176 ROUND THE YEAR 



rately the form impressed upon it by the growing 

 parts within. The larva in the latter part of its 

 history has accordingly three skins, one outside 

 another, larval, pupal, and imaginal. 



Insects furnish examples of every degree of com- 

 plication of such imaginal folds. Where no change 

 of form is to be effected, the imaginal skin is closely 

 moulded upon the larval skin. Slight changes in 

 length of leg or mouth-parts are readily brought 

 about either by wrinkling of the new integument, 

 which becomes extended as soon as it is freed, or 

 by shallow infolding. Many intermediate cases of 

 various complexity occur in different organs or in 

 different Insects. The maximum of complexity is 

 found in the Blow-fly and other Insects of the same 

 family. Here the complete want of correspondence 

 between the structure and mode of life of the larva 

 and the fly, together with the high and special 

 development of the organs of the fly, have led to 

 an extraordinary elaboration of the imaginal folds, 

 which are numerous, intricate and deep. Closely 

 connected with this complexity of the new growth 

 is the completeness of the resting-stage. The pupa 

 has no external mark of a living thing ; internally 

 it is at one time reduced to simple elements, and 

 consists of a kind of pulp, except for the unde- 

 veloped imaginal folds. Protected by the hardened 

 larval skin, which forms a firm smooth capsule about 

 it, it goes through the evolutions which are to trans- 

 form a sluggish and voracious larva, destitute of 

 limbs and almost destitute of senses, into a swift and 

 adroit fly. 



