AQUATIC MIDGES AND SOME RELATED INSECTS. 1 5 



there may be available. These silken sheets are held extended by the thin branching 

 threads of silk (figs. 36 and 37). The whole aggregation of silk sheets and threads 

 is held under tension by silk layers attached in a spiral position. The supporting 

 threads are then originally used as attachment fibers to hold a section of the tube 

 extended and are later pulled into a position nearly at right angles to the lining by the 

 tension exerted by the addition of another section. In this way the lining or tube 

 appears slung in the centre of a cavity with numerous threads radiating in various 

 directions (fig. 26). 



SILK STRUCTURES. 



The completed silk lining shows relatively little structure as far as the tube itself is 

 concerned, but the supporting lines thrown out when the larva fastens this lining between 

 two parallel glass surfaces are quite interesting. In studying the structure the tube is 

 seen to be of a fairly uniform diameter and to be composed of a thick layer of silk, which 

 shows no definite layers or strands. At intervals the silk is pulled out into conical en- 

 largements. At these points the tube is seen to be made up of more than one layer, for 

 the lining continues straight on leaving a space. The lining is held extended in the 

 form of a cylinder by very interesting branched threads. These threads are often more 

 or less sheetlike next to the tube, but divide and subdivide toward their point of attach- 

 ment where they are much more widely spread out than at their origin (fig. 37). These 

 structures show unquestionably the use of the prolegs, for it is inconceivable that such 

 fine threads often ending in more than one plane could have been attached in any other 

 way. 



The structure of the conical net is not easily made out even under high powers 

 of the microscope, but the addition of powdered carmine to the water passing through 

 the net gives it such a uniform coat that the author is inclined to think the entire 

 structure porous. At times Protozoa and other relatively large organisms are seen 

 to be forced into one of these nets and to escape by a circuitous route, which would 

 suggest a breach between ribbons or sheets of silk. In other cases relatively large 

 gaps, opening directly through one side of the net, are indicated by the escape of 

 particles. When the net is collapsed, as it always is when the larva is not forcing 

 water through it (the condition always existing in stained material), none but the 

 grosser structures are visible (fig. 27). 



The net, as explained above, is spun in sections, but the position of the threads 

 attached to its sides, as well as the observed behavior of the larva, shows these sections 

 to be less regular than one might infer from the previous description. The arrange- 

 ment of the net in sections in a manner similar to that of the lining of the gallery suggests 

 the possibility of narrow slits in its surface of the same nature as those in the attach- 

 ments of the tube (fig. 36). It is probable, however, that the impact of the current 

 is necessary to open them wide enough to allow water to pass through. 



The conical net is spun exceedingly thin, as one would expect from the frequence 

 with which it is consumed and replaced. This is doubtless correlated with the speed 

 of the movements involved in its construction. In fact, it seems reasonable to conclude 

 that the nature of the silk rather than the psychology of the larva dictates the speed 

 of its movements. The spinning of one part of the net upon another in such rapid 

 80285°— 22 3 



