VII THE COLOURS OF THE LEPIDOPTERA 153 



are wholly or in part due to pigments deposited in 

 the tissues : — white (in part), yellow, red (at least in 

 part), very rarely green, brown, black (in part). 

 White as a pigmental colour is probably rare outside 

 the Pieridae ; it often exhibits a changing iridescence 

 like that of mother-of-pearl, and is then structural. 



In the Marbled White {Ar£-e galathea), an insect 

 with a simple colour-scheme in black and yellowish 

 white, a white pigment occurs which turns bright 

 yellow with alkalies. Of yellow pigments in butter- 

 flies not included in the Pieridae very little is known ; 

 none give the murexide reaction. Many red pig- 

 ments, on the other hand, give a reaction the same 

 as that displayed by the red pigment of the Pieridae. 

 That is to say, it has been noticed by several 

 observers that in many cases scales of a bright red 

 or scarlet colour yield z. yellow solution when treated 

 with hot water, but if the solution be evaporated to 

 dryness a red residue remains. Similarly, the red 

 colour is turned yellow by the application of acid, 

 but the red colour may be restored by the addition 

 of ammonia. This is the so-called "reversion effect '' 

 of Mr. Perry Coste, and strongly recalls the relation 

 described by Mr. Hopkins as existing between the 

 red and yellow pigments of the Pieridae. Red 

 pigment showing this peculiar character occurs, for 

 example, in Deilephila elpenor. 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 



