122 BERTHA'S VISIT TO HER 



composed of silk, spun from its mouth almost 

 as soon as it comes out of the egg, arid as it 

 increases in size it enlarges the tube, by slitting 

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

 terials. To preserve the perpendicular posture 

 of its tent, this ingenious insect attaches several 

 silken threads from the protuberance at the base 

 to the surface of the leaf; but it has a still more 

 singular device to protect the tent against any 

 violence : it forms a vacuum in the protu- 

 berance at the base, which fastens it to the leaf 

 as effectually as if an air-pump had been em- 

 ployed. This vacuum is caused by the insect's 

 retreating on the least alarm up the tube, which 

 its body so completely fills that the space below 

 is free of air, and the tube is pressed down like 

 the exhausted receiver of an air-pump. 



Mary easily convinced me of this when she 

 seized it suddenly while the insect was at the 

 bottom, the silken cords readily gave way, and 

 the tube was detached by a very slight force ; 

 but when she touched it gently, giving the 

 insect time to retreat, we found that a much 

 stronger effort was required to loosen it. As if 

 aware of the effect of the admission of air from 

 below, this little philosopher carefully avoids 

 gnawing quite through the leaf; and when he 

 has eaten as deeply as he can venture, he cuts 

 the cords of his tent and pitches it on a fresh 



