435 
1880.] The Laws of Emphasis and Symmetry. 
the colour may have been derived. On the contrary, the 
two principles seem to support each other. To take an 
especial instance : — It has been explained that larvae which 
feed upon the leaves and stems of plants in open daylight 
assume a green colour, like the objects by which they are 
surrounded ; whilst those which prey upon roots, bark, and 
wood, and especially such as pass their existence in the 
dark,— whether under ground or in the interior ol fruits, 
seeds, or trunks of trees, — are of a pale grey or livid colour. 
All these faCts are doubtless intelligible on the hypothesis of 
“ protective colouration.” But they are also by no means 
antagonistic to a chemical explanation. The present writer 
has pointed out, on a former occasion,* that larvae feeding 
upon leaves may readily be coloured green by retaining in 
their tissues, in an undecomposed state, the chlorophyll con. 
tained in their diet. On the other hand, larvae which feed 
upon substances devoid of chlorophyll may be considered 
much less likely to display a green colouration. Thus the 
caterpillars of Cossus ligniperda and of Zeuzera cesculi are not 
green ; but they have not been consuming a green pigment. 
The same rule holds good with the larvae of the Elateridae, 
the Buprestidae, Lamellicornes, Longicornes, &c. It may 
surely be submitted that before a mimetic or otherwise pro- 
tective colouration can be developed by the agency of Natural 
Selection the material for the production of such colour 
must be present. It is, of course, admitted that many 
caterpillars consuming chlorophyll are not green ; but in 
these cases their colours are such as may be formed by the 
decomposition or modification of chlorophyll, — colours such 
as we see actually produced in the leaf when fading. The 
difficulty remaining is to know why in some species the 
green pigment remains unchanged till the inseCt reaches 
maturity, and why in others it is immediately decomposed, 
or at least disappears after some time. Why is the green 
colour in Chczrocampa elpenov sometimes absent in the larva, 
and why does it appear in the perfect inseCt ? We may say 
that the chlorophyll is altered by oxidation, and restored to 
its original condition by a process of reduction. But why 
do these changes take place in some species and not in 
others whose vital conditions are, as far as we can judge, 
essentially identical ? At the same time, when we seek to 
account for these peculiarities of colour and design on the 
principles of mimetism and protection, the same questions 
still lurk in the back-ground : how are the colours requisite 
* Quarterly Journal of Science, vol. viii. (1878), p. 48. 
