562 



AN INTRODUCTION TO ENTOMOLOGY 



decorates the parchment with filaments of Spirogyra, arranged 

 concentrically over the sides in a single ex- 

 ternal layer." (Needham and Lloyd.) Wlien 

 moving about, the larva usually drags its case on 

 one edge. There is one species, Ithytrichia conjiisa, 

 which cements its case firmly to rocks in flowing 

 water. These cases are common ; they are parch- 

 ment-like, elliptical, with a small opening at each 

 end (Fig. 691, 2), and measure from 5 mm. to 

 6 mm. in length. They are incomplete, being ce- 

 mented along the edges to the rock, with no floor 

 below the larva. The larva is very remarkable in 

 form (Fig. 691, i). When feeding, it protrudes 

 the narrower part of its body from its case and 

 gathers food from the surface of the rock; the 

 expanded abdominal segments are much wider 

 than the openings in the case. 



Family HYDROPSYCHID^ 



The family Hydropsychidas of the older authors 



Fig. 691. —Ithytrichia ^^^ ^gg^^ divided into four families by Uhner,— 



2! case.' '' (After Hydropsychidcs, Philopotamid^, PolycentropidcE, 



Lloyd.) and Psychomyidae. It is to this group of families 



that the net-spinning cadd ice-worms belong. 



The best-known of these are species of the genus Hydropsyche, the 



nets of which have been described by many writers. 



The larvee of Hydropsyche live only in rapid streams and on the 

 wave-beaten shores of lakes. They are campodeiform, and do not 

 build portable cases, but live in tubes composed of silk and debris, 

 and fastened permanently in place; sometimes they establish them- 

 selves in old worm-holes in submerged 

 wood. The most striking feature in 

 their habits, however, is the fact that 

 each one builds a net for the capture 

 of its food. This net is built adjacent 

 to the tube in which the larva lives; it 

 is funnel-shaped and has at its down- 

 stream end an opening in which is built 

 a strainer. This is a beautiful object, 

 consisting of two sets of regularly 

 spaced strands of silk extending across 

 the opening at right angles to each 

 other (Fig. 692). These nets are often built in crevices between 

 stones ; but fully as often they are built up from a flat surface, as on 

 the brink of a waterfall. In this case they are in the form of semi- 

 elliptical cups, which are kept distended by the current. Much of 

 the coating of dirt with which rocks in such places are clothed in 

 summer is due to its being caught in these nets. Sometimes when 



692. — Net of Hydropsyche. 



