124 THE COLOUKS OF ANIMALS 



The period during which the colours of pupae are 

 determined 



A very large number of experiments and the 

 closest and most frequent observations were devoted 

 to the determination of the time during which these 

 organisms are sensitive to surrounding colours. It 

 was first necessary to observe everything that happens 

 to a caterpillar between the cessation of feeding and 

 the change into a chrysalis, for I felt sure that the 

 time of susceptibility lay somewhere within these 

 limits. When one of these caterpillars is full-fed, 

 it descends from its food-plant (nettle) and wanders 

 about in search of some suitable surface upon which 

 to pass the pupal period. This is stage i., and its 

 length varies greatly, according to the proximity of 

 suitable surfaces. Then the caterpillar, having found 

 the surface, rests motionless upon it, generally in a 

 somewhat curved position. This is stage ii., and it 

 is also variable in length, but fifteen hours may be 

 accepted as a fair average of the time spent in this 

 position. Finally the caterpillar hangs, head down- 

 wards, suspended by its last pair of claspers (larval 

 legs), which are attached to a boss of silk spun at the 

 close of the second stage. This is stage iii., which 

 lasts for about eighteen hours, at the end of which 

 time the skin splits along the back behind the head, 

 and the chrysalis is exposed by the skin being worked 



