272 THE BIOLOGY OF INSECTS 



correspondence with their dwelling-places and with the 

 conditions of their lives. 



The adaptation of many insects to their haunts, in such 

 a manner that they resemble in appearance, to a greater or 

 less extent, the objects among which they live, offers a 

 subject of great interest. Examples have been, however, 

 frequently described and discussed, and some contribution 

 to the question will be attempted in a later chapter (XII), 

 when the fascinating problems of insect evolution generally 

 will be considered. For the present, therefore, it may 

 suffice to recall a few cases illustrative of these relations 

 between insects and their surroundings. Caterpillars feed- 

 ing on leaves are frequently green in colour because the 

 pigment of the plant tissues within the food canals passes 

 into the fat-body and becomes apparent through the 

 translucent skin and cuticle. But dark pigment may be 

 developed in the skin, and as this happens to an increasing 

 degree, the caterpillar tends to become darker and may 

 approximate in appearance to the brownish bark of the twigs 

 of its food plant or the dark soil. E. B. Poulton (1890) and 

 others have shown that the appearance of the caterpillars 

 is a response to the nature of the light reflected from their 

 surroundings. If these be predominantly green, the 

 reflected yellow rays penetrating the skin of the caterpillar, 

 inhibit the development of the dark pigment, so that the 

 insect appears green, because that colour shows through the 

 body- wall from material in the deeper tissues. It is found, 

 therefore, that many caterpillars are notably variable as to 

 colour, appearing dark or green according to the pre- 

 dominant colour of their haunts or food-plants. Thus 

 they are endowed with what is termed '' protective re- 

 semblance," because it seems likely that their appearance 

 serves to hide them, so that they escape the observation of 

 possible enemies which w^ould devour them were they 

 detected. Protective resemblance becomes more marked 

 in cases where the insect is of such form that it resembles 

 a twig of its food plant ; this condition is well known in 

 many of our " looper " caterpillars (Plate X) and in the " stick 



