CATERPILLARS 57 



but it soon hardens up and there is no more growing. 

 Many of the details of the liberation of the butterfly or 

 moth are extremely interesting, thus there are often lids 

 which open neatly ; there is sometimes a transitory head- 

 organ that helps the escape, just like the " egg-tooth " 

 with which young birds break through the egg-shell ; the 

 puss-moth secretes caustic potash from its mouth which 

 dissolves away the pupa-case. 



The ranks of the caterpillars are thinned by the weather, 

 by many birds, and by other harassing enemies, such as 

 the Ichneumon flies which lay their eggs in the juicy 

 body. The risks that caterpillars run are so many that 

 their survival is sometimes surprising. In the main it is 

 secured in two ways by sheer force of numbers on the 

 one hand, and by numerous protective adaptations on the 

 other. The importance of the latter, which are often very 

 subtle, will be better realised when we remember, what 

 Alfred Russel Wallace pointed out, that it is essential for 

 most caterpillars to escape even a tentative peck. The 

 soft-walled tense character of the body is " extremely 

 dangerous, for a slight wound entails great loss of blood, 

 while a moderate injury must prove fatal." 



The protective adaptations are manifold. Hairy cater- 

 pillars are left alone by most birds, though the cuckoo seems 

 to relish them. Not a few secrete offensive fluids, such as 

 formic acid, when they are touched. Some have an un- 

 pleasant smell, and others are unpalatable, as experiment 

 has shown. Some hide during the day, others play 'possum 

 when touched. Some lash with their tail whips and strike 

 " terrifying attitudes/' But the subtler line of adaptation 

 is that seen in the numerous instances of protective re- 

 semblance. Thus some caterpillars are extraordinarily like 

 stunted twigs or little knobs on a stem ; others are like 

 little splinters of wood or the curled margins of withered 



