The Colours of Animals. 83 



II. 



1. In fact, whenever in any group we find differ- 

 ences in form or colour, we shall always find them 

 associated with differences in habit. Let us take the 

 case of Caterpillars. The prevailing colour of cater- 

 pillars is green, like that of leaves. The value of 

 this to the young insect, the protection it affords, 

 are obvious. We must all have observed how dif- 

 ficult it is to distinguish small green caterpillars 

 from the leaves on which they feed. When, how- 

 ever, they become somewhat larger, their form 

 betrays them, and it is important that there should 

 be certain marks to divert the eye from the outlines 

 of the body. This is effected, and much protec- 

 tion is given, by longitudinal lines (fig. n), which 

 accordingly are found on a great many caterpillars. 

 These lines, both in colour and thickness, much 

 resemble some of the lines on leaves (especially 

 those, for instance, of grasses), and also the streaks 

 of shadow which occur among foliage. If, how- 

 ever, this be the explanation of them, then they 

 ought to be wanting, as a general rule, in very 

 small caterpillars, and should prevail most among 

 those which feed on or among grasses. 



2. Now, similar lines occur on a great number of 

 caterpillars belonging to most different groups of 



G 2 



