iv.] METAMORPHOSES OF INSECTS. 69 



gradual, the mouth of the so-called larva resembles 

 that of the perfect insect, and the principal difference 

 consists in the presence of wings. 



Similar considerations throw much light on the 

 nature of the chrysalis or pupa state that remark- 

 able period of death-like quiescence which is one of 

 the most striking characteristics of insect metamor- 

 phosis. The quiescence of the pupa is mainly owing 

 to the rapidity of the changes going on in it. In 

 that of a butterfly, not only (as has been already 

 mentioned) are the mouth and the digestive organs 

 undergoing change, but the muscles are in a similar 

 state of transition. The powerful ones which move 

 the wings are in process of formation ; and even the 

 nervous system, by which the movements are set 

 on foot and regulated, is in a state of rapid change. 1 



It must not be forgotten that all insects are in- 

 active for a longer or shorter space of time after 

 each moult. The slighter the change, as a general 

 rule, the shorter is the period of inaction. Thus, 

 after the ordinary moult of a caterpillar, the insect 

 . only requires a short rest until the new skin is 

 hardened. When, however, the change is great, the 

 period of inaction is correspondingly prolonged. 

 Most pupae indeed have some slight powers of 

 motion; those which assume the chrysalis state in 

 wood or beneath the ground usually come to the sur- 

 face when about to assume the perfect state, and the 

 aquatic pupae of certain Diptera swim about with 

 much activity. Among the Neuroptera, certain families 

 have pupae as quiescent as those of the Lepidoptera ; 



1 See Newport, Phil. Trans., 1832. 



