INTRODUCTION. 



The way in which Eumia cratcegata, Venilia macnlata, and many 

 other species, become pale yellow or white on exposure to weather, 

 foreshadows the soluble and unstable character of many of the yellow 

 pigments, although others are apparently much more stable. Amongst 

 those which are easily soluble I have already mentioned Euchlve carda- 

 mines, and there are beside, Colias edusa, Gonopteryx rhamni (male), 

 G. deopatra, Hepiahts humuli ? , Fidonia atomaria (male), Abraxas 

 grossidariata &c., whilst some species which have rather more insoluble 

 pigment, lose it and become bleached on continued exposure to light 

 and air. I have seen the yellow of T. pronuba entirely removed in 

 this way. But perhaps the fact that several of the yellow species may 

 have their pigment absolutely changed in a progressive direction to red 

 by the direct action of cyanide of potassium is still more interesting. It 

 has long been known that certain Colias were so affected, as also Gonop- 

 teryx rhamni, such artificially produced vars. having been exhibited at our 

 Societies from time to time, as if these were the congeneric species 

 deopatra, which has such a colour naturally, and which itself may be 

 acted on (and its colour intensified) in the same direction. The yellow 

 or orange is changed to red and, in most species, shades off regularly 

 through orange, thus showing the complete genetic relationship. The 

 natural development of red scales as a seasonal form of variation in 

 Colias ccesonia, a normally yellow species, is very interesting (vide, f Ent. 

 Record,' ii., pp. 271-272). 



It may be thus seen that all yellows from the pale whitish yellow 

 of Urapteryx sambucata to the rich orange of Colias edusa and the 

 orange-red of G. deopatra are all of one genetic significance leading 

 from white to red ; and when one traces the derivative coloration of 

 edusa through the intermediate forms which connect it with its var. 

 helice, and sees the pale primrose of Eumia cratcegata, lose its colour 

 under the action of the weather, one recognises that they have their 

 origin equally in white. The normally white markings on the fore- 

 wings of Arctia caia are frequently creamy or yellowish in colour, and 

 are not infrequently tinted with red. There is also a tendency in some 

 specimens of Nemeophila plantaginis (especially females), for the yellow 

 of the underwings to become deep orange and more rarely red, a pro- 

 gressive development in direct opposition to the retrogressive var. 

 hospita. In fact, it is highly suggestive, that whilst hospita is a 

 retrogressive variety of our typical yellow form, this is, in its turn, a 

 retrogressive condition of a still more highly developed red form. 

 That the red in the hind wings of Arctia caia is directly derived from 

 yellow, may be noticed when the black spots on the hind wings of this 

 species tend to become partially obsolete. The edges of the reduced 

 spots which remain, and frequently the whole area in which the black 

 has been suppressed, are usually filled up, not with red corresponding 

 to the rest of the area of the wing, but with yellow, shading off im- 

 perceptibly into the red ground colour. In this instance, I do not 

 think the black is in any way reduced to yellow, but that the area 

 previously occupied by the black scales, which in this species are 

 probably not pigmental has been replaced by pigmental scales, the 

 latter showing their recent development in the less highly developed 

 colour of their pigment compared with the remainder of the area of 

 the wing. 



Perhaps one of the most interesting results with an alkali is to get 



