164. INVERTEBRATE ANIMALS. 
skin of the larva; whilst in other cases the larva—immediately 
before entering the pupa stage—spins for itself a protective case 
of silken threads, which surrounds the chrysalis, and is known 
as the “cocoon.” Having remained for a variable time in this 
inanimate, quiescent pupa stage, during which rapid changes 
have been going on in the interior of the animal, the insect now . 
frees itself from the envelope which obscured it, and appears as 
the perfect winged adult or imago. 
