HABITATIONS OF INSECTS. 



397 



The case of Leptocerus bimaculatus, which is less artificially 

 constructed of a mixture of mud and sand, is pyriform, and 

 has its end curiously stopped by a plate formed of grains of 

 sand, with a central aperture.^ Other species construct 

 houses which may be called alive, forming them of the shells 

 of various aquatic snails of different kinds and sizes, even 

 while inhabited, all of which are immoveably fixed to it, and 

 dragged about at its pleasure — a covering as singular as if a 

 savage, instead of clothing himself with squirrels' skins, 

 should sew together into a coat the animals themselves. 

 However various may be the form of the case externally, 

 within it is usually cylindrical, and lined with silk ; and 

 though seldom apparently wider than just to admit the body 

 of the insect, some species have the power of turning round 

 in it, and of putting out their head at either end.^ Some 

 larvae constantly make their cases of the same materials ; 

 others employ indifferently any that are at hand ; and the 

 new ones which they construct as they increase in size (for 

 they have not the faculty, like the larva of the moth, of en- 

 larging them) have often an appearance quite dissimilar to 

 that of the old. Even those that are most careless about the 

 nature of the materials of their house are solicitously at- 

 tentive to one circumstance respecting them, namely, their 

 specific gravity. Not having the power of swimming, but 

 only of walking at the bottom of the water by aid of the six 

 legs attached to the fore part of the body, which is usually 

 protruded out of the case, and the insect itself being heavier 

 than water, it is of great importance that its house should be 

 of a specific gravity so nearly that of the element in which it 

 resides, as while walking neither to incommode it by its 

 weight, nor by too great buoyancy; and it is as essential 

 that it should be so equally ballasted in every part as to be 

 readily moveable in any position. Under these circumstances 

 our caddis-worms evince their proficiency in hydrostatics, 

 selecting the most suitable substances ; and, if the cell be too 

 heavy, glueing to it a bit of leaf or straw ; or, if too light, a 

 shell or piece of gravel. It is from this necessity of regu- 

 lating the specific gravity, that to the cases formed with the 



1 De Geer, ii. 564. 2 ibid. 



