Protective Resemblance 319 



markings which recall the appearance of lichens. Most 

 of these insects have the power of adapting their colour 

 in response to their surroundings. The caterpillars of 

 Amphidasys betularia, for instance, are brown when 

 first hatched from the eggs ; but those reared among 

 green leaves and shoots turn green as growth proceeds, 

 while when dark twigs are present with the food, a 

 corresponding dark colour is induced in the caterpillars. 

 The presence of light is necessary for the production 

 of these colour changes, and the caterpillars are found 

 to be no longer sensitive in their later stages. The 

 dark pigment to which the brown or grey colours are 

 due is deposited in the cells of the outer skin, while 

 the green-colouring matter is found in the underlying 

 fat ; and experiments have shown that the presence or 

 absence of both kinds of pigment is determined by the 

 surrounding objects through the quality of the light 

 reflected from them, the suppression of the superficial 

 dark pigment allowing the deeper green to show 

 through the skin and thus give its hue to the caterpillar. 

 It has been shown that the formation of the dark 

 pigment is hindered by the action of certain yellow 

 rays of the solar spectrum, which are absorbed by dark 

 objects, but reflected from green leaves and shoots. 

 There can be no doubt that the power possessed by 

 these caterpillars of modifying their appearance in ac- 

 cordance with their surroundings must be of great value 

 in ensuring their preservation from enemies (184). 



There are other insects which, without the exact 

 resemblance to twigs shown by the loopers, are 

 coloured and marked in such harmony with their 

 surroundings as to be inconspicuous when at rest. 

 The caterpillars of Hawk-moths for instance are by 

 no means easily seen on their food-plants, the green 

 surface of their bodies broken up by dark sloping 

 lines agreeing well with the effect of light and 



