1896. THE PIGMENTS OF ANIMALS. 175 



acid body with a weak base. Red pigment showing this pecuHar 

 character occurs, for example, in Delias eucharis and Deilephila elpenov. 

 The phenomenon is of common though not universal occurrence, some 

 reds being quite unchanged by acids. 



Green in butterflies presents many difficulties. It may be entirely 

 structural, and arise by surface markings or by the superposition of 

 scales, as in the species of Nematois (Spuler). Again, from the green 

 scales of Papilio euryviedes, Urech extracted a yellow pigment which 

 was almost insoluble in water, but which dissolved readily in hydro- 

 chloric acid. He adds as a note, however, that the scales retained 

 their green colour after treatment with acid and ammonia. It is 

 almost impossible to doubt that in this case the green colour is 

 structural, the part played by the yellow pigment being uncertain. 

 Further, he found that the green scales of Thecla rnbi are yellow by 

 transmitted light, and almost colourless when the light falls from the 

 base of the scale upwards, while to hydrochloric acid they yield a yellow 

 pigment. These two cases seem to suggest that in butterflies, as in 

 birds, green may be produced by a combination of a yellow pigment 

 and a structural modification. On the other hand, from the green 

 scales of Sphinx nerei Urech extracted a pigment which was slightly 

 soluble in water and readily soluble in acid and ammonia. Of the 

 three solutions the first was greeensh yellow, the second orange 

 yellow, and the third green. The addition of ammonia to pigment 

 turned yellow by acid restored the green colour. This fact would 

 suggest that the green pigment of Sphinx nerei is derived from a yellow, 

 in much the same way as is the red pigment of Delias eucharis. 



So far we have treated of four colours which are either always 

 or occasionally produced by pigment, viz., white, yellow, red, and 

 green. Of these the white pigment is uric acid itself (?) ; the yellow 

 (lepidotic acid of Hopkins) is, at least in the best known cases, un- 

 doubtedly a derivative of uric acid, and in the Pieridse has been found 

 to occur as one of the normal waste-products of the organism. 

 Further, the relations of red and green respectively to yellow render 

 it probable that these pigments also are derivatives of the uric acid 

 group. It must not be forgotten, however, that in various butterflies 

 there occur yellow pigments which do not give the murexide reaction, 

 and whose affinities have yet to be determined. For example, 

 the yellow pigment of Papilio machaon when tested with nitric 

 acid and ammonia gave a green instead of a purplish colour. In 

 view of this and similar facts, it would seem to be premature to assert 

 that all yellow pigments, even in butterflies, are directly derived from 

 uric acid. 



We have in the foregoing account omitted all reference to the 

 black or brown pigments. Black is usually due in part to the 

 sculpturing of the surface, and in part to a dark granular pigment. 

 All these dark-coloured pigments Urech found to be insoluble 

 in water and organic solvents, but to dissolve in most cases 



