The life cycle of the common flea is interesting in that the larvae actually spins a cocoon... read here:
"The larval stage actually has three developmental stages within this stage. Larvae are about 1/4" (6.35 mm) long, and semi-transparent white. They have small hairs along their body and actively move. They eat the feces of adult fleas (which is mostly dried blood) and other organic debris found in the carpet, bedding, and soil. Depending on the amount of food present and the environmental conditions, the larval stage lasts about 5 to 18 days (longer in some cases) then the larva spins a silken cocoon and pupates.
PUPA (plural = pupae): The pupa is the last stage before adult. The adult flea can emerge from the cocoon as early as 3 to 5 days, or it can stay in the cocoon for a year or more, waiting for the right time to emerge. When is the right time? (Never, say pet lovers everywhere!) Stimuli such as warm ambient temperatures, high humidity, even the vibrations and carbon dioxide emitted from a passing animal will cause the flea to emerge from the cocoon faster. This brings us back to the adult flea."
(Source: Veterinary Medicine, Janet Tobiassen Crosby, DVM, )