When they first hatch, the caterpillars are not more than a few millimetres long and nearly transparent. Only later do they become coloured, often with spots, spines or hairs, and are then characteristic for the species.
The caterpillars differ in shape. (Map 02, Araschnia levana). They are mostly worm-like. However, those of the hairstreaks, coppers and browns are flat and relatively short, looking more like woodlice or small slugs.

Caterpillars are mostly solitary, but some live gregariously in silken shelters for at least part of their life. They feed, growing and moulting until fully grown and finally pupate, turning into a chrysalis or pupa. They hibernate communally, but later separate, pupating alone. (003.MOV, Inachis io, caterpillars eating)
Caterpillars that grow quickly, such as the Small White, may sometimes pupate only a month after hatching. However, when a species hibernates as a caterpillar, it can take about ten months from hatching to pupation. The slowest-growing caterpillars are those of some ringlets (Erebia spp.), that are nearly two years old when they pupate.