148 " THE COLOURS OF ANIMALS 



protective; the larva resembles the under side of a 

 rolled- up leaf, and when the food-plant bears leaves 

 with white and downy under sides (apple, Salix 

 viminalis, &c.) the larva is usually whitish ; while it 

 is generally yellowish-green upon trees of which the 

 leaves have green under sides (Salix triandra^ S, bahy- 

 lonica, S. rubra, S.fragilis, &c.). I remember, when 

 a boy, finding the two varieties of larva, and being 

 much astonished at the difference between them. 



I began working at the species in 1884, and have 

 bred large numbers of the larvae for every season since 

 that year. Only the results of the earlier experiments 

 are published.^ The eggs of each female moth were 

 kept separate, and the caterpillars of each batch were 

 fed upon a variety of food-plants, and manifested de- 

 cided differences in their shade of green. At the same 

 time remarkable exceptions occasionally occurred : 

 sometimes, also, when collecting I have found bright 

 green individuals upon apple. Blinding experiments 

 like those upon the Small Tortoiseshell led to negative 

 results. These experiments were very laborious, for 

 a caterpillar changes its skin four times, and with it 

 the covering to its eyes and the opaque varnish. Hence, 

 before each change of skin the caterpillars were sepa- 

 rated from the food, and, after changing it, were re- 

 blinded before being restored. 



Before this investigation had been begun, it waa 

 believed that such variability in caterpillars was due to 



' Proc. Boy. Soc. vol. xxxviii. p. 269 ; vol. xl. p. 135. 



