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, which include the insects and their wingless relatives.

As you saw at the opening of this chapter, more than 1 million of the 1.8 million described living species on Earth are insects, but that is a small fraction of the number of species of insects that are thought to exist on Earth. One of Terry Erwin’s experiments is shown in more detail in Investigating Life: How Many Unknown Species? This and similar experiments show that we have much to learn about the biodiversity of our own planet.

The wingless relatives of the insects—the springtails, two-pronged bristletails, and proturans—are probably the most similar of living forms to insect ancestors (Figure 31.25). These hexapods 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 m²) in soil, leaf litter, and on vegetation and are the most abundant hexapods in the world in terms of number of individuals (as opposed to number of species).

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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 growing from their abdominal segments (Figure 31.26). Insects are 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 48.4).

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Table 31.2 lists the major insect groups. Two groups—the jumping bristletails and silverfish—are wingless and have simple life cycles, like the springtails and other non-insect hexapods. The remaining groups are all pterygote insects. Pterygotes have two pairs of wings, except in some groups in which one or both pairs of wings have been secondarily lost. These secondarily wingless groups include the parasitic lice and fleas, some beetles, and the worker individuals of many ant species.

Hatchling pterygotes do not look like adults; 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 stages are gradual, an insect is said to have incomplete metamorphosis. If the change between at least some stages is dramatic, an insect is said to have complete metamorphosis (see Figure 30.10). In many insects with complete metamorphosis, different stages are specialized for different functions and use different food sources. In many species the larvae are specialized for feeding and growing, whereas the adults are specialized for reproduction and dispersal.

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 halteres (which are derived from the second pair of wings). In winged beetles, one pair of wings—the forewings—forms heavy, hardened wing covers.

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Two groups of pterygotes, the mayflies and dragonflies (Figure 31.27A), 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 both 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. Adult mayflies live only about a day, just long enough to mate and lay eggs.

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 31.27B), roaches, mantids, stick insects, termites, stoneflies, earwigs, thrips, true bugs (Figure 31.27C), aphids, cicadas, and leafhoppers. These groups 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 31.2), which undergo complete metamorphosis (Figure 31.27D). The many species of beetles account for almost half of this group (Figure 31.27E). Also included are lacewings and their relatives; caddisflies; butterflies and moths (Figure 31.27F); true flies (Figure 31.27G); and bees, wasps, and ants, some species of which display unique and highly specialized social behaviors (Figure 31.27H).

Molecular data suggest that insects began to diversify about 450 million years ago, about the time of the appearance of the first land plants. These early hexapods evolved in a terrestrial environment that lacked any other similar organisms, which in part accounts for their remarkable success. But the success of the insects is also due to their wings. Pterygote insects were the first animals in evolutionary history to achieve the ability to fly. 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 (Figure 31.28). 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.

Question

The uppermost dorsal branch on the ancestral arthropod limb is thought to have functioned in gas exchange. The Hox gene expression data suggest that this structure may have been homologous to the insect wing.

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Flight opened up many new lifestyles and feeding opportunities that only the insects could exploit, such as pollination of (and coevolution with) flowering plants. Flight is almost certainly one of the reasons for the remarkable numbers of both insect species and individual insects, and for their unparalleled evolutionary success.