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Life cycle
Egg
Most insects begin their lives as fertilized eggs. The chorion, or eggshell, is commonly pierced by respiratory openings that lead to an air-filled meshwork inside the shell. For some insects (e.g., cockroaches and mantids) a batch of eggs is cemented together to form an egg packet or ootheca. Insects may pass unfavourable seasons in the egg stage. Eggs of the springtail Sminthurus (Collembola) and of some grasshoppers (Orthoptera) pass summer droughts in a dry shrivelled state and resume development when moistened. Most eggs, however, retain their water although they may pass the winter in a state of arrested development, or diapause, usually at some early stage in embryonic development. However, dried eggs of Aedes mosquitoes enter a state of dormancy after development is complete and quickly hatch when placed in water.
The hatching of young larvae is achieved in several ways. Some, such as caterpillars, bite their way out of the egg. Many, such as the flea, have hatching spines with which they cut a slit in the shell. Some insect eggs have a preformed “escape cap” that the larva pops from the shell by increasing the pressure inside the egg. Depending on the species, this may be accomplished either by swallowing air and then constricting muscles in the body to exert pressure on the cap or by having an expandable region on the head (many Diptera have a ptilinum) that can be extended by hydraulic (blood) pressure. After hatching, the larva continues to distend itself in this way, although the ptilinum collapses back into the body, until the cuticle hardens.
Once formed, the insect cuticle cannot grow. Growth can occur only by a series of molts (ecdyses) during which new and larger cuticles form and old cuticles are shed. Molting makes possible large changes in body form.
Types of metamorphosis
In the most primitive wingless insects (apterygotes) such as the silverfish Lepisma saccharina, there is almost no change in form throughout growth to the adult. These are known as ametabolous insects. Among insects such as grasshoppers (Orthoptera), true bugs (Heteroptera), and homopterans (e.g., aphids, scale insects), the general form is constant until the final molt, when the larva undergoes substantial changes in body form to become a winged adult with fully developed genitalia. These insects, called hemimetabolous, are said to undergo incomplete metamorphosis. The higher orders of insects, including Lepidoptera (butterflies and moths), Coleoptera (beetles), Hymenoptera (ants, wasps, and bees), Diptera (true flies), and several others, are called holometabolous because larvae are totally unlike adults. These larvae undergo a series of molts with little change in form before they enter into complete metamorphosis, which includes molting first into pupae and then into fully winged adults.
Types of larvae
Larvae, which vary considerably in shape, are classified in five forms: eruciform (caterpillar-like), scarabaeiform (grublike), campodeiform (elongated, flattened, and active), elateriform (wireworm-like), and vermiform (maggot-like). The three types of pupae are: obtect, with appendages more or less glued to the body; exarate, with the appendages free and not glued to the body; and coarctate, which is essentially exarate but remaining covered by the cast skins (exuviae) of the next to the last larval instar (name given to the form of an insect between molts).