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Obtect, Exarate or Coarctate
Insects that undergo the life stages of embryo, larva, pupa and imago (adult) are called holometabolous insects. At the pupa stage, these insects may appear in one of three forms: coarctate, obtect or exarate. Insects with coarctate pupae, such as soldier flies, develop inside of the larval skin. Obtect pupae have legs and appendages completely enclosed (butterflies and moths). Exarate pupae have legs and appendages extended (mosquitoes and many beetles have exarate pupae).
Chrysalides
A chrysalis (plural chrysalides) is the pupa of a butterfly. It is also called a nympha. When forming a chrysalis, a caterpillar, the butterfly larva, will produce special silk that it uses to attach itself to a leaf or twig. The caterpillar wraps itself in a leaf or some other material and remains trapped in the chrysalis until it ecloses (emerges) as an adult butterfly. This process varies according to the species and may take anywhere from a few weeks to several years. Chrysalides can be easily identified by their common, gold-colored interior (chrysos is Greek for gold).
Cocoons
Other insect larvae, particularly moth caterpillars, house their pupae in cocoons. Cocoons are special protective coverings made of natural fibers called silk. They vary widely in size, shape, texture, hardness and color, but the presence of silk is always the primary identifying feature of a cocoon. Caterpillars use plant matter and their own hairs to camouflage the cocoons. You can also identify cocoons by their location. Unlike butterfly chrysalides, which are often highly visible, caterpillars make every effort to hide and protect cocoons by making them in tree trunks or underground.
Other Information
Insect pupae can be identified by their size; small insects tend to have smaller pupae, larger insects tend to have larger pupae. Coarctate pupae are rare and extremely unique. Because coarctate pupae develop inside of the larvae, scientists have given them a different name -- puparium.