Concept 23.4: Arthropods Are Diverse and Abundant Animals

Arthropods and their relatives are ecdysozoans with paired appendages. Arthropods are an extremely diverse group of animals in numbers of species. Furthermore, the number of individual arthropods alive at any one time is estimated to be about 1018, or a billion billion. Among the animals, only the nematodes are thought to exist in greater numbers.

Several key features have contributed to the success of the arthropods. Their bodies are segmented, and their muscles are attached to the inside of their rigid exoskeletons. Each segment has muscles that operate that segment and the jointed appendages attached to it. Jointed appendages permit complex movements, and different appendages are specialized for different functions. Encasement of the body within a rigid exoskeleton provides the animal with support for walking in the water or on dry land and provides some protection against predators. The waterproofing provided by chitin keeps the animal from dehydrating in dry air.

Representatives of the four major arthropod groups living today are all species-rich: the chelicerates (including the arachnids—spiders, scorpions, mites, and their relatives), myriapods (millipedes and centipedes), crustaceans (including shrimps, crabs, and barnacles), and hexapods (insects and their relatives). Phylogenetic relationships among arthropod groups are currently being reexamined in light of a wealth of new information, much of it based on gene sequences. These studies suggest that the chelicerates are the sister group to the remaining arthropods, and that the crustaceans may be paraphyletic (some crustaceans are more closely related to hexapods than to other crustaceans). There is strong support for the monophyly of arthropods as a whole.

The jointed appendages of arthropods gave the clade its name, from the Greek words arthron, “joint,” and podos, “foot” or “limb.” Arthropods evolved from ancestors with simple, unjointed appendages. The exact forms of those ancestors are unknown, but some arthropod relatives with segmented bodies and unjointed appendages survive today. Before we describe the modern arthropods, we will discuss those arthropod relatives.

Arthropod relatives have fleshy, unjointed appendages

Until fairly recently, biologists debated whether the velvet worms (onychophorans) were more closely related to annelids or arthropods, but molecular evidence clearly links them to the arthropods. Indeed, with their soft, fleshy, unjointed, claw-bearing legs, velvet worms may be similar in appearance to the arthropod ancestor (FIGURE 23.25A). The 180 species of velvet worms live in leaf litter in humid tropical environments. Their soft, segmented bodies are covered by a thin, flexible cuticle that contains chitin. They use their fluid-filled body cavities as hydrostatic skeletons. Fertilization is internal, and large, yolky eggs are brooded within the female’s body.

Figure 23.25: Arthropod Relatives with Unjointed Appendages (A) Velvet worms have unjointed legs and use the body cavity as a hydrostatic skeleton. (B) Tardigrades can be abundant on the wet surfaces of mosses and plants and in temporary pools of water.

Tardigrades (water bears) also have fleshy, unjointed legs and use their fluid-filled body cavities as hydrostatic skeletons (FIGURE 23.25B). Tardigrades are tiny (0.5–1.5 mm long) and lack both a circulatory system and gas exchange organs. The 1,200 known species live in marine sands and on temporary water films on plants, as well as in ephemeral pools. When these films and shallow pools dry out, the animals also lose water and shrink to small, barrel-shaped objects that can survive for at least a decade in a dormant state. Tardigrades have been found in densities as high as 2 million per square meter of moss.

Chelicerates are characterized by pointed, nonchewing mouthparts

In the chelicerates, the head bears two pairs of pointed appendages modified to form mouthparts, called chelicerae, which are used to grasp (rather than chew) prey. In addition, many chelicerates have four pairs of walking legs. The 114,000 described species are placed in three major clades: sea spiders, horseshoe crabs, and arachnids.

The sea spiders (pycnogonids) are a poorly known group of about 1,000 marine species (FIGURE 23.26A). Most are small, with leg spans less than 1 cm, but some deep-sea species have leg spans up to 60 cm. A few sea spiders eat algae, but most are carnivorous, eating a variety of small invertebrates.

Figure 23.26: Two Small Chelicerate Groups (A) Although they are not spiders, it is easy to see why sea spiders were given their common name. (B) Horseshoe crabs are an ancient group that has changed very little in morphology over time.

There are four living species of horseshoe crabs, but many close relatives are known from fossils. Horseshoe crabs, which have changed very little morphologically during their long fossil history, have a large horseshoe-shaped covering over most of the body. They are common in shallow waters along the eastern coast of North America and the southern and eastern coasts of Asia, where they scavenge and prey on bottom-dwelling animals. Periodically they crawl into the intertidal zone in large numbers to mate and lay eggs (FIGURE 23.26B).

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Arachnids are abundant in terrestrial environments. Most arachnids have a simple life cycle in which miniature adults hatch from internally fertilized eggs and begin independent lives almost immediately. Some arachnids retain their eggs during development and give birth to live young.

The most species-rich and abundant arachnids are the spiders, scorpions, harvestmen, mites, and ticks (FIGURE 23.27). More than 60,000 described species of mites and ticks live in soil, leaf litter, mosses, and lichens, under bark, and as parasites of plants and animals. Mites are vectors for wheat and rye mosaic viruses and cause mange in domestic animals as well as skin irritation in humans.

Figure 23.27: Arachnid Diversity (A) Known for their web-spinning abilities, spiders are expert predators. (B) A desert scorpion assumes the animal’s aggressive attack position. These arachnids are fearsome predators, and their sting can be dangerous to humans. (C) Harvestmen, also called daddy longlegs, are scavengers. (D) Mites include many free-living species as well as blood-sucking, external parasites.

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Spiders, of which about 50,000 species have been described, are important terrestrial predators with hollow chelicerae, which they use to inject venom into their prey. Some species have excellent vision that facilitates elaborate courtship displays and allows the spiders to hunt their prey. Others spin elaborate webs made of protein threads in which they snare prey. The threads are produced by modified abdominal appendages connected to internal glands that secrete the proteins, which solidify on contact with air. The webs of different groups of spiders are strikingly varied, and this variation enables the spiders to position their snares in many different environments for many different types of prey.

Mandibles and antennae characterize the remaining arthropod groups

The remaining three arthropod groups—the myriapods, crustaceans, and hexapods—have mouthparts composed of mandibles rather than chelicerae, so they are together called mandibulates. Mandibles are often used for chewing as well as for biting and holding food. Another distinctive characteristic of the mandibulates is the presence of sensory antennae on the head.

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The myriapods comprise the centipedes, millipedes, and their close relatives. Centipedes and millipedes have a well-formed head with the mandibles and antennae characteristic of mandibulates. Their distinguishing feature is a long, flexible, segmented trunk that bears many pairs of legs (FIGURE 23.28). Centipedes, which have one pair of legs per segment, prey on insects and other small animals. In millipedes, two adjacent segments are fused so that each fused segment has two pairs of legs. Millipedes are detritivores. More than 3,000 species of centipedes and 9,000 species of millipedes have been described, and many more species probably remain unknown. Although most myriapods are less than a few centimeters long, some tropical and subtropical species are ten times that size.

Figure 23.28: Myriapods (A) Centipedes have modified appendages that function as poisonous fangs for capturing active prey. They have one pair of legs per segment. (B) Millipedes, which are scavengers and plant eaters, have smaller jaws and legs than centipedes do. They have two pairs of legs per segment.

Crustaceans are the dominant marine arthropods today, and they are also common in freshwater and some terrestrial environments. The most familiar crustaceans are the shrimps, lobsters, crayfishes, and crabs (all decapods; FIGURE 23.29A) and the pillbugs (isopods; FIGURE 23.29B). Additional species-rich groups include the amphipods, ostracods, copepods (FIGURE 23.29C), and branchiopods (FIGURE 23.29D), all of which are found in freshwater and marine environments.

Figure 23.29: Crustacean Diversity (A) This decapod crustacean is a “Sally Lightfoot” crab from the Galápagos. (B) This pillbug, a terrestrial isopod, can roll into a tight ball when threatened. (C) Minute copepods such as this one are an important link in aquatic food chains. (D) Tadpole shrimp are branchiopods, not to be confused with the decapod crustaceans commonly called shrimp, or with the brachiopods seen in Figure 23.15. (E) Barnacles are sessile, attaching to a substrate by their muscular stalks and feeding with retractable feeding appendages (the delicate, tentacle-like structures seen here).

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Barnacles are unusual crustaceans that are sessile as adults (FIGURE 23.29E). Adult barnacles look more like mollusks than like other crustaceans, but as the zoologist Louis Agassiz remarked more than a century ago, a barnacle is “nothing more than a little shrimp-like animal, standing on its head in a limestone house and kicking food into its mouth.”

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Most of the 67,000 described species of crustaceans have a body that is divided into three regions: head, thorax, and abdomen (FIGURE 23.30A). The segments of the head are fused together, and the head bears five pairs of appendages. Each of the multiple thoracic and abdominal segments usually bears one pair of appendages. The appendages on different parts of the body are specialized for different functions, such as gas exchange, chewing, capturing food, sensing, walking, and swimming. In many species, a fold of the exoskeleton, the carapace, extends dorsally and laterally back from the head to cover and protect some of the other segments.

Figure 23.30: Two Segmented Body Plans The bodies of crustaceans and insects are divided into three regions, the head, thorax, and abdomen. (A) In crustaceans, each body region bears specialized appendages, and a shell-like carapace covers the head and thorax. (B) The thorax of insects bears three pairs of legs, and in most groups, two pairs of wings. Unlike other arthropods, insects have no appendages growing from their abdominal segments.

The fertilized eggs of most crustacean species are attached to the outside of the female’s body, where they remain during their early development. At hatching, the young of some species are released as larvae; those of other species are released as juveniles that are similar in form to the adults. Still other species release eggs into the water or attach them to an object in the environment.

More than half of all described species are insects

During the Devonian period, more than 400 million years ago, some mandibulates colonized terrestrial environments. Of the several groups (including some crustacean isopods and decapods) that successfully colonized the land, none is more prominent today than the six-legged hexapods: the insects and their relatives. Insects are abundant and diverse in terrestrial and freshwater environments. Only a few insects live in salt water.

The wingless relatives of the insects—the springtails, two-pronged bristletails, and proturans—are probably the most similar of living forms to insect ancestors. These insect relatives have a simple life cycle; they hatch from eggs as miniature adults. They differ from insects in having internal mouthparts. Springtails can be extremely abundant (up to 200,000 per square meter) in soil, leaf litter, and on vegetation and are the most abundant hexapods in the world in terms of number of individuals (rather than number of species).

More than 1 million of the 1.8 million described species of living organisms are insects. As we discussed at the beginning of this chapter, biologists have estimated that many millions of insect species remain to be discovered. Like crustaceans, insects have a body with three regions—head, thorax, and abdomen. They have a single pair of antennae on the head and three pairs of legs attached to the thorax. In most groups of insects, the thorax also bears two pairs of wings. Unlike other arthropods, insects have no appendages associated with their abdominal segments (FIGURE 23.30B).

Insects can be distinguished from springtails and other hexapods by their external mouthparts and by antennae that contain a motion-sensitive receptor called Johnston’s organ. In addition, insects have a derived mechanism for gas exchange in air: a system of air sacs and tubular channels called tracheae (singular trachea) that extend from external openings called spiracles inward to tissues throughout the body (see Figure 31.10). Insects use nearly all species of plants and many species of animals as food.

TABLE 23.2 lists the major insect groups. Two groups—the jumping bristletails and silverfish—are wingless and have simple life cycles, like the springtails and other close insect relatives. The remaining groups are all pterygote insects. Pterygotes have two pairs of wings, except in some groups where one or both pairs of wings have been secondarily lost. Secondarily wingless groups include the parasitic lice and fleas, some beetles, and the worker individuals in many ants.

The Major Insect Groupsa

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Hatchling pterygotes do not look like adults, and they undergo substantial changes at each molt. The immature stages of insects between molts are called instars. A substantial change that occurs between one developmental stage and another is called metamorphosis. If the changes between its instars are gradual, an insect is said to have incomplete metamorphosis. If the change between at least some instars is dramatic, an insect is said to have complete metamorphosis. In many insects with complete metamorphosis, the different life stages are specialized for different environments and use different food sources. In many species, the larvae are adapted for feeding and growing, whereas the adults are specialized for reproduction and dispersal.

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Insect metamorphosis is under the control of hormones, as described in Concept 35.5

Pterygote insects were the first animals in evolutionary history to achieve the ability to fly. Flight opened up many new lifestyles and feeding opportunities that only the insects could exploit, and it is almost certainly one of the reasons for the remarkable numbers of insect species and individuals, and for their unparalleled evolutionary success.

Molecular data suggest that insects began to diversify about 450 million years ago, about the time of the appearance of the first land plants (although earliest known fossil insects are only about 400 million years old). The earliest hexapods evolved in a terrestrial environment that lacked any similar organisms, which in part accounts for their remarkable success. But the success of the insects is also due to their wings. Homologous genes control the development of insect wings and crustacean appendages, suggesting that the insect wing evolved from a dorsal branch of a crustacean-like limb:

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APPLY THE CONCEPT: More than half of all described species are insects

The following data were used by entomologist Terry Erwin to estimate the undescribed diversity of insects. Review the design of Erwin’s experiment in the opening story of this chapter. Then use Erwin’s data to answer the questions and consider how changes in the assumptions may affect the estimates.a

  1. From the data above, estimate the number of insect species in an average hectare of Panamanian forest. Assume that the data for beetles on L. seemannii are representative of the other tree species, and that all the species of beetles that are not host-specific were collected in the original sample. Remember to sum your estimates of the number of (a) host-specific beetle species in the forest canopy; (b) non-host-specific beetle species in the forest canopy; (c) beetle species on the forest floor; and (d) species of all insects other than beetles.
  2. Use the following information to estimate the number of insect species on Earth. There are about 50,000 species of tropical forest trees. Assume that the data for beetles on L. seemannii are representative for other species of tropical trees, and calculate the number of host-specific beetles found on these trees. Add an estimated 1 million species of beetles that are expected across different species of trees (including temperate regions). Estimate the number of ground-dwelling beetle species based on the percentage used in question 1. Now estimate the number of species of insects found worldwide, based on the percentage of beetles among all insect species.
  3. The estimates in questions 1 and 2 are based on many assumptions. Do you think these assumptions are reasonable? Why or why not? Would you argue for a different set of assumptions? How do you think these changes in assumptions would affect your calculations? Can you think of ways to test your assumptions?

The dorsal limb branch of crustaceans is used for gas exchange. Thus the insect wing probably evolved from a gill-like structure that had a gas exchange function.

The adults of most flying insects have two pairs of stiff, membranous wings attached to the thorax. True flies, however, have one pair of wings and a pair of stabilizers called haltares. In winged beetles, one pair of wings—the forewings—forms heavy, hardened wing covers. Flying insects are important pollinators of flowering plants.

Two groups of pterygotes, the mayflies and dragonflies (FIGURE 23.31A), cannot fold their wings against their bodies. This is the ancestral condition for pterygote insects, and the mayflies and dragonflies are not closely related to one another. Members of these groups have predatory or herbivorous aquatic larvae that transform into flying adults after they crawl out of the water. Dragonflies (and their relatives the damselflies) are active predators as adults. In contrast, adult mayflies lack functional digestive tracts. Mayflies live only about a day, just long enough to mate and lay eggs.

Figure 23.31: Diverse Winged Insects (A) Unlike most flying insects, a dragonfly cannot fold its wings over its back. (B) Orthopteran insects such as grasshoppers undergo several larval molts (instars), but the juveniles resemble small adults (incomplete metamorphosis). (C) Hemipterans are “true” bugs. (D–H) Holometabolous insects undergo complete metamorphosis. (D) A larval caddisfly (right) emerges from its case constructed from silk and sand. (E) Coleoptera is the largest insect group; beetles such as this rose chafer account for more than half of all holometabolous species. (F) The swallowtail butterfly is found in most parts of North America. (G) This bee fly is a true fly. Although bee flies cannot sting, most potential predators avoid them because of their resemblance to bees. (H) This western paper wasp is a hymenopteran, a group in which most members display social behaviors.

All other pterygote insects—the neopterans—can tuck their wings out of the way upon landing and crawl into crevices and other tight places. Some neopteran groups undergo incomplete metamorphosis, so hatchlings of these insects are sufficiently similar in form to adults to be recognizable. Examples include the grasshoppers (FIGURE 23.31B), roaches, mantids, stick insects, termites, stoneflies, earwigs, thrips, true bugs (FIGURE 23.31C), aphids, cicadas, and leafhoppers. They acquire adult organ systems, such as wings and compound eyes, gradually through several juvenile instars.

More than 80 percent of all insects belong to a subgroup of the neopterans called the holometabolous insects (see Table 23.2), which undergo complete metamorphosis (FIGURE 23.31D). The many species of beetles account for almost half of this group (FIGURE 23.31E). Also included are lacewings and their relatives; caddisflies; butterflies and moths (FIGURE 23.31F); sawflies; true flies (FIGURE 23.31G); and bees, wasps, and ants, some species of which display unique and highly specialized social behaviors (FIGURE 23.31H).

CHECKpoint CONCEPT 23.4

  • What features have contributed to making arthropods among the most abundant animals on Earth, both in number of species and number of individuals?
  • Describe the difference between incomplete and complete metamorphosis.
  • Can you think of some possible reasons why there are so many species of insects?

The majority of Earth’s animal species are protostomes, so it is not surprising that the protostome groups display a huge variety of different body plans and other characteristics. We will now consider the diversity of the deuterostomes, a group that contains far fewer species than the protostomes but which is even more intensively studied and, not incidentally, is the group to which humans belong.

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