470 HABITATIONS OF INSECTS. 



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 baUasted 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 Avith the 

 greatest regularity we often see attached a seemingly 

 superfluous piece of wood, leaf, or the like. 



A larva of one of the aquatic TipididcE lives in cases 

 somewhat similar to those of some Phryganece. Se- 

 veral 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 Chirononuts, 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 larvas prepare for themselves previously to as- 

 suming the pupa, and intended for their protection in 

 that defenceles 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 



* Rcaum. iii. 179. 



