tJie colours of certain Lepidoptcra. 859 



out in the mR.nner previously described, the appearance, 

 as seen from the internal surface, after removing the 

 digestive tract and most of the deep part of the fat-body, 

 is shown in Plate XIV., fig. 18, where abdominal segments 

 1 to 4 are represented. The tracheal system is only in- 

 dicated on the left side. The anterior direction is shown 

 by the arrow. In each segment the anterior brown band 

 prolonged from the dorsal stripe is well shown, and here 

 the epidermis is not underlaid by green fat, although this 

 effect is probably in [)art due to the stretching. A mass 

 of bright yellow fat lies on each side of the dorsal stripe 

 anteriorly in each segment. This belongs to a deeper 

 part of the fat-body heloiv the muscles of the body-walls. 

 Over the green fat which forms the posterior band in 

 each segment, it has been already stated that the 

 epidermis is not brown but pale yellowish in tint, and 

 quite transparent. 



It is therefore clear that the surroundings determine 

 not only the presence or absence of true pigment in the 

 epidermic cells, but also its constitution and therefore 

 colour when present. And the range of possible tints 

 and combinations is very wide, including all shades of 

 brown and grey, passing into black on the one side and 

 white on the other, and comprising uniform tints as well 

 as the most complex combinations, as when these larvae 

 resemble the appearance of lichen. But the surroundings 

 also determine the presence of the green colour in the 

 superficial layer of fat. These are the results, and some 

 quality in the light reflected from surrounding objects 

 forms the cause, but the physiological chain which con- 

 nects the two has yet to be discovered. 



Direct Evidence of a Colour-relation between the 

 Larv^ of a. betularia and their Natural Sur- 

 roundings. 



Nearly all the colours obtained in these experiments 

 are well known in the field, and the others will doubtless 

 be found if looked for on plants of the appropriate colour. 

 Thus the white varieties, the only ones I have not seen 

 wild, would probably be found upon food-plants with 

 white pubescent or glaucous shoots. Such a wide power 

 of colour-adaptation is especially necessary for a larva 

 which feeds, like A. betularia, on almost any shrub or 



