The chrysalis of a Bright Shield Skipper Butterfly
Image: Ross Field
Source: Museum Victoria
Question: What is the difference between a cocoon and a chrysalis?
Answer: Like all insects, caterpillars must shed their skins as they grow. When they become too big for the skin they’re in, caterpillars develop a new skin under their old skin. The old skin then splits and they crawl out of it. When a caterpillar is large enough to enter the next stage of its development – the pupal stage – it stops eating and finds a safe place to pupate.
The caterpillar then prepares to undergo a very special moult. This time when the old skin splits, the skin that emerges from under the old skin is a chrysalis. Both butterflies and moths develop a chrysalis in their pupal stage, but the chrysalises of moths are usually contained inside a woven structure made of silk – a cocoon.
When it is first secreted, the chrysalis is soft and sticky but it soon hardens to form a protective outer later around the developing pupa. During the next few weeks or months, the animal inside the chrysalis gradually turns into a butterfly or moth. When it is ready, the pupal skin splits and a wet, crumpled butterfly (or moth) crawls out. Moths that build cocoons either cut their way out or secrete a special fluid that softens the structure. Once it has emerged, the butterfly or moth then hangs from its resting place until its wings stretch out and dry and its new body hardens.
Chrysalises and cocoons are highly varied in their appearance, but most blend in with their surroundings. As chrysalises and cocoons cannot move around, effective camouflage is their greatest defence. Chrysalises usually resemble leaves (living or dead), but some are brightly coloured. These species are usually poisonous and the bright colours advertise their poison to deter predators. Some species of caterpillars incorporate sticks, dung and bits of leaves into their silken cocoons to disguise them. Others deter predators by weaving the irritating hairs from their own bodies into their cocoons.