160 Proceedings of the Royal Physical Society. 



conditions in modifying the form and colour of the larva for 

 protective purposes. He says : Lepidopterous larvae are those 

 best suited for an inquiry into the subject ; because besides 

 living exposed to all the attacks of birds and predatory 

 insects, it is frequently found that individuals of the same 

 genus, or even of the same species, feed on different plants, 

 and so become subject to the influence of different conditions. 

 In these larvse the ground colour is usually some shade of 

 green, grey, or brown. It is easy to understand the way in 

 which a green ground colour helps to protect a caterpillar, by 

 its resemblance to the colour of the leaves of the food plant. 

 It is a peculiar fact, however, that many larvae are born with 

 a green ground colour, but eventually change this for some 

 shade of brown. In some species, indeed, we find two varieties, 

 as it were, the first composed of individuals retaining the 

 green colour throughout the whole larval state, and the second 

 of others which become brown after their second or third 

 moult ; this phenomenon occurs most commonly in larvae 

 which feed on low herbs, and which attain a large size when 

 full grown. The reason for this change undoubtedly is that 

 the larva, so long as it was young, passed the whole of its 

 time among the leaves (as do also those individuals which 

 remain green), but when larger it changes its habits and only 

 feeds at night, hiding during the day amid the soil and dead 

 leaves at the roots of the plant, where, of course, some shade 

 of a brownish or yellowish colour is the most adapted to con- 

 ceal it. There are other caterpillars, however, which are brown 

 or grey in colour through life, and which pass their time among 

 the leaves and branches of their food plant : in some of these, 

 as in the Geometers generally, the colouring greatly resembles 

 that of a young shoot or a dry twig, and this resemblance is 

 further strengthened by the attitudes they assume when at rest ; 

 others, however, being of a grey hue, rest at full length on 

 the larger limbs, where it is almost impossible to see them 

 amongst the lichens which they so strongly resemble in colour. 

 With regard to markings occurring on this ground colour, those 

 on grey and brown tree-feeding caterpillars often resemble 

 the slight roughnesses and other marks found on twigs 

 and shoots, and are often accompanied by excrescences and 



