When the caterpillar is fully grown, it prepares for pupation. It first looks for a suitable place in the vegetation or on the ground. Pupation can take from several hours to days. A caterpillar of the last larval instar does not eat any more, slowly becoming stiff. Then its skin splits open, and the chrysalis emerges, at that moment still very soft and mobile.

The chrysalis may be suspended in the vegetation, either by its tip, such as that of the Peacock (004.MOV, pupation of Inachis io), or supported by a silken girdle, as that of the Orange-tip. However, sometimes, it is not attached to a plant, but just found deep in the vegetation, under stones, or in the topmost layer of soil, such as the reddish-brown chrysalis of the Grayling in a small hollow under the leaf litter.

The pupa or chrysalis cannot move away or, of course, feed. To lessen the chance of predation, it is mostly very well camouflaged, sometimes resembling a withered leaf, like the Comma, or a bird dropping, such as the Black Hairstreak.
In this apparent state of rest, many big changes are taking place. The organs of the caterpillar are being broken down and, under hormonal control, the butterfly is being built up again. In butterflies that pupate in the summer, this process takes from ten days to three weeks, but when a chrysalis is also in a state of hibernation, metamorphosis lasts more than ten months. (Map 03, Araschnia levana)