Cocoon stage is the most fascinating stage in insect development. Cocoons vary in shapes and design in different groups.
Cocoons may be soft or tough and hard. They may be semitransparent or opaque. Closely woven solid, or loosely structured mesh. They may be single layered or multiple layered containers. Their colors vary from dirty dull to sparkling shine or glittering gold.
A concealed location is the most often selected spot for cocoon construction. Underside of a leaf, a crevice near tree trunk base, a hanging twig, leaf litters serve as safest spots. These are most preferred by caterpillars before they begin construction of cocoons.
There are three broad shapes of cocoons based on their appearance.
Objects: This is most characteristic of butterflies. It is a tight fitting vase shaped case. The pupa inside, holds its legs and developing wings close to body.
Exarate: This is found in bees and wasps. The pupa is loosely enclosed. Legs and wings appear free from body.
Co-arctate: This is characteristic of flies. The pupa uses skin, shed in the last molt, as its protective cover.
Apart from silk, the basic material of cocoon, several other items are added to cocoon wall for strength and protection.
Some moth caterpillars shed their larval hairs and fix them on the cocoon. The hairs are irritating to touch. Predators avoid such cocoons.
Small twigs, fecal pellets, pieces of vegetation such as dry leaves are attached to outer surface. Such cocoons are well disguised and seldom noticed by predators.
The inside of cocoon is also taken special care of during construction. This is necessary to enable the insect escape without difficulty. Certain areas in the inner wall are made soft enough to be cut or chewed by the emerging insect. Some cocoons have exit holes that allow only one way out.
The architectural design of cocoon involves several factors that enables pupa to undergo rest, development and most importantly escape!