When we watch a caterpillar spinning a cocoon or a butterfly emerging from its chrysalis, we are witnessing part of the remarkable life cycle of an insect. Insects live their lives in stages, changing form as they develop from egg to adult. In some, young and adult are so different, it’s hard to believe they could be the same species. A caterpillar becomes a butterfly, a garden grub becomes a shiny beetle, a wingless creature crawling on the bottom of a pond becomes a dragonfly skimming above it. Is there a connection between insect life cycles and the incredible success of this group of animals?
Insects, members of the class Insecta, have a hard outer skin or exoskeleton to which their muscles and tendons are attached and which protects the soft organs inside their bodies. They grow by molting, splitting open and shedding the exoskeleton when they get too big for it, then growing a new, larger one. Most insects molt between four and eight times during their lives, though some have more molts. Once they become adults, they do not molt again.
For insects, growing up is not just a matter of getting bigger. As they mature, insects undergo metamorphosis, a change in form controlled by hormones in the body. There are two main kinds of metamorphosis in insects. Some insects have simple metamorphosis with three stages: egg, nymph, and adult. A nymph lacks wings, but it has legs, eyes, and antennae. Most insects have complete metamorphosis with four stages: egg, larva, pupa, and adult. The larva is very different from the adult in both appearance and habits. The pupa is a resting stage during which the insect transforms into the adult.
In simple metamorphosis, the nymph stage often looks like a miniature, wingless version of the adult. It undergoes several molts, gradually developing the adult features as it grows larger. Wings develop on the outside of the body, and in some insects the beginnings of wings can be seen as flat, bud-like growths on the upper back. The wings and reproductive organs become fully formed and functional only after the final molt, when the insect becomes an adult.
Grasshoppers and crickets go through simple metamorphosis, as do earwigs, aphids, and squash bugs, to name a few. A grasshopper lays her eggs in the soil, and tiny nymphs hatch out, able to hop away from predators within minutes. They feed on grasses and other plants and grow quickly, molting four or five times, to reach the adult stage with wings. Several generations hatch during the summer, so adults and nymphs of different sizes are often found at the same time. Grasshoppers, like all insects with simple metamorphosis, do their growing in the nymph stage, although the adults continue to feed, sometimes with voracious appetites – like locusts, so to speak! Because they share the same habitat and eat the same food, nymphs and adults compete with each other for the same resources.
Dragonflies, damselflies, stoneflies, and mayflies also have simple metamorphosis, but with these insects the nymphs lead very different lives from the adults. They live at the bottom of streams and ponds, breathing with gills and feeding on freshwater animals or plants. Mayflies live for a year as aquatic nymphs and only a few hours as winged adults. During their brief flight they find mates, lay their eggs, and then die. Dragonflies and damselflies may live for up to four years as nymphs underwater and a single summer as winged adults, hunting for other flying insects on the wing. The lives of these aerial predators are strikingly different from those of their aquatic nymphs.
Most insects undergo complete metamorphosis, including, among others, the beetles, butterflies, moths, bees, ants, wasps, flies, mosquitos, lacewings, and caddisflies. Insects with complete metamorphosis do most of their growing as larvae, the stage that is specialized for eating and which usually bears little resemblance to the adult. Caterpillars are the larvae of butterflies or moths, maggots are fly larvae, grubs and wireworms are beetle larvae. In these insects, the adult usually doesn’t eat the same food as the larva and may not eat at all. June bugs, actually beetles, feed on grass roots as larvae, tree leaves as adults. Monarch caterpillars eat milkweed leaves, while the adult butterflies drink nectar from flowers with a long tube-like proboscis. Luna moths and other giant silkworm moths have no mouthparts and don’t feed at all as adults. In general, larvae are specialized for feeding and growing, adults for reproduction, so there is often little competition for resources between stages and each is suited to its part of the life cycle.
Perhaps most remarkable of all the life stages in complete metamorphosis is the pupa stage, between the larva and the adult. The pupa is generally quiescent, not eating or moving about, though some will twitch or wiggle when touched, and mosquito pupae, called “tumblers,” somersault into deeper water if disturbed. Inside the pupa, dramatic changes take place. Larval genes are suppressed and adult genes are activated. Groups of cells that were present but inactive in the larva now grow into different body parts, like legs, eyes, and wings. The larva’s tissues are broken down, rearranged, and rebuilt into other structures. When the adult insect emerges from the pupa, it has compound eyes, antennae, jointed legs, reproductive organs, and usually wings.
In cold climates, insects enter a resting stage called diapause to survive the winter. Some insects, like woolly bear caterpillars, spend the winter as larvae, under tree bark or in the leaf litter. Larvae may build up antifreeze chemicals in their bodies to protect them from frost. A few insects overwinter as adults, like the mourning cloak butterflies that appear when the first spring flowers come into blossom. Many insects overwinter as eggs or pupae, for these stages require no food, don’t need to move around, and are well protected by their exoskeletons.
The monarch butterfly’s way of surviving the winter is unique in the insect world. In early spring, females in the southern part of the United States lay eggs on young milkweed plants. The caterpillars feed on the milkweed and eventually become butterflies. Each new generation of butterflies moves generally northward to lay their eggs, along with the warm weather. By the third or fourth generation the monarchs have spread as far as the northern states and southern Canada. The final generation of adults, those that emerge at the end of the summer, differ from previous generations in that their reproductive organs do not fully develop at this time. These butterflies migrate, some traveling two thousand miles or more, along a route they can know only by instinct, to spend the winter in a few small forests in Mexico or California. Clustered on tree branches with millions of other monarchs, they enter a dormant state and rest through the winter. The following spring they complete their development, find partners and mate, and then fly northwards to lay their eggs on young milkweed shoots. Though some other insects undertake seasonal migrations, none compares to the epic journey of the monarch butterfly.
Adult insects have one main purpose: to find mates and lay eggs. Because of their small size, insects rely on a variety of signals – scent, color, movement, light, sound, or a combination of these – to meet the opposite sex. Many female moths release a scent or pheromone into the air that attracts males from great distances. A male giant silk moth has been known to travel as far as thirty miles to a female. The twinkling of fireflies on a summer night may seem random, but a female firefly, perched on a stem of grass, is on the alert for the particular pattern and color of flashes, at the right height in the sky, that identify a male of her species. She sends out a reply flash and waits for him to alight. Mayfly males form swarms and perform a synchronized dance to attract females. Among insects that chirp, or stridulate, like crickets, grasshoppers, and katydids, the males sing to attract females. Crickets scrape the hard edge of one wing across a row of pegs on the other wing, while grasshoppers rub a wing against a row of pegs on their back legs, each species producing its own special song.
Insects far outnumber all other kinds of animals on earth, and they’ve been around for hundreds of millions of years. Their small size, large number of offspring, and ability to fly have all played a part in their amazing success. Perhaps, too, living life in stages where each stage is suited to a particular niche gives insects the competitive edge to outclass every other class of animals.
Suggested Reading
Borror, D.J. and R. E. White. A Field Guide to the Insects. Peterson Field Guide. Boston: Houghton Mifflin, 1970.
Cottam, Clarence and Herbert S. Zim. Insects: A Golden Guide. New York: St. Martin’s Press, 2001.
Stokes, Donald. A Guide to Observing Insect Lives. Boston: Little, Brown, 1983.
Triplehorn, Charles A. and Norman F. Johnson. Borror and DeLong’s Introduction to the Study of Insects. 7th ed. Belmont, CA: Thomson Brooks/Cole, 2005.
Life cycle of monarch butterflies: http://www.monarchwatch.org/biology/cycle1.htm
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