HABITATIONS OF INSECTS. 265 



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 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. 1 



A larva of one of the aquatic Tipularm lives in cases somewhat similar 

 to those of some Pkryganece. Several of these of a fusiform shape, and 

 brown colour, composed partly of silk, and partly perhaps of fragments of 

 leaves, and inhabited by a red larva, apparently of a Chironomus, were 

 found by Reaumur upon dead leaves in a pool of water in the Bois de 

 Boulogne. 2 



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

 scribed 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^ insects, those formed by 

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

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

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

 by the Grylli, Cicindcla, and a few others, are the only ones properly 

 belonging to it. 



The habitation of one of these (Cteniza ccementaria} is subterraneous ; 

 not a mere shallow cavity, but a tube or gallery upwards of two feet in 

 length, and half an inch broad. This tunnel, so vast compared with the 

 size of the insect, it digs by means of its strong jaws in a steep bank of 

 bare clay, so that the rain may readily run off without penetrating to its 

 dwelling. Its next operation is to line the whole from top to bottom with 

 a web of fine silk, which serves the double purpose of preventing the earth 

 that composes the walls from falling in, and, by its connection with the 

 door of the orifice, of giving information to the spider of what is passing 



1 For a description of various other habitations of this tribe, and of peculiarities 

 in their construction, see M. Pictet's valuable work, JKecherches pour servir a CHis- 

 toire et a VAnatomie des Phryganides, in which the Linnean genus Phryganea is 

 divided into seven genera, and the metamorphoses of fifty-two species are described. 



2 Reaum. iii. 179. 



