chrysalis or cocoon?
opened cocoon showing chrysalis of Chelepteryx collesi, ANTHELIDAE
The emergence of a moth or butterfly (Lepidoptera) from its pupa (chrysalis) or cocoon is hard to predict.
Many species have one generation per year and over-winter as the pupa. For these: the 52 weeks are apportioned approximately:
Some species have more than one generation a year, so in summer, these only spend maybe 2 or 3 weeks as a pupa. Nevertheless, being cold blooded, their metabolism is much slower in winter, so across winter they will stay as a pupa for maybe 30 weeks.
Some species have one generation every two years, so for those you need to be very patient.
However, these timings are not fixed. They depend on weather, time of year, genetic variations, locality, food availability to the larva, and whether the animal inside feels agoraphobic.
Looking after a pupa is fairly simple, as no food is required, but some simple principles need to be followed to keep it alive. A pupa must not get too wet (or it gets attacked by mould) or too dry (or the animal inside dies of desiccation). Also it should be disturbed as little as possible, as pupae bruise easily.
Provide some twigs for the new adult, when it does emerge, so that it can hang upside down, as most Lepidoptera need to do this for their wings to expand properly.
Sadly, your patience may also go unrewarded as a high proportion of caterpillars get infected with parasitoids (parasites that kill their host). These tend to make a caterpillar upset and wander about, instead of hiding on its food plant. So infected caterpillars are the ones that are most often found. So if a bunch of flies or wasps come out the pupa, do not be too surprised.
Note: caterpillars 'hatch' from eggs, but an adult Lepidoptera 'emerges' from its pupa.
Once you have an adult, make sure it does not beat itself to a frenzy (butterflies and moths basically do not like being kept in jamjars): a spell in the refrigerator (NOT the freezer) is a good way of putting them to sleep. Then you may more easily photograph it, and finally be able to identify it from pictures in these web pages, or in books, or with the help of your local museum.
(updated 3 May 2012, 5 January 2014)