HABITATIONS OF INSECTS. 160 



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 a . Other 

 species construct houses which may be called alive, form- 

 ing them of the shells of various aquatic snails of different 

 kinds and sizes even while inhabited, all of which are im- 

 moveably 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 cylin- 

 drical 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 put- 

 ting out their head at either end b . Some larvae constantly 

 make their cases of the same materials ; others employ in- 

 differently 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 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 

 o-ravity. 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 pro- 

 truded 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 



" Dc Gcer, ii. 564. b Ibid. 



