fkb. 1899] COLOURS AND PIGMENTS OF BUTTERFLIES 139 



clearly point out the differences between the Pieridae and other butter- 

 flies. 



As regards the optical colours of butterflies, Baer, like Spuler, finds 

 that the particular tint may be due to the sculpturing of the individual 

 scales, or to the combination of different scales. Scales displaying 

 optical colours usually contain pigment in addition, but this pigment 

 does not produce any marked effect on the colour, although it is no 

 doubt a factor in its production. Other apparently optical colours, such 

 as emerald-green, peacock-blue, violet, etc., are, according to the author, 

 produced by the superposition of scales containing pigment, and scales 

 displaying optical colours. The exact physical cause of the optical 

 colours was not ascertained. 



Of more importance for our immediate purpose is the treatment of 

 pigmental colours. Histologically, the pigments were found to be of 

 two types — diffused and granular. Pigments of the former type occur 

 diffused through the chitin of the scale, are usually present in very 

 small amount, and include the dark pigments, most yellow, orange, and 

 red pigments except in the Pieridae, and the white pigment (uric acid) 

 of the Pieridae. Contrary to the results of Hopkins, Baer finds that 

 some of the orange and yellow pigments of the Pieridae are diffuse 

 and not granular. Granular pigments occur exclusively in the Pieridae, 

 and are yellow or red in colour. They colour the scales in which they 

 occur very deeply, and associated with this fact Baer finds that such 

 scales are few in number, and almost without surface sculpturing. The 

 superposition of dark -coloured scales upon scales deeply tinted by 

 yellow granular pigment may, as in Anthocharis cardamines, produce a 

 greenish tint. 



Another recent paper by the Countess Maria von Linden is chiefly 

 concerned with the development of the markings of the wings of butter- 

 flies, but incidentally also discusses the order of development of the 

 colours. The author, like preceding observers, finds that those parts of 

 the wing in the pupa which are ultimately black, are first colourless, 

 then yellow or yellowish- white, then orange and orange-red, and finally 

 attain their black tint. The last stage, the development of the dark 

 tint, is not due to a replacement of the lighter pigments, but to a 

 superposition of darkly tinted scales over light ones. Optical colours, 

 such as blue, appear later than the dark tints, and are only exhibited 

 by scales which contain dark pigment. Though the paper is not 

 primarily concerned with pigment, it nevertheless is of importance as 

 tending to emphasise the distinction between the dark pigments and 

 the others, and the late appearance in ontogeny both of these dark 

 pigments and of the optical colours. 



The two preceding papers may serve as an introduction to a 

 third, a much more elaborate treatise, by Herr M. C. Piepers, on 

 " Colour-evolution in the Pieridae." Under this title the author 

 discusses a variety of topics connected with the colours of butterflies 



