The life span of an insect varies greatly. Some insects, like grasshoppers, complete their cycle in several years. Others, like fruit flies, live for only a few days. Between the time an insect lays eggs and the time for the insect that hatches from this egg to mate itself, a series of spectacular transformations takes place.
Insects have an external skeleton, or exoskeleton, that looks like a shell or armour. This protective envelope has many advantages, but also certain disadvantages, particularly in terms of growth. Because it is so rigid, the exoskeleton is not very extendable, and quickly becomes too small. The insect has to replace it from time to time as it grows. This phenomenon of changing skin is known as moulting.
When an insect is ready to moult, it finds a resting place. A new exoskeleton has already begun forming under the old one. When the time is right, the old tegument bursts at the thorax and in a few moments, the insect wiggles out of the shell that has grown too small. It emerges in its new cuticle, which hardens quickly once it comes in contact with the air. The insect returns to its activities a few hours later, leaving behind its old exoskeleton, or carcass.
Insects generally moult between four and eight times, but dragonflies may moult 10 or 12 times and mayflies sometimes moult as many as 30 times. With only some exceptions, like silverfish, insects stop growing once they reach adulthood and no longer need to moult.
A large majority of insects change shape during their growth. This transformation is called metamorphosis, which may be classified as incomplete or complete depending on the importance of the changes that take place during development.
Insects that do not go through metamorphosis
Only a few groups of insects do not go through metamorphosis. They are known as ametabolous insects. The larvae of these apterous (wingless) insects look like adults, except for their size, glands and genital organs. They also moult several times as they grow. Silverfish are among the insects that are ametabolous.