404 
tannin formed in the organism of an animal, has obtained 
15 lbs. of tannin from 500 lbs. of weevils, which he proved 
to be such not merely by analysis, but by its action 
upon animal skins and metallic salts, especially those of 
iron. i\bout the same time, Mr. J. W. Slater, whilst 
engaged in studying the generation of colouring matters 
in the bodies of insects, was struck by the fact that a very 
large proportion of insects, especially beetles which prey 
on wood, bark, roots, &c., display colours closely resembling 
those yielded by tannin and its modifications in contact 
with animal matter. He, therefore, took the wing-cases 
of some cockchafers, and having freed them from fatty 
matters, he steeped them in solutions of iron, chrome, and 
copper. The changes of colour produced were exactly 
those which would happen if a slip of tanned leather or 
other matter, impregnated with tannin, were similarly 
treated. 
From these facts there seems to be but one legitimate 
inference. The cause of simultaneous change of colour is 
the tannin of the leaves : which changes the colour of the 
leaves when they are exposed to light; which is taken 
into the system of the leaf-eating insects and acts in the 
same way in their bodies as in the leaves. The insects 
that eat the tannin-leaves are changed ; the insects which 
are carnivorous do not show these results, for the obvious 
reason that they have not eaten the leaves.* 
The explanation of the colours of desert animals is to 
be found in the fact that dryness bleaches. 
“ The colour of the wings of the beautiful Indian Noon-moth 
(Actias selene) is very susceptible to dryness and moisture. The 
normal colour is a delicate pale green ; pieces of the wing of a 
specimen, chloroformed for the purpose of this experiment, became 
very soon straw-coloured in dry air of a temperature of a little under 
Scientific News. vol. i., new series, p. 30.