The Caddis- Worm 



which are always heavier than water, are able 

 to keep on the surface by means of a tempo- 

 rary air-balloon which decreases the density of 

 the whole structure. 



This apparatus works in the simplest man- 

 ner. Consider the rear of the sheath. It is 

 truncated, wide-open and supplied with a 

 membranous partition, the work of the spin- 

 neret. A round hole occupies the centre of 

 this screen. Beyond it lies the interior of the 

 scabbard, which is smoothly lined and wadded 

 with satin, however rough the exterior may be. 

 Armed at the stern with two hooks which bite 

 into the silky lining, the animal is able to move 

 backwards and forwards at will inside the 

 cylinder, to fix its grapnels at whatever point 

 it pleases and thus to keep a hold on the cylin- 

 der while the six legs and the fore-part are 

 outside. 



When at rest, the body remains indoors en- 

 tirely and the grub occupies the whole of the 

 tube. But let it contract ever so little towards 

 the front, or, better still, let it stick out a part 

 of its body: a vacuum is formed behind this 

 sort of piston, which may be compared with 

 that of a pump. Thanks to the rear-window, 

 a valve without a plug, this vacuum at once 

 fills, thus renewing the aerated water around 



207, 



