vi DEFENCES OF INSECTS 227 



the species is equivalent to a change of character in 

 the imitated object. Among insects in this class may 

 be mentioned the larva of Sphinx ligustri. When 

 full-fed previous to pupation, it deserts its food- 

 plant and descends to the ground to find a suitable 

 burying-place, where its bright green coat would be 

 conspicuous and rife with danger. Accordingly the 

 caterpillars lose their original colour, and become 

 brown, bringing them into consonance with their new 

 surroundings. Professor Meldola was the first to 

 perceive the true meaning of this change. An in- 

 stance is afforded by Geometrse of exactly the op- 

 posite alteration at a corresponding period. In its 

 young days the larva (Ennomos angularia) is a brown 

 " stick-caterpillar," and receives special protection 

 from its resemblance to twigs. Eventually it forms 

 a cocoon of green leaves loosely put together, so that 

 its body is plainly visible through the leafy screen 

 But at the same time its brown colour disappears, and 

 its surface becomes green, due to its green blood, 

 which is visible through the transparent skin. 



The tendency in caterpillars is to become dull in 

 colour on the approach of pupation, and these inci- 

 dental changes, in all probability, have constituted 

 the accidental resemblance to the soil, the initial start 

 as it were, from which the definite alteration has been 

 developed by natural selection.* That the change 

 must sometimes prove disastrous to the individual is 

 unquestionable. If at this crisis of life a caterpillar 

 of the Privet Hawk Moth were to descend upon turf, 

 or ground covered with other green vegetation, it 

 would still turn brown ; although its green dress would 



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