11.1.1 11.11: Flight and metamorphosis produced the greatest adaptive radiation ever.

Two characteristics of insects that have been central to their success are flight and the way they cope with the change in body size as they grow. Wings and the ability to fly are adaptations that first appeared in insects. With flight, insects are able to avoid many predators and can efficiently search for food and for mates. Flight also enables organisms to more quickly leave one location and disperse to another that is potentially more hospitable.

As arthropods, insects have a modular body plan and a rugged exoskeleton, to which their muscles are attached. Like a suit of armor, the exoskeleton provides excellent protection from injury and predators. It also helps individuals conserve water and resist drying out. One challenge associated with having an exoskeleton, however, is that it prohibits growth. Nearly all insects (and a small number of other arthropod species) overcome this constraint with a process of growth (shared by the roundworms, of phylum Nematoda) that is very different from the pattern seen in other animals, such as mammals. After hatching, their life is divided into three completely different stages (FIGURE 11-18).

• 1. Larva. An egg hatches into a larva (pl. larvae), which looks completely different from the adult form. A caterpillar is the larval stage of a butterfly or moth, and a grub is the larval stage of a beetle. A larva’s job is to eat and grow large enough to enter the next life stage, the pupa. Larvae are able to grow by passing through numerous stages (usually 4–8) in which they shed their exoskeleton, increase in size, and develop a new, slightly larger exoskeleton, in the process of molting.
• 2. Pupa. Eventually, a larva completely covers itself with a casing. Covered by the case—which has different names (such as cocoon or chrysalis) in different insect groups—the insect is now called a pupa. In the pupa, genes are activated that code for the adult body form, and the proteins that made up the larva are broken down to amino acids, which are recycled to synthesize adult proteins and reassembled into the adult form. This rebuilding process is called metamorphosis, from two Greek words meaning “change of form.” The entire process requires several days, and by the end of the pupal stage, the adult insect is curled tightly inside the pupal case.
• 3. Adult. When the adult form hatches from the pupal case, it is at its full size. Insects do not grow after they emerge from their pupae—for this reason, some adults do not even eat. Rather than eating and growing, the job of an adult insect is reproduction.

The developmental process called complete metamorphosis, which includes these three stages, occurs only in insects and allows the larva and adult to act as if they were animals from different species, each optimized to perform very specialized tasks. Caterpillars eat leaves, for example, and spend their entire larval period on a single plant. In contrast, butterflies feed on nectar that they collect by flying from flower to flower, saving up enough resources to lay or fertilize eggs. As a larva, the animal feeds and grows. As an adult, it reproduces. The genes that control the larval body form are different from the genes that determine the adult form, so natural selection can act on the larval and adult stages independently.

About 83% of insect species go through complete metamorphosis, and this separation of the life stages has been an important factor in helping insects diversify into nearly 20 times as many named species as vertebrates. Among those insect species that do not undergo the dramatic changes of complete metamorphosis, typically the eggs hatch as nymphs (the juvenile form), which resemble a smaller version of the adult. These juveniles do not have wings or reproductive organs, but they live in the same habitats as adults and eat the same foods. They then undergo several molts as they grow, and stop molting when they reach adult size. This pattern of growth and development is called incomplete metamorphosis, and it occurs in grasshoppers and cockroaches, among many other species.