Insect Life Cycles

Because insects have a tough, non-living outer covering or exoskeleton, they cannot grow steadily, but have to grow in stages by periodically shedding the exoskeleton. This process is called moulting or ecdysis. The stages between moults are called instars.

Although some insects give birth to active young (notably the summer generations of aphids), the vast majority of insects lay eggs. When the young insect is ready to leave the egg it either chews its way out or bursts the egg shell by muscular action, sometimes assisted by sharp spines on its body covering.

The apterygote (or wingless) insects, and some of the species which have lost their wings secondarily during the course of evolution, hatch in a form very like that of the adult except for the lack of reproductive organs. Apart from an increase in size there is little visible change in appearance as the insect grows (i.e., little or no metamorphosis).

The number of young stages varies considerably depending on species.

In contrast, the development of pterygote (or winged) insects is more complex. The insect that hatches from the egg is often very different from the adult stage; most obvious is the lack of wings in the young insect, as well as its smaller size. Also, there may be differences in general body form, mouthparts, legs and other features. It follows that most young pterygote insects must undergo considerable changes before reaching the adult state. These changes are collectively called metamorphosis.

Winged insects fall into two groups according to how their wings develop during the young stages. In the first group, including such insects as cockroaches, grasshoppers, dragonflies and bugs, the wings develop gradually on the outside of the body and get larger at each moult until they are fully formed. The young stages of these insects are called nymphs and they usually resemble the adults in general appearance, often inhabiting the same places and eating the same kinds of food. This group of insects is classified as the Exopterygota, in reference to the external development of the wings. Since there are no dramatic changes in body-form during development, the insects are said to undergo a partial or simple metamorphosis.

The number of nymphal stages (or instars) varies depending on species. Note the short developing wings in the nymph, usually called 'wing-pads' at this immature stage (saddle-shaped area just behind the thorax), in contrast to the long fully-developed wings of the adult insect which completely cover the abdomen (the species illustrated is the desert locust, Schistocerca gregaria).

The second group of winged insects includes beetles, butterflies, moths, flies, bees, wasps and so on, where the young stages are very unlike the adults. These young stages are called larvae (or grubs and caterpillars). They often occupy completely different habitat niches and exist on quite different food sources from those of the adults. Instead of undergoing a series of small changes to reach the adult form, the larvae undergo one very dramatic change that requires a special resting and non-feeding stage during which the transformation can occur. This resting stage is called the pupa (or chrysalis). In this group, the wing buds develop internally and are not visible until the pupal stage, so that in classification the group is known as the Endopterygota. Due to the large change from larval to adult form, the insects are said to undergo complete or complex metamorphosis.

The number of larval stages (or instars) varies depending on species. The larva or caterpillar of this particular moth (the garden tiger moth, Arctia caja) is covered by long, dense hairs. The pupa or chrysalis is shown in side-view (head-end at the top) with the outline of one of the developing forewings visible on the surface.