300 HABITATIONS OF INSECTS. 



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 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 spexijic 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 regulating the specific gravity, that to the cases formed with the greatest 

 regularity we often see attached a seemingly superfluous piece of wood, 

 leaf, or the like.^ 



A larva of one of the aquatic Tipularice lives in cases somewhat 

 similar to those of some Phryganece. Several of these of a fusiform 

 shape, and brown color, composed partly of silk, and partly perhaps of 

 fragments of leaves, and inhabited by a red larva, apparently of a Chiro- 

 nomus, were found by Reaumur upon dead leaves in a pool of water in 

 the Bois de Boulogne.^ 



In concluding this head I may observe, that here might have been 

 described the various abodes which solitary larvae prepare for themselves 

 previous to assuming the pupa, and intended for their protection in that 

 defenceless stage of existence ; but as I shall have occasion again to refer 

 to them in speaking of the larva state of insects, I shall defer their descrip- 

 tion to that letter, to which they more strictly belong. 



From the next division of the habitations of insects, those formed by 

 solitary perfect insects for their own accommodation, I shall select for 

 description only two, both the work of spiders, and alluded to in a former 

 letter ; which indeed, with the exception of the inartificial retreats made 



» De Geer, ii. 564. 



** For a description of various other habitations of this tribe, and of peculiarities in their 

 construction, see M. Pictet's valuable work, Eechercfies pour servir tVHistoire et d, VAnatomie 

 des Phryganides, in which the Linnean genus Phryganea is divided into seven genera, and 

 the metamorphoses of fifty-two species are described. 



3 Reaum. iii. 179. 



