134 A BOOK OF INSECTS 



ing out of shadows, the result being that the appearance 

 of solidity is destroyed. To illustrate his theory in its 

 bearing upon the colours of large animals, Mr. Thayer 

 constructed models (replicas of which may be seen in the 

 Natural History Museums of London, Oxford, and Cam- 

 bridge) by means of which he proved that a bird which 

 is dark on the back, shading through increasing paleness 

 on the sides to white beneath, is far less conspicuous than 

 one that is uniformly coloured — even though its tints 

 may exactly accord with its surroundings. Both the 

 dummy birds are covered with the same grey material 

 with which the box is lined. One is otherwise uncoloured, 

 and is rendered very conspicuous by the illumination of 

 its back and the heavy shading of its under surface — thus 

 showing that mere identity in the colour of an animal 

 and its surroundings does not in itself afford protection. 

 The other model is skilfully painted with a dark tint 

 above, shading to white beneath, with the result that an 

 effect of flatness and unreality is produced. At a distance 

 of four feet it is invisible. 



The principle of shadow neutralisation, or obliterative 

 colouring, was recognised by Professor Poulton a number 

 of years before Mr. Thayer formulated his theory. After 

 describing the wonderful stick-likeness of the Geometrid 

 caterpillars, he has the following passage : " In order that 

 the resemblance may be complete, it is essential that the 

 caterpillar should appear to grow out of the branch in a 

 natural manner. The two pairs of claspers assist in pro- 

 ducing this effect, for they partially encircle the branch, 

 and appear to be continuous with it. Between the two 

 pairs there is necessarily a furrow, where the body of 

 the larva lies along the cylindrical branch. This furrow, 

 which, if apparent, would greatly interfere with the 

 resemblance, is rendered inconspicuous in the following 



