128 



THE SPINY ELM CATERPILLAR 



the caterpillars. At the end of the week they moult or cast 

 their skins, a process in which the skin of each insect splits 

 open along the back, and the caterpillar crawls out of it, 

 being covered with a new skin that had been formed beneath 

 the old one. This new skin stretches somewhat after the 

 caterpillar emerges, so that the insect is able to increase con- 



Fig. 44? Colony of Caterpillars feeding upon willow. 



siderably in size. At the period of moulting the caterpillars 

 remain quiet for a short time, but they soon become active 

 again and begin feeding with increased voracity. 



During the next three weeks this moulting process is re- 

 peated three times, the caterpillars becoming larger each time, 

 and leaving their cast skins upon the denuded twigs, as shown 

 in Fig. -13. They soon scatter more or less over neighboring 



