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 
js 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 da/lasted 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. [t 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 Tipularie lives in cases somewhat similar 
to those of some Phryganee. 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.? 
In concluding this head I may observe, that here might have been de- 
scribed the various abodes which solitary larve 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 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, Cicindele, and a few others, are the only ones properly 
belonging to it. 
The habitation of one of these (Cteniza cementaria) 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 
__ | For a description of various other habitations of this tribe, and of peculiarities 
in their construction, see M. Pictet’s valuable work, Recherches pour servir a 0 His- 
toire et & V Anatomie des Phryganides, in which the Linnean genus Phryganea is 
divided into seven genera, and the metamorphoses of fifty-two species are described. 
* Reaum. iii. 179. 
