There are two main ways in which immature insects develop into adults:
- Gradual metamorphosis - this occurs in hemimetabolous insects such as grasshoppers, cockroaches, mantids, stick insects, earwigs, thrips, termites and true bugs.
- Abrupt metamorphosis – this occurs in holometabolous insects such as flies, bees, ants, wasps, butterflies, moths, lacewings, fleas, caddisflies and beetles.
Metamorphosis: Working wings are borne only by adult insects. After hatching from the egg as a wingless, immature stage, insects undergo a change of form (metamorphosis) involving a sequence of feeding and moulting before the final winged adult form appears.
With gradual metamorphosis the wings begin to appear as small buds on the exterior of the immature insect as it grows larger with successive moults. Generally these immature stages, called nymphs, resemble the adults in form and habits and the wing buds are always obvious on the thorax of older nymphs.
An example of gradual metamorphosis is shown for a grasshopper (images not to scale):
In insects with abrupt metamorphosis the immature stages, which are known as larvae (singular larva), have no trace of external wing buds and never resemble the final adult insect. They are usually known by terms such as grub, caterpillar or maggot. When the larva is fully grown it moults into a resting, non-feeding stage called the pupa. Inside the pupa the larval tissues are completely rearranged into the form of the adult. At the end of pupal life, a perfect winged insect emerges, totally unlike the larva which preceded the pupa. Thus metamorphosis from immature to adult stage in these Orders is radical and abrupt.
An example of abrupt metamorphosis is illustrated for a butterfly (images not to scale):
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