spiders. 3. scorpions. 4. horseflies. 19. Which of the following are hymenopterans? 1. bees 2. flies 3. beetles 4. horseshoe crabs 20. The Diptera are generally characterized by 1. fangs that inject poison into prey. 2. a closed circulatory system. 3. one pair of wings. 4. a lack of sexual reproduction. 21. In nature, Drosophila 1. sucks blood from warm-blooded animals. 2. is a parasite of bees. 3. is a scavenger on rotting meat. 4. eats yeast. 22. The expression "bug" for an error in computer software originated when a moth flew into a switch on an early Navy computer and physically blocked it, causing the computer to crash. The Navy technicians who called the moth a bug were not shipshape on their entomology, however. The moth was a member of the order..., not a bug of the order Lepidoptera... Hemiptera. 2. Coleoptera... Hemiptera. 3. Diptera... Orthoptera. 4. Arachnida... Hymenoptera.

15 Biology 1110 Arthropoda Correct. And if we knew the large number of unnamed insects in the tropics, the percentage might go even higher Correct Correct. The exoskeleton is a hard outer shell with a waxy coating. 2. No. The exoskeleton is a mineral reservoir, particularly in large crustaceans, but this didn't assist the move to land Correct. Arthropods (and mollusks) have a hemocoel and a very reduced true coelom. Annelids have a very extensive true coelom that acts as a hydrostatic skeleton. Both annelids and arthropods have complete guts, bilateral symmetry, and have reached the organ level of organization No. Hirudinea is a class of the Annelida, not a phylum. 3. Correct. The Tardigrada or water bears are also an ecdysozoan, protostome phylum. 4. No. The Echinodermata are deuterostomes, and are very distantly related to the arthropods Correct. This is the only ecdysozoan phylum listed. The Platyhelminthes and Mollusca are lophotrochozoans, and the Chordata aren't even protostomes No. Trilobites had appendages on almost every segment. 2. Correct. 3. No. They had compound eyes, antennae, and all the usual sensory equipment on their heads. 4. No. Trilobites were to Paleozoic seas what insects are to land today. They were everywhere, and they ate almost everything Correct. The pincers and fangs are the chelicerae. 2. No. Pedipalps are found on the chelicerates, not the crustaceans and insects No. That sounds like the crustaceans. 2. Correct. 3. No. That sounds like the insects Correct Correct No. Both centipedes and millipedes (and insects) have mandibles. 2. No, they don't have nearly as many legs as their names suggest. 3. Correct. 4. No. There are no arthropods that reproduce exclusively asexually Correct Correct. One of the earliest insects of which we are aware was like a dragonfly. 4. No, all insects have compound eyes, and only chelicerates have chelicerae No. An insect circulatory system has almost nothing to do with supplying oxygen to muscles. 3. Correct. This is why insects can increase their metabolism so much more than mammals can. They are delivering oxygen directly to tissues without going through a circulatory system Correct No, myriapods are the centipedes and millipedes. 2. Correct.

16 Correct. Horseflies are insects (hexapods) Correct. 2. No, Diptera. 3. No, Coleoptera. 4. No, Chelicerata No, that sounds like a spider (Chelicerata). 2 and 4. No, there are no arthropods with those characteristics. 3. Correct No, although some species eat yeasts on rotting mushrooms. 4. Correct Correct. 4. No. Arachnida are spiders and Hymenoptera are bees, wasps and ants.