28 



THE COLOURS OF ANIMALS 



of absolute stillness would be far too great to be 

 borne did not the caterpillar spin a thread of silk, 

 which is attached at one end to the stem, while the 

 other end remains fixed to the head of the animal. 

 How great the strain would be without such a support 

 may be well understood by any one who has tried to 

 hold out the arm straight at right angles to the body 

 for five minutes. There is considerable tension upon 

 the thread of silk, so that, if it be 

 cut, the larva falls back with a jerk, 

 making a more obtuse angle with 

 the stem ; and it then tries to remain 

 rigid in the new position : this is im- 

 possible because of the strain, and 

 after again falling backwards once or 

 twice, and making one or two more 

 attempts to keep firm and motionless, 

 it is obHged to give up the twig-Hke 

 position while it fixes a new sup- 

 porting thread. In some cases the 

 caterpillar gains support by holding 

 a leaf or twig with one of its three 

 pairs of true legs, or legs which will persist in the 

 perfect insect (see fig. 3; also figs. 40 and 41, page 152). 

 These pairs of legs are placed one on each of the 

 body-rings behind the head. 



It is very interesting to notice how the head of 

 these caterpillars is modified from the usual shape 

 into one which suggests the end of a twig. It is very 



Fig. 3.— The larva of 

 Peppered Moth (^m- 

 phidasis betularia) ; 

 last stage ; natural 

 size. 



