Butterflies and moths belong to a group of insects which undergo complete metamorphosis. This means that there is a drastic change in form during the course of their life cycle which passes through the stages of egg, larva, pupa and adult. Butterflies usually lay their eggs on the leaves of the plant on which their caterpillars feed. The Peacock butterfly, for example, lays its eggs on nettles while the Large White chooses cabbages, brussel sprouts or other members of the cabbage family and the Painted Lady butterfly chooses thistles.
When the caterpillars hatch from the egg they feed continuously on the leaf of their food plant by biting off pieces with their jaws. In their large numbers they may cause extensive damage to the plant.
In order to grow, the caterpillars have to repeatedly shed the outer layer of their ‘skin’ (cuticle) and expand their bodies before the new cuticle hardens. After several moults, the caterpillar attains full size and migrates from the food plant to a sheltered position, e.g. a wall or branch of a shrub. Here the caterpillar sheds its final larval skin to become a pupa (or chrysalis). In some species, the caterpillar also spins a cocoon of silk round itself before pupating. The pupal stage appears to be a ‘resting’ phase but internally the larval organs are being dissolved and replaced with the structures of an adult butterfly, though still confined within the pupal case. Many butterflies spend the winter months in the pupal stage.
The butterfly eventually splits its pupal cuticle and emerges. Its wings expand and harden before it can fly away. The form of the butterfly is in complete contrast to that of the larva which gave rise to it. There are two pairs of wings, three pairs of legs, antennae and compound eyes. The feeding method is also totally different from that of the caterpillar. The mouthparts take the form of a tube (proboscis) which is rolled up beneath the head when the butterfly is not feeding. During feeding, the butterfly unrolls the proboscis and uses it to probe into flowers in order to suck up the nectar that the flowers secrete.