466 HABITATIONS OF INSECTS. 



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 sal- 

 vage, 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 turn- 

 ing 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 enlarging them) have often an ap- 

 pearance quite dissimilar to that of the old. Even those 

 that are most careless about the nature of the materials 

 of their house, are solicitously attentive to one circum- 

 stance 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 pro- 

 ficiency in hydrostatics, selecting the most suitable sub- 

 " De Geer, ii. 564. 



