Chrysalis or Cocoon?

 

Most moth caterpillars spin a cocoon made of silk within which they metamorphose into the pupal stage. Most butterfly caterpillars, on the other hand, form an exposed pupa, also termed a chrysalis.

 

Chrysalis

When the caterpillar is fully grown, it makes a button of silk which it uses to fasten its body to a leaf or a twig. Then the caterpillar's skin comes off for the final time. Under this old skin is a hard skin called a chrysalis. Like other types of pupae, the chrysalis stage in most butterflies is one in which there is little movement. However, some butterfly pupae are capable of moving the abdominal segments to produce sounds or to scare away potential predators. Within the chrysalis, growth and differentiation occur. The adult butterfly emerges (ecloses) from this and expands its wings by pumping haemolymph into the wing veins.  Although this sudden and rapid change from pupa to imago is often called metamorphosis, metamorphosis is really the whole series of changes that an insect undergoes from egg to adult. When emerging, the butterfly uses a liquid, sometimes called cocoonase, which softens the shell of the chrysalis. Additionally, it uses two sharp claws located on the thick joints at the base of the forewings to help make its way out. Having emerged from the chrysalis, the butterfly will usually sit on the empty shell in order to expand and harden its wings. However, if the chrysalis was near the ground (such as if it fell off from its silk pad), the butterfly would find another vertical surface to rest upon and harden its wings (such as a wall or fence).

 

Coccon

A cocoon is a casing spun of silk by many moth caterpillars, and numerous other holometabolous insect larvae as a protective covering for the pupa.

Cocoons may be tough or soft, opaque or translucent, solid or meshlike, of various colors, or composed of multiple layers, depending on the type of insect larva producing it. Many moth caterpillars shed the larval hairs (setae) and incorporate them into the cocoon; if these are urticating hairs then the cocoon is also irritating to the touch. Some larvae attach small twigs, fecal pellets or pieces of vegetation to the outside of their cocoon in an attempt to disguise it from predators. Others spin their cocoon in a concealed location—on the underside of aleaf, in a crevice, down near the base of a tree trunk, suspended from a twig or concealed in the leaf litter. The silk in the cocoon of the silk moth can be unravelled to get silk fibre which makes this moth the most economically important of all Lepidopterans. The silk moth is the only completely domesticated Lepidopteran and does not exist in the wild. Insects that pupate in a cocoon must escape from it, and they do this either by the pupa cutting its way out, or by secreting fluids, sometimes called cocoonase, that soften the cocoon. Some cocoons are constructed with built-in lines of weakness along which they will tear easily from inside, or with exit holes that only allow a one-way passage out; such features facilitate the escape of the adult insect after it emerges from the pupal skin.