HABITATIONS OF INSECTS. 469 



the work of an insect. The case of P. bimaculata, 

 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 cen- 

 tral aperture^. Other species construct houses which 

 may be called alive, forming them of the shells of va- 

 rious 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 attentive to one circumstance respecting 

 them, namely, their specific gravity. Not having the 

 j>ower 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 iinportance that its house should be of a 



* De Gcer,ii. 554. "Ibid, 



