THE TRICHOPTERA. 217 



The tubes are open at both ends to allow of the free 

 passage of water^ which is absolutely necessary to 

 enable this fluid to come in contact with the gills^ as 

 without this_, respiration could not be effected. To 

 retain them firmly in their tubes^ the larvse are pro- 

 vided with a pair of jointed^ hook-like appendages at 

 the extremity of the body, which are assisted in most 

 instances by three fleshy tubercles on the first segment 

 of the abdomen. 



From the mouth or anterior opening of this curious 

 habitation the larva protrudes its head and the first 

 three segments of its body bearing the legs, and as 

 the case is generally composed of materials which 

 are scarcely, if at all, heavier than water, it has little 

 difiiculty under these circumstances in keeping its 

 house over its tail at all events, wherever it goes. 

 Moreover, by the action of the terminal hooks it can 

 withdraw itself completely within its case at a mo- 

 ment's notice, should any prying fish, or other dis- 

 turber of its quiet existence, exhibit a desire of 

 becoming too intimate ; and as soon as the danger has 

 passed away, the leg-bearing segments are protruded 

 as before, and the creature recommences its slow 

 I perambulation of the river's bed. 



Although the majority of these larvse live in port- 



i able cases as above described, there are a few which 



i attach their residence to the surface of stones under 



|j water, sometimes, as in Hydropsy die maculicornis, a 



li small species observed by M. Pictet, forming a long 



' tortuous tube, composed of fine sand, not unlike the 



dwellings formed by some Annelides. The larvse 



I' which live in cases of this description are more active, 



and have the segments of the body of a firmer texture 



than those which pass their whole existence in the 



L 



