vi DEFENCES OF INSECTS 235 



Poulton exposed larvae to one colour during stages I. 

 and II., and transferred them to another colour for 

 stage III. ; other larvae were exposed to each of 

 the colours for all three stages ; and the effects were 

 compared. Stages II. and III. both showed them- 

 selves sensitive, but stage II. is more sensitive than 

 the other. For the larvae which Mr. Poulton exposed to 

 gilt surroundings during stage II. and to black after- 

 wards, "were lighter than those which had been exposed 

 to black during stage II. and to gilt afterwards." 

 That both stages are sensitive is proved by the fact 

 that " the black and gilt surroundings produced still 

 greater effects when they operated for the whole 

 period before pupation." 



Thus the old theory of photographically sensitive 

 chrysalides falls to the ground. As Mr. Poulton 

 points out, the adjustment of the pupal colours 

 to their surroundings is due to larval susceptibility, 

 and the larva itself has ceased to be highly sensitive 

 many hours before pupation. That stage II. is the 

 great period of susceptibility, probably explains the 

 true meaning of the motionless rest of the larva on 

 the surface upon which it will pupate ; while stage 

 III. has other meanings connected with the rapid 

 pupal changes that are taking place. 



The next question was to determine the organ 

 or part of the larva which is affected by surrounding 

 colours. Mr. Poulton thought that the eyes (ocelli*}, 

 or the bristles which cover the caterpillars, might 

 be sensitive. Neither of these proved to have 

 anything to do with the influence. In one set of 

 experiments, out of a number of larvae placed in the 



