The Influence of External Conditions 35 



It is not difficult to starve caterpillars, but it is difficult to 

 make them take more food than they do normally; yet by 

 indirect means this can be done by giving them food containing 

 an excessive amount of nutritive substances. 



Reviewing the results of the effects produced by changes in 

 food, Pictet points out that, in general, the variations produced 

 are either in the direction of lighter (albinistic) or darker (mel- 

 anistic) shades. If the kinds of food that produce the former 

 effect are compared with those that produce the latter effect, it 

 will be seen that the lighter colors result from feeding on plants 

 which, owing to their anatomical structure, present obstacles to 

 mastication either as a result of a thick epidermis or of the pres- 

 ence of crystals or of hairs, etc. In consequence of the imperfect 

 nutrition of the caterpillar, the butterfly is less pigmented. On 

 the other hand, the melanistic or black variations result from 

 those plants that offer no obstacles to mastication or nutrition. 

 The caterpillars develop rapidly, acquire a size greater than the 

 normal, and the butterfly shows a greater development of pig- 

 ment. 



The influence of different food plants in causing an albinistic 

 or melanistic change in the moths can be traced, according to 

 Pictet, to the length of the pupal stage, and this in turn to that 

 of the caterpillar stage, which again is due to the amount of 

 nourishment received. Thus, those food plants that give in- 

 sufficient nourishment prolong the caterpillar stage ; but this 

 leads to a shortening of the pupal stage, and the albinistic effect 

 is produced, because the dark pigments do not have time to 

 develop extensively. Conversely, an abundant nourishment 

 shortens the caterpillar stage ; but this causes a prolongation of 

 the pupal stage during which the pigments have time to develop 

 even more fully than normally, and the melanistic variation is 

 the result. 



Not only the colors of the moth or butterfly are affected by 

 the character of the food, but the colors of the caterpillars them- 

 selves may sometimes be affected, although to a less degree. 

 Pictet finds, in fact, that there is a correlation between the pig- 



