The Crane-Flies of New York — Part II 801 



of the black fly (Simuliidae) , Both larvae and pupae spend their entire 

 lives in cases on stones in water — usually in running, well-aerated water, 

 and often in the most rushing torrents. 



The larvae, as already stated, lack spiracles, the entire respiration being 

 carried on thru tracheal gills, four in number, and the rich tracheal 

 development in the elongate caudal lobes. Thus the tracheal system is 

 truly closed, and represents the maximum of specialization in the reduc- 

 tion in size and final loss of the spiracles. In air-breathing, terrestrial 

 forms, the spiracles are large and situated comparatively close together, 

 gradually becoming smaller and more removed from one another as the 

 creature becomes more and more dependent on blood gills or tracheal 

 gills for respiration. This is the only truly closed tracheal system known 

 to the writer to occur in the Tipulidae. It should be noted that the loss 

 of the spiracles is accompanied by great enlargement of the gills and the 

 taking on of the gill function by the two caudal lobes. The haunt of the 

 larvae is in silken cases on rocks, often in the swiftest part of the stream, 

 where it is impossible to breathe thru spiracles and where the associated 

 forms of life (Ephemerida, Plecoptera, Trichoptera, and Diptera) all, 

 or practically all, depend entirely on gills for respiration. 



As a rule, the larval cases are made on rubble or rounded stones, a 

 crevice or a groove caused by inequalities of the rock surface covered 

 over being the simplest and commonest place chosen. The inequalities 

 in the rock are bridged over by a silken, mud- or silt-coverod case, which 

 is very delicate and laterally fimbriated with the young larva but becomes 

 much firmer, thicker, and more compact with the older larva and 

 pupa. The larval case is open at both ends and the larva passes back- 

 ward and forward freely, showing considerable agility when disturbed. 

 When the larva is still small, the case is correspondingly small and 

 insignificant; but the case of the matured larva is conspicuous, measur- 

 ing from 4 to 5 centimeters in length and about 1.2 centimeters across 

 the lateral " wings," or fimbriations. The insect moves freely along 

 the tube but is very loath to leave it unless actually ejected. As 

 stated above, the late larval and pupal covering is very different from the 

 flims3% silt-covered tube of the young larva, being smooth, compact, 

 hard, and often covered with pebbles. The pupa has two powerful hooks 

 at its caudal end, enabling it to fasten to the case. In most cases the pupa 

 hangs with the current, head downstream, like the pupa of Blepharocera 



