How Insects Grow
Most insects develop from an egg and, upon hatching, have a form different from that of the adult. The series of form changes as an insect develops from egg to adult is called metamorphosis. The young insect is covered with a more or less firm skin called the exoskeleton. As the insect feeds, it grows inside this skin, but it cannot increase in volume because it is restricted by its exoskeleton. A new elastic exoskeleton then forms under the old rigid exoskeleton. The old exoskeleton splits along the back and the insect crawls out of its old skin and expands to its new size. After exposure to air for a short time, the new exoskeleton becomes hardened and the insect is ready to resume activity and grow some more. The process of shedding the old skin is called molting. Molting occurs several times over varying periods of time until the final stage is reached.
With each molt insects change their form to varying degrees, depending on the kind of metamorphosis that insects may have. Most vegetable garden insect pests have either gradual (Figure 1) or complete metamorphosis (Figure 2). Examples of gradual or incomplete metemorphosis, in which the very young resemble the adults, include plantbugs, grasshoppers, stink bugs, squash bugs, aphids and leafhoppers. Examples of pests with complete metamorphosis are Mexican bean beetles, cabbage loopers, hornworms, flies, June beetles, cutworms and armyworms.
Gradual metamorphosis (Figure 1). Generally these young insects resemble the adults. In proportion to the rest of the body, the legs and head become relatively smaller in each instar. This is because the head and legs do not grow as fast as the rest of the body. In insects which are winged, there is also a gradual development of the wings with each molt. There are no more molts after the fully developed, winged, adult emerges. Not all of these insects develop wings. The young are called nymphs. Nymphs and adults inhabit the same places and eat the same kind of food.
nymph Figure 1
Complete metamorphosis (Figure 2). All four stages of development - egg, larva, pupa and adult - are present. All increases in size occur during the larval stage. Some people erroneously think that small flies will grow to be big flies. At the end of the larval stage, the insect transforms into a pupa which does not feed or move about. It is sometimes called a resting stage, but inside the pupal skin drastic changes are taking place. More alteration of form is going on during the pupal stage than during any other period of the insect's development. Out of the pupa emerges the fully formed adult, complete with wings. No further molts occur. The larvae and adults of these insects may live in different habitats, eat different food, have different kinds of mouthparts and have many other differences. The larval stage of some orders of insects are called maggots, grubs or caterpillars.
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