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



extreme yellowish variety, on the other hand, does not present the 

 same marked line of demarcation at the subdorsal level ; the whole 

 larva is bright yellowish-green, the tint being more pronounced on 

 the under- surface, but very bright and distinct over the whole 

 larva. It is here also obvious that the main colour is deeply placed, 

 and is to some extent modified by the shagreen dots and by 

 subcuticular whitish masses, but these latter have not the same effect 

 as in the other variety (and are probably developed to a much less 

 extent) . 



A less extreme form of the yellowish variety differs in the dorsal 

 tint being more completely obscured by white (the result being a 

 yellowish- white in this region). The line of demarcation is not sharp. 

 In an intermediate variety (such as that found upon 8. Ferru- 

 ginea), the upper surface resembles that of the whitish form, with 

 the same sharp contrast at the subdorsal, but below this the tint is 

 green, without any marked tendency towards yellowish or bluish, the 

 under surface being of course the most deeply coloured. Such a 

 variety is not at all uncommon, and I have described it as inclining 

 towards the whitish side of intermediate. It is truly intermediate in 

 the colour of the lateral and lower surfaces, inclining towards the 

 whitish variety on the upper surface, and in the sharp demarcation at 

 the subdorsal level. It is at once clear that the green tints, whether 

 yellowish or bluish, are due to derived pigments, which have already 

 been described as present in the blood of the pupa, and which have 

 been proved to exist in the subcuticular tissues, as well as in the blood 

 of the larva. The white masses are evidently due to a substance of 

 true larval origin, and the effect of the food-plant upon the latter 

 shows at once that its influence extends beyond the derived colours. 

 The yellowish tint of the bright variety is due to predominant xantho- 

 phyll, as is proved by comparing the blood of pupae of which the 

 history is known. The greener variety must conversely be due to a 

 larger proportion of chlorophyll, and the blue tinge may be caused 

 by a slight change in its constitution ; but this needs investigation in 

 the larva, for although the papal blood of yellow varieties is brighter 

 in colour, that of the others does not retain the blue tinge. The 

 difference is delicate, but distinct; and much must be lost in the 

 redistribution accompanying pupation. It is likely that the blood of 

 the larvae is not very different, but that the chief distinctions are due 

 to the subcuticular derived pigments, and these are largely destroyed 

 in the changes preceding pupation. It is unfortunate that this inquiry 

 did not suggest itself to me when I possessed the living larvae in 

 abundance. 



The case of 8. Ligustri is more distinct, for the differences in the 

 green seem to be entirely retained in the pupal blood. I have now 

 many times compared the blood of lilac and privet pupae, and that 



