HABITATIONS OF INSECTS. 461 



tent from one part of the leaf to the other, and eating 

 away the space immediately under it. It touches no 

 other part; and when these insects abound, as they 

 often do to the great injury of pear-trees'*, you Aviil per- 

 ceive every leaf bristled with them, and covered with 

 little withered specks, the vestiges of their former meals. 

 The case in which the caterpillar resides, and which is 

 quite essential to its existence, is composed of silk spun 

 from its mouth almost as soon as it is excluded from the 

 egg. As it increases in size, it enlarges its habitation 

 by slitting it in two, and introducing a strip of new ma- 

 terials. But the most curious circumstance in the his- 

 tory of this little Arab is the mode by which it retains 

 its tent in a perpendicular posture. This it effects partly 

 by attaching silken threads from the protuberance at 

 the base to the surrounding- surface of the leaf But 

 being not merely a mechanician, but a profound natu- 

 ral philosopher well acquainted with the properties of 

 air, it has another resource when any extraordinary 

 violence threatens to overturn its slender turret. It 

 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 caused by the in- 

 sect's retreating on the least alarm up its narrow case, 

 which its body completely fills, and thus leaving the 

 space below free of air. In detaching one of these cases 

 you may easily convince yourself of the fact. If you 

 seize it suddenly while the insect is at the bottom, you 

 will find that it is readily pulled off, the silken cords 

 giving way to a very slight force ; but if, proceeding" 

 gently, you give the insect time to retreat, the case will 



* Yoxi-^ilionFfuit I'rees, 4tocdit, 2TI. 



