274 Mr. E. B. Poulton. Essential Nature of the [Apr. 23. 



transparent and opaque alternately. Even in the most opaque 

 Geometer larvae, exactly resembling the dark twigs of their food- 

 plants, the inner surfaces of the large posterior claspers are often 

 green or yellowish. This is the one spot in the body which is always 

 hidden, and here only is the pigment absent and the tint of the blood 

 visible. 



In some cases the green colour of the blood disappears, and is 

 replaced by a tint which helps towards producing the brown coloura- 

 tion. Thus the larva of Chcerocampa JElpenor is at first green, becom- 

 ing brown at the beginning of the fifth stage (as a rule). Examin- 

 ing the blood of a pupa of this species, I found that it was of a reddish- 

 brown colour, and must therefore have helped in causing the ground 

 colour of the last stages. Although no traces of the green colour were 

 to be seen, the spectra of modified chlorophyll and of xanthophyll 

 were distinct in a considerable thickness of the fluid. Evidently the 

 larva had not destroyed the pigments of the previous stages beyond 

 the point at which they ceased to interfere with the effect of the 

 later stage. The brownish tint in the blood of this brown larva 

 plays a far less important part in the total colouration than the 

 green colour of the blood in green larvae, as is proved by the following 

 considerations : — It is not uncommon to find green larvae of G. Elpenor 

 in the last stage, and it may be assumed that the blood has then 

 retained the green tint. But even in these green larvae there is a 

 considerable amount of dark pigment in and near the four eye-like 

 terrifying marks on the first and second abdominal segments, and 

 the brown colour (where it occurs) is not obviously different from 

 that of a normal brown variety. Again, in blown specimens of 

 Chcerocampa Porcellus the brown colour remains distinct and dark, 

 although the blood has, of course, been removed. And yet, con- 

 sidering the very complete resemblances between the larvae of G. 

 Elpenor and G. Porcellus throughout their life-history, it is practi- 

 cally certain that the blood of the latter is similar in colour to that of 

 the former. 



I will now give instances of the segregation of the derived pigments 

 in the subcuticular tissues (probably the hypodermis cells). This 

 must be true of Smerinthus Ocellatus, inasmuch as this larva remains 

 green after dissection when the blood has escaped. Again, the green 

 ground colour of the larvae of Sphinx Ligustri, S. Ocellatus, &c, turns 

 brown on the dorsal surface before pupation, but is comparatively 

 unchanged elsewhere, while the green blood is also unchanged in the 

 pupa. This latter fact is also true of the pupa of Smerinthus Tilice, 

 although the adult larva becomes purplish-brown without a trace 

 of green remaining (even upon the paler under-surf ace) . Hence 

 these larvae must be coloured by superficial segregation of the 

 derived pigments probably strengthened by the colour of the 



