218 THE TRICHOPTERA. 



interior of their cases ; and as tliey are also of a more 

 predaceoiis disposition, it is probable tliat they pro- 

 trude themselves fm-ther from the mouth of their 

 dwellings, or perhaps quit them altogether when in 

 search of food. 



The food of most of these larvae consists principally 

 of vegetable matter, for the comminution of which 

 their jaws are well adapted, but a slight admixture of 

 animal food appears to suit the taste of the majority, 

 as M. Pictet found that the larvae which fed freely 

 upon willow-leaves, also occasionally made a meal 

 upon other aquatic insects. The larvae of some species 

 also seem to be entirely confined to an animal diet, 

 not even sparing their own kind when there is a 

 scarcity of other more legitimate food. 



Unlike the Sialis lutaria, the larvae of the Tiicho- 

 ptera, when about to pass into the pupa state, do not 

 quit the water, but undergo this change within their 

 cases, which furnish them with a sufficient protection 

 against the accidents which might befall them in 

 their helpless condition. Before performing this im- 

 portant operation, however, the larva takes the pre- 

 caution of fixing his dwelling to some immovable 

 body at the bottom of the water, and then wisely 

 closes the two ends of his tube against his numerous 

 predatory neighbours by means of silken gratings, 

 which, whilst they keep off the unwelcome visits of 

 the piratical inhabitants of the water, allow that 

 element to flow freely into the case, to supply the air 

 necessary for the respiration of its inmate, which, like \ 

 the larva, is provided with external branchiae. In 

 the chamber thus formed, the insect, which already 

 exhibits pretty distinctly the structure proper to it in 

 the perfect state, although the wings, legs, and an- 



