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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.