i 7 8 
COLOUR IN NATURE 
CHAP. 
that is, two molecules of the orange pigment might 
combine, and form one molecule of red and one of 
yellow pigment. The chief interest of this suggestion 
lies in the'fact that such a splitting of colours has 
been described in the males as compared with the 
females in both birds and butterflies. It may quite 
well be that this is in reality due in some such way 
to a chemical change. In butterflies, as we have 
already seen, there is certainly a strong tendency for 
colours like yellow, orange, and red to show oscillation 
in amount. 
Into the numerous other problems connected 
with the colours of insects space does not permit us 
to enter here. Many of them are indeed subjects 
which can only be adequately treated by ento¬ 
mologists, and reference should be made to the works 
of Wallace, Poulton, Weismann, Meldola, Beddard, 
and others for details. It is, however, hoped that 
the foregoing summary will be of use in calling 
attention to aspects of the subject which are less 
familiar, but are undoubtedly of great importance. 
The Colours of Spiders 
Among other Arthropods the colours of spiders 
merit at least a brief notice. In them the pigments 
do not seem to have been investigated at all, and 
the structural colours only to a very slight degree, 
but there are some interesting observations on the 
colours themselves. 
As compared with insects, the characteristics of 
spiders which are important for our purpose are the 
practical absence of obvious segmentation in the 
