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).

Q

Question 11.4

Mammals get bigger and bigger the more they eat. Why don’t insects?

Figure 11.18: Metamorphosis. Some insects have a life history divided into unique stages, each with a distinct purpose.

468

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.

TAKE-HOME MESSAGE 11.11

The ability to fly and the developmental process of metamorphosis as a means of overcoming the constraints of having a rugged exoskeleton have contributed to the enormous ecological diversity of insects. The life cycle of most insects includes a larval stage that is devoted to feeding and growth, a pupal stage during which metamorphosis occurs, and an adult stage in which the insect reproduces.

How has metamorphosis led to the wide ecological diversity of insects?