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 (Arge 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 Pieridse. 
That is to say, it has been noticed by several 
observers that in many cases scales of a bright red 
or scarlet colour yield aj 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 
