PEAR-TREE. 141 



part of the leaf to another, and eating away the spaces 

 immediately beneath. The case in which the caterpillar 

 resides, and which is essential to its existence, is composed 

 of the finest silk ; and as the creature increases in size, it 

 enlarges its mimic tent by slitting it in two, and introducing 

 a slip of new material. In order to retain the tent in a 

 perpendicular posture, the little Arab attaches silken threads 

 from the base to the surrounding surface of the leaf, and, 

 lest any violence should overturn its slender dwelling, the 

 creature forms a vacuum in the protuberance at the base, 

 and thus as effectually fastens it to the leaf, as if an air-pump 

 had been employed. This vacuum is produced by the rapid 

 retreating of the insect, on the least alarm, up its narrow 

 case, which it completely fills up, and thus leaves the space 

 below free ; and, as if aware that should the air get admis- 

 sion from below, a vacuum would be impracticable, and 

 the strongest bulwark of its fortress be destroyed, the little 

 philosopher carefully avoids gnawing a hole in the leaf, 

 contenting itself with the pasturage afforded by the paren- 



