35 
BEETLE COLORATION. 
(Read May 3rd, 1898, by Mr. H. HEASLER.) 
Ever since the researches of such eminent naturalists as Darwin, 
Bates, Wallace, and others have shewn how important a part colora¬ 
tion has played in the struggle for .existence, and in the life-history of 
a species, it has been one of tlie most successfully studied aspects of 
nature from a morphologist’s point of view. The facts brought to our 
notice of the importance of coloration in the numerous examples of 
mimicry now so well-known have largely been instrumental in drawing 
our attention to the importance of the subject. True, our knowledge 
of the physiological value or the chemical nature of colour is still in 
its infancy, but that does not prevent us from seeing the enormous 
importance the colours of a species are to itself, and of seeing dimly 
the record they are of the life-history of a species or group. 
In Coleoptera we have three main physical causes of colour, 
namely, absorption or pigment colours, interference colours, or those 
due to the light waves of certain length interfering with one another, 
and white, due to dispersion. 'Phe absorption colours are those due to 
a substance which we call the pigment, whose chemical condition is 
such that it absorbs all the colours of white light except that which it 
reflects ; thus, a red pigment is a substance which absorbs all the light 
waves dift'ering in length from the red waves which it alone reflects, and 
which causes in our eyes the .sensation of red; similarly, a pigment 
which produces in our eyes the sensation of green is one which absorbs 
all the waves of light whose length difters from that of the green 
waves. 
Interference colours, on the other hand, are purely mechanical, and 
have nothing to do with the chemical nature, but only the mechanical 
structure, of the substance which shows the colours, and are simply due 
to the fact that the waves of light of a certain length striking a thin 
film of transparent matter are reflected from both the top and bottom 
surface, and that colours of a certain wave length reflected from the 
first surface are nullified by the waves of the same length reflected 
from the second surface, which to do this must arrive half a wave 
length or a certain number of odd half-wave lengths behind the ones 
from the first surface. The only point is that the substance must be 
transparent for a certain depth so as to get two reflecting surfaces. 
Take this diagram, for instance, 
which is the one most commonly used to demonstrate this kind of 
colour interference. Here we have a ray of light A C striking the 
surface at C, a portion is reflected as usual, which we will ignore so as 
not to complicate the diagram, and a portion enters the transparent 
