76 BUTTERFLIES 



all appeared at the same time, hence the advantage 

 of appearing a few at a time in successive years. 



Pupae are of various forms : (i) Underground 

 pupae, which are found in crevices in walls and 

 the roots of trees, the holes in which they lie 

 being often carefully lined. These pupae have 

 some power of motion, and work their way to 

 the surface before the imago emerges from the 

 pupa. (2) Suspended pupce. A good example of 

 these is found in the pupa of the common tor- 

 toiseshell butterfly. The larva having chosen a 

 suitable place, by means of the glands which open 

 on the under lip, spins a little button of silk strong 

 enough to support its weight ; it then thrusts its 

 tail into the button and swings head downwards. 

 It has now to get out of its skin without letting go 

 its hold, and this it does by gradually working out 

 of its skin, which has previously been split along 

 the back, and pushing it towards its tail. Before 

 completely escaping, it gets the tip of its tail free 

 while the part in front of the tail is still fixed, the 

 lining of the trachea and intestine helping to 

 suspend it ; it then stretches its tail up to the 

 button of silk, fixes itself and spins round several 

 times to secure a firm hold, finally casting away 

 its larval skin. Other larvae sling themselves up 

 by a girth of silk, and some are flexible enough 

 to attach the thread on one side, carry it over and 

 fix it on the other side, repeating this process 

 several times. Others again spin the girth of 

 silk first and then slip their heads under it ; some, 

 such as the swallow-tail, hold the silk in their 



