VAKIABLE PROTECTIVE RESEMBLANCE IN INSECTS 153 



with the leaves upon which they fed (see fig. 41) : 

 about one or two per cent., however, took their colour 

 from the latter. The food-plants were the same in 

 both experiments. 



The change of colour is not due to the food seen through 

 a transparent skin 



Some authorities have supposed that the change 

 of colour under such circumstances is a comparatively 

 simple thing, that the younger green leaves eaten 

 and seen in the alimentary canal through the more or 

 less transparent tissues cause a brighter appearance, 

 while the older leaves produce in the same manner a 

 darker appearance. This cause of colour is certainly 

 efficient in many transparent caterpillars (see p. 79), 

 such as some of the Noctua, but it does not account 

 for any of the results obtained in my experiments. 



As a precaution against such an error, I reversed 

 the surroundings of a few larvae of most of the species 

 experimented upon. The new conditions were main- 

 tained for some days, during which the contents of the 

 alimentary canal must have been changed many times, 

 but no perceptible effect was produced. This result 

 also serves to show that the influences act very slowly, 

 and that the processes of adjustment are totally dif- 

 ferent from those which cause the rapid changes of 

 colour considered in Chapter VII. 



