Wild Side: Metamorphosis

Evolution helps insects increase their efficiency.


Most everyone knows the basics of insect metamorphosis: It’s the way a caterpillar turns into a butterfly. We don’t often think, though, of how thoroughly the fact of metamorphosis governs the lives of insects. But the complete change of form — or, for other groups of insects, the lack of a complete change in form — dictates the outline of an insect’s natural history.

The most dramatic type of metamorphosis is known as holometaboly, or complete metamorphosis. In effect, insects exhibiting complete metamorphosis act like two completely different species at different times of their lives. (Some early attempts to explain the origins of metamorphosis actually involved the notion of unrelated species somehow mating and merging into a single species.)

Think about the larva, or caterpillar, of a moth or butterfly. Sluglike, possessed of short legs that can do little more than grip stems and pull the caterpillar along, the larval stage is structurally very simple (though it contains tracts of cells that will eventually form the more elaborate structures of the adult). Basically, it’s an eating machine: You can think of caterpillars as digestive tracts with just enough mobility to move to fresh food once they eat everything within reach.

Ordinarily a caterpillar hatches right on a suitable species of plant for it to eat; adult females sniff out plants from a short list of species their instincts tell them are right for laying eggs on. Most of the food that a moth or butterfly consumes during its lifetime is eaten during the larval stage, and energy stored from this larval eating binge remains critical for the successful functioning of the adult.

If the caterpillar is optimized for eating, the design of adult moths and butterflies places much less emphasis on eating and much more on mobility. In most species, adult males and females alike are capable of flight, allowing dispersal to new areas; in some cases, like the monarch and American lady butterflies, adults may migrate hundreds or even thousands of miles.

Adults of many species of moths and butterflies do eat, of course, sipping nectar from flowers through tubelike mouthparts. But generally speaking, these nectar meals are optional, not essential, prolonging life and increasing fecundity, but not being necessary for successful reproduction. In many moths, adults eat little or not at all, sometimes even lacking functional mouthparts. The adult form in this group, in other words, is designed primarily for mobility, so adults can find each other to mate, and so that fertilized females can search for appropriate plants to lay their eggs on.

By partitioning two essential functions — eating for energy uptake, and mobility for dispersal — into separate life stages, evolution has allowed these insects to increase their efficiency. Larva or adult, each main life state is optimized for a particular part of the work of survival and reproduction. And larvae and adults, with completely different eating habits, don’t compete against each other for resources.

Many other groups of insects have retained an older, simpler form of development, known as hemimetaboly, or partial metamorphosis. In this process, a young insect, or nymph, loosely resembles its adult form as soon as it hatches. Its development to adulthood will involve a series of molts as it outgrows its exoskeleton, and with each molt, it will resemble its adult form more closely. In the case of grasshoppers, for example, a sequence of five molts takes a hatchling through a total of six stages, known as instars, with the sixth instar being the adult form.

Generally, it’s hard to identify grasshopper nymphs with any precision, but even very early instars generally show the traits of a family, subfamily, or genus. In contrast to the extreme division of labor one finds among the life stages of complete metamorphosis, grasshoppers and other hemimetabolous insects often show continuity in behavior, food, and habitat preference as they mature. It seems likely that very young grasshoppers depend more on tender, young plant growth for food, if only because their mouthparts are not as powerful as those of an adult. But I have often watched the maturation of a cohort of young grasshoppers over a period of weeks, with steady development toward their adult form, and it seems clear that the youngest nymphs must eat the same species of plants and live in the same area as the adults they eventually become.

In contrast with caterpillars, grasshopper nymphs are quite agile, almost from the moment they hatch. They certainly can’t leap as far as adults can, but they have the same instinctual awareness of threats, and the same inclination to jump to safety when threatened. If they lack the specialized efficiency of a caterpillar, they benefit from having some of the mobility of their adult form.

I’m not nearly enough of a biologist to fully understand the biochemistry behind metamorphosis, or the elaborate evolutionary history that dictated what pattern of development a group of insects follows. But I watch with amazement as the process happens. Science can explain it, but it looks like a miracle.