AND OF LIGHT. 259 



able range, Dr. A. R. Wallace * considers Professor 

 Poulton's suggestion rather improbable; still it should 

 be noted that the Small Tortoiseshell almost invariably 

 seeks mineral surroundings for the pupal period, and 

 very rarely becomes a chrysalis on its food plant. 



The time at which the colours are determined was 

 found to be especially during the resting stage of the 

 caterpillar, just before pupation, and to a less degree 

 during the onset of the pupal stage, when the caterpillar 

 hangs head downwards, suspended by its last pair of 

 claspers. The former stage lasts about 15 hours, and 

 the latter about 18, and at the end of it the skin splits 

 along the back of the head, and the chrysalis becomes 

 exposed. The reaction of the skin of the larva to the 

 colour of its surroundings was proved by some ingenious 

 experiments to be an indirect one, effected probably 

 through the medium of the nervous system. Thus 

 when a larva, during the onset of the pupal stage, was 

 so placed that part of it was illuminated by a gilded 

 background, and part by a black one, parti-coloured 

 pupae were never obtained. The effective results were 

 produced by that colour to which the larger area of skin 

 had been exposed. 



We see, then, that in the development of certain Lepi- 

 doptera there is a period, lasting only a day or two, dur- 

 ing which an extreme sensitiveness to the colour of the 

 surroundings is present, and we have also seen that dur- 

 ing the pupal period there may be a great sensitiveness 

 to the temperature of the surroundings. These cases 

 therefore form exceptions to the conclusion arrived at 

 in the last chapter, viz., that reaction to environment 



* " Darwinism," p. 198. 



