VAKIABLE PROTECTIVE EESEMBLANCE IN INSECTS 125 



up towards the boss of silk. Then the tail of the 

 chrysalis is withdrawn from the interior of the skin 

 and is forced up the outside of the latter, until it 

 comes in contact with the boss of silk. Contact 

 immediately causes some of the numerous hooks on 

 the end of the chrysalis to be entangled in the silk. 

 During this apparently perilous operation the chrysa- 

 lis is suspended to the larval skin, although different 

 opinions obtain as to the exact method of its attach- 

 ment. The sight is extremely interesting and beautiful, 

 and the operation is almost always performed with 

 precision and success. As soon as the pupa is firmly 

 attached to the silk, it endeavours, by the most violent 

 movements, to get rid of the skin, and generally 

 succeeds in detaching it. 



Exact determination of the period of susceptibility 



The whole of the period before pupation, including 

 the three stages, may be estimated at about thirty-six 

 hours. Even if the caterpillars were susceptible 

 during stage i., no effective results could be obtained ; 

 for they are then wandering over surfaces of various 

 colours, of which few can be the same as that which 

 will form the environment of the chrysalis. Many 

 experiments were conducted with the object of as- 

 certaining the exact period of susceptibility. Larvae 

 were exposed to one colour during stages i. and ii., 

 and then transferred to another colour for stage iii., 

 7 



