You’ve spread out your picnic blanket and are about to feast on potato salad and fried chicken. Then along come the ants. And bees. And mosquitoes.

	James Truman and Lynn Riddiford

There’s a reason why these pests seem to be everywhere. They are part of four insect groups—beetles, bees/ants, moths/butterflies, and flies/mosquitoes—that make up more than half of the world’s known animal species. The secret to their success? Metamorphosis.

Metamorphosis is a process in which larval, pupal, and adult stages differ greatly, allowing each to occupy a different habitat and consume different food sources. Now two University of Washington zoology professors, the husband-and-wife team of James Truman and Lynn Riddiford, are proposing a novel hypothesis for how metamorphosis evolved.

The researchers believe that a change in hormonal function during embryonic development led to the evolution of a unique larval stage, an innovation that allowed a virtual population explosion among these species in the last 250 million years. “Metamorphosis really opened up niches that weren’t available to insects before that,” says Riddiford.

The earliest insects, which strongly resembled today’s silverfish, lacked metamorphosis. Their juveniles looked very much like adults, minus functioning genitalia. After the evolution of flight, more advanced species—such as cockroaches and grasshoppers—developed incomplete metamorphosis. Their immature stages still resembled the adults, but in addition to lacking genitalia they bore wing buds that transformed into functional wings during the molt to the adult stage.

In contrast, the higher insects—species with complete metamorphosis—spend their juvenile life as larvae that bear no resemblance to the adults. Truman and Riddiford explain that what allows them to change from, say, a caterpillar into a butterfly, is the way a group of insect hormones, juvenile hormones (JH), and ecdysteroids interact during embryonic, larval, and pupal stages.

Juvenile hormones, not present in other insects, suppress the development of adult structures. These hormones remain as the larva grows, then disappear to allow growth of imaginal discs, which will give rise to specific adult structures. A complex interplay between JH and ecdysteroids then allows the larva to progress to a pupa, and finally ecdysteroids alone drive the transformation to adult.

Juvenile hormones play such an important role in the embryonic and larval development of metamorphosing insects that they have been used as the basis for insecticides. For instance, JH mimics are used to treat ponds where mosquitoes breed, thereby blocking their metamorphosis. Such treatment also prevents eggs from hatching.

The four major insect groups with complete metamorphosis all are thought to descend from a common ancestor, so it appears the development of metamorphosis in the insect world has occurred only once. “There are indications that another group, called thrips, has evolved toward complete metamorphosis but so far has fallen short,” says Truman.

In insects with complete metamorphosis, the lack of competition between juveniles and adults for food is a major factor in their success and diversification. Adults can feed on one source, such as nectar or blood, and only lay eggs when there is appropriate food for their young, such as dung, carcasses, fruit, and other relatively temporary sources.

Truman and Riddiford believe that metamorphosis will provide a valuable model for researchers to understand the molecular basis for how shifts in the timing of protein production can lead to the creation of different body forms. That, in turn, could shed greater light on how life patterns have evolved.

“Any innovation that helps you generate species that account for more than half of all living animals is not a trivial innovation,” says Truman.

[Winter 2000 - Table of Contents]