170 HABITATIONS OF INSECTS. 



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 pro- 

 ficiency in hydrostatics, selecting the most suitable sub- 

 stances ; 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 Tijnilidtf lives in cases 

 somewhat similar to those of some Phryganecs. 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 Chironomns, 

 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 previously to assuming the 

 pupa, and intended for their protection in that defence- 

 less 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 description 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 ac~ 



* Reaum.iii. 179. 



