140 A BOOK OF INSECTS 



pupa?, while white or gilded surroundings induced pale 

 ones — many of the latter being remarkably metallic. He 

 showed further that the period of greatest susceptibility 

 is when the caterpillar rests motionless upon the surface 

 from which it will subsequently hang as a pupa. The 

 utility of metallic spots on the chrysalides of butterflies 

 is not at first very obvious ; but Professor Poulton points 

 out that these would harmonise with recently fractured 

 rock-surfaces which presented such glittering minerals as 

 mica ; and that the caterpillars of the small tortoiseshell 

 butterfly, as well as certain of its allies, commonly leave 

 their food-plants and resort to mineral surroundings before 

 assuming the pupa state. " In England " (he writes) " we 

 very rarely see a brightly metallic pupa, because in our 

 moist climate exposed rock-surfaces quickly weather and 

 become lichen-covered. If, however, the bright appear- 

 ance of many recently fractured rocks were retained, as 

 they are in drier countries, they would cause the produc- 

 tion of a similar appearance in the pupa? of those larvae 

 which sought them." 



Owing to the increase of vegetable life, and the con- 

 sequent covering up of mineral surfaces, there is reason 

 for believing that species with metallic chrysalides are 

 gradually adapting themselves to the changed conditions. 

 In the case of the peacock butterfly (Vanessa io) "there 

 is a dark variety which is formed when pupation takes 

 place upon dark rock-surfaces ; but the golden form has 

 been replaced by a green variety, which is produced when 

 the chrysalis is suspended from the leaves of its food- 

 plant. The green variety still retains the metallic appear- 

 ance, and exhibits it to a much greater extent than the 

 dark variety." Professor Poulton found that the green 

 variety of this chrysalis could also be produced artificially 

 by means of a gilt or a white environment. With regard 



