1885.] Colouring of Phytophagous Larvce. 279 



(black and yellow) as a signal of distastefulness, and the appearance 

 is entirely dne to cuticnlar pigment, and persists in the empty shell. 

 The pupae of E-hopalocera are nearly all exposed, and protectively 

 coloured. Most of the colours are due to the above- described causes, 

 derived or proper pigment ; but in some cases they cannot be the 

 result of pigment at all, and have not yet been sufficiently investigated 

 (e.g., the metallic colours of Vanessa pupae). I have worked at the 

 colour in the pupa of P. Machaon, which appears in two chief forms 

 connected by intermediate varieties, the forms being coloured by 

 derived and proper pigments respectively. Those coloured by derived 

 pigments are yellow or green and yellow, the green being pre- 

 dominant in the anterior part of the body, especially the wings, <fcc, 

 but also occurring in minute lines and masses mixed with the 

 yellow. The other form is grey with dark markings. The blood 

 is always bright yellow, with contained xanthophyll, but this does 

 not aid the superficial colour, as the cuticle is opaque. The derived 

 pigments are contained in a remarkable and very complicated layer 

 beneath the true cuticle. This layer is very finely lamellated, but at 

 the same time possesses a prismatic structure, being composed of 

 columns arranged vertically to the surface, and themselves made up 

 of parallel fibrillae. It is cuticularised, and stains deeply in picric acid, 

 but it is sharply marked off by a fine homogeneous membrane from 

 the true cuticle above, and the subcuticular tissues below. The 

 whole structure needs further investigation, but I believe that the 

 layer is a cuticularised and very complex result of the differentiation 

 of the hypodermis cells. The pigments are contained in the outer 

 lamellae, while the inner remain white. Although the layer is very 

 firm and completely united to the true cuticle (which it resembles in 

 consistence), neither itself nor its contents are beyond the reach of 

 the physiological processes of the organism, for the pigments are 

 withdrawn shortly before the emergence of the imago, and the wings 

 of the latter are visible through the now transparent covering. At 

 the same time the whole inner layer must itself disappear, for the 

 empty pupa cases of this species are thin, and evidently composed of 

 the true cuticle only, as in other pupae ; while the inner layer makes 

 the whole pupal covering very thick, being itself many times thicker 

 than the true cuticle. I have no doubt that this layer will be found 

 commonly in Rhopalocera, and it is probably correlated with the 

 extreme development of protective colours in the pupae of this group. 

 The pupal pigments upon the dark varieties, and in some parts of 

 most of the green varieties, are placed in the true cuticle. 



7. The Ultimate Use of the Derived Pigments. 

 In many species there is a great positive gain in retaining the 

 derived pigments in the blood (or other part) of the papa, for by their 

 VOL. XXXVIII. x 



