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This chapter is from the book

Reinventing oneself

The dramatic remodeling of an insect's body that transforms a crawling, food-minded larva into a walking, flying, reproducing adult is not the result of changes in the insect's DNA. Similar to us, each insect has a unique DNA profile and the cells in its various organs share the same genetic blueprint.

Specialized cells differ in which genes are switched on and, therefore, what proteins are produced. An egg cell contains a concentration gradient of signaling molecules that assigns the initial pattern of cell identities in an embryo. Throughout development, a plethora of chemical signals prompts genes to switch on and off.

Two hormones, juvenile hormone and ecdysone, are the master signals for insect metamorphosis. A larva morphs into an adult in response to declining levels of juvenile hormone and simultaneous spikes in ecdysone. The hormonal changes cause some larval cells to die and also lead to the production of new cells and the repurposing of other cells.

Among the cells that die during metamorphosis are the larval muscles. Adult muscles use remnants of the larval muscle fibers, but mostly they develop from cells in imaginal disks—hollow sacs of unspecialized cells that form in the embryo and remain throughout larval life. Flight muscles can constitute more than 10 percent of an adult insect's mass and contract about 60 times faster than the larval muscles; they also differ in protein content.

Most larval sensory nerves die and are replaced, but motor neurons—the nerves that move the muscles—are reused. The larval motor neurons retract their branches as the larval muscles die, and serve as templates for the developing adult muscles before growing new branches to them.

Economizing by recycling instead of disposing of larval cells also occurs in the central nervous system. Although larval and adult behaviors are divergent, some behaviors survive metamorphosis. One study showed that a moth can remember what it learned as a caterpillar. Caterpillars that were exposed to an odor paired with a small shock learned to avoid the odor and continued to avoid it as adults, whereas untrained caterpillars did not.

Only holometabolous insects—including butterflies, beetles, bees, and flies—go through complete metamorphosis. Hemimetabolous insects—including cockroaches, grasshoppers, crickets, and dragonflies—do not have a larval stage. They hatch as mini adults that lack wings and reproductive organs. The benefit of being holometabolous is that larvae and adult insects can exploit different habitats and food sources.

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