( li ) 
mologists, as it opens up most fruitful suggestions witii 
respect to the supposed "direct action" of the environment 
in modifying the colours of insects. It is only right to state 
that the author himself has not made use of his observations 
in this way, but I hope to have the opportunity of returning 
to this subject on some future occasion. The paper to which 
I refer is by Mr. F. Gowland Hopkins on the pigments of the 
Pieridse, of which an abstract appeared in the Proceedings of 
the Koyal Society for January 14th, 1895. As far back as 
1871 I mentioned at a meeting of this Society that the yellow 
colouring matter in the wings of GoneptenjA' rhamni was of an 
acid character, being dissolved out by alkalies and pre- 
cipitated by acids. A similar observation was at the same 
meeting announced by Mr. A. G. Butler.^'^ The subject 
received no further attention till recently taken up by 
Mr. Hopkins, whose results I may state are the outcome of 
several years' work, and represent the sacrifice of large 
numbers of butterflies' wings in order to get the necessary 
amount of material. In the paper referred to it is shown 
that there is a chemical relationship between the white and 
coloured pigments of Pierine butterflies and an excretory 
product, viz., uric acid, which acid the author has succeeded 
in isolating from the wings of the white species, while a 
derivative of this acid is present in the wings of the coloured 
species, Mr. Hopkins has thus for the first time established 
that among butterflies an excretory product is utilized for 
purposes of ornamentation. The artificial production of the 
yellov/ pigment by the action of water on uric acid at a high 
temperature is also claimed, but the evidence on this point is 
not yet complete. As the outcome of this interesting piece 
of work, we are provided with an actual chemical test for 
mimetic species of whites, because, however modified in 
external colour and pattern, the Pierids always give the uric 
acid derivative while the models which they resemble belong 
to groups which do not contain these pigments. 
* Proc. Ent. Soc, 1871, p. sxiv. 
