37 
the light meets when attempting to pass through it ; again, milk owes 
its colour to the innumerable fatty globules held in suspension, and 
even a highly-coloured substance will lose its colour under similar 
conditions, such as stout or porter, the froth of which is nearly white. 
In Coleoptera the whites are nearly always hairs or scales, the colour 
being probably due to air spaces in them. 
I have shewn as concisely as possible the main causes of colour in 
Coleoptera, and in doing so I mentioned that the altered angle of 
reflection produced an alteration in the colour of the insect, which was 
of importance to the insect itself. It will be seen that as the angle of 
incidence becomes greater, that is, that the insect comes nearer to the 
level of the eye, that the colour invariably darkens. Take a Cetonia, 
for instance, and arrange in this fashion and lower the 
insect till it shows a golden-red colour. Now, on lifting the insect 
higher and higher, the colour alters to green or greenish-blue, and in 
all our metallic beetles the same result occurs. If the insect is green 
to start with it changes to a blue or violet, and never does an insect 
reverse this order and change from blue to green, or from green to 
orange, when placing it nearer and nearer to the level of the eyes. 
Now, the importance to metallic l)eetles is this : I'heyhave in common 
with other insects two classes of foes, those which look down on their 
prey like the birds, and those who hunt on a level with their prey like 
the lizards and other ground-feeding insectivorous animals, whose eyes 
are nearly on a level Avitli the beetles on which they feed. 
Take some of our British ground beetles, for instance. We liavc 
several which looked down upon are a lu’illiant green, and these 
amongst grass are by reason of their colour protected from the birds, 
as they are not readily seen amongst the grass on the commons and in 
the fields they frequent. But tliis green colour Avould In- no means 
protect them from the ground lizards, to whom the grass does not look- 
green, but dark, against the brilliant sky; and a metallic green beetle 
would on the ground be comparatively conspicuous, and it is here where 
the changing colour comes into play. We have seen that the beetle when 
on a level, or nearly on a level, with the eye looks a bluish or violet- 
black, in fact, about the best colour the insect could have for its own 
protection from the ground-feeding animals, and so by this double 
adaptation, due entirely to the simile physical laws Avhich govern 
interference colours, the beetles are enabled to escape from their foes 
in Avhatever position they are approached. 
In our common green tiger beetle, Cirlndela va)iij>C!itns, these pro¬ 
tective advantages seem to me to be still further developed. This 
well-known insect is familiar to everyone, and looked down upon is a 
bright grass-green, the metallic coloration being so broken up that, 
instead of getting a metallic green like the ordinary interference 
colours, we see a green more like a pigment green. Now, supposing 
this species to l)e preyed upon by birds and lizards, which, although 1 
have no direct evidence to support it, seems a reasonable supposition, it 
is obvious to everyone that a grass-green insect amongst grass is even 
better protected than a metallic-green insect would be, and our tiger 
beetle would have little to fear from the birds. But Avhen looked at 
on a level Avith the eye the insect is almost black, OAving to the peculiar 
structure of the elytra, for in this beetle a further step has been 
reached from this point of vieAv also, as all the points of metallic 
