HABITATIONS OF INSECTS. 461 



part ; and when these insects abound, as they often do 

 to the great injury of pear trees a , you will perceive 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 in- 

 creases in size, it enlarges its habitation by slitting it in 

 two, and introducing a strip of new materials. But the 

 most curious circumstance in the history of this little 

 Arab is the mode by which it retains its tent in a per- 

 pendicular 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 natural philosopher well 

 acquainted with the properties of air, it has another re- 

 source 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 insect'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 offj the silken cords giving way to a very slight 

 force ; but if, proceeding gently, you give the insect time 

 to retreat, the case will be held so closely to the leaf as 

 to require a much stronger effort to loosen it. As if 

 11 Forsyth on Fruit Trees, 4to edit. 271. 



