244 NATURAL HISTORY OF AQUATIC INSECTS CH. 



find one in which some shred does not spoil the look 

 of the sheath by projecting beyond the rest. This 

 irregularity, as we shall presently see, is really 

 necessary to the complete suitability of the sheath. In 

 the cases of the larvae of Phryganeidae the bits of 

 leaves are arranged with great regularity in a spiral 

 manner. (See Fig. 82, I, p. 256.) 



" A larva sometimes picks up a reed, split into two. 

 If its sheath is entirely made up of small bits, and 

 therefore wants firmness, it will fasten these bits of 

 reed to the outside, bringing them as close together 

 as possible. Some sheaths are made up of such 

 fragments of reed ; in others small straws, such as 

 those of Equisetum, are arranged transversely to the 

 axis of the body [Limnophilus rhombicus], so that 

 the cross-section shows a circle within a polygon, the 

 angles of the polygon being produced. Such cases 

 are naturally very rough, but they often exhibit a kind 

 of symmetry. 



" Lastly there are sheaths which are built up, partly 

 longitudinally and partly transversely, the fragments 

 bcino; ill-arranged and wanting in symmetry. Here a 



O O O J * 



large irregular piece of wood has been fastened on, 

 there a bit of stone, or it may be a shell. Some are 

 entirely made of bits of shell, all the shells belonging 

 to one species. [This is often the case with Limno- 

 philus flavicornis.] I have seen some which are made 

 up of little bits of the shells of water-snails, and others 

 made of bivalve shells, the t\vo valves being still 

 united. Singularly enough, they are sometimes built 

 up of living animals. What should we think of a 

 savage who should cover his body, not with furs, but 



