876 



THE AMERICAN NATURALIST 



[Vol. XL 



the unbranched filaments of Simulium meridionale Riley, and the 

 branched structures of Simulium. pecuarum Riley. Curiously 

 enough, although figuring the gills in their correct position, Riley 

 states in his general discussion that they occur on the under side 

 of the body — a statement made by Osten Sacken sixteen years 

 earlier, although even he contradicted himself by copying the fig- 

 ures of Verdat. In addition to these there have been figured, by 

 Meinert ('86), who refers to them as anal papillae, the unbranched 

 gills of Simulium ornatum Meigen, by Townsend ('93) the branched 

 type of a species of SimuUum from the Grand Canon of Colorado, 

 by Lugger ('96) the simple type of Simulium tribulatum Lugger, 

 and by Johannsen (:03) the unbranched type of Simulium hirti- 

 pes Fries. 



]\liall and Hammond seem to have been the first to call attention 

 to the fact that these structures are retractile into the rectum, a 

 fact which may be verified by study of living, or better, sectioned, 

 specimens. 



Most writers agree that these filaments are gills but there is a 

 divergence of opinion regarding the method by which they func- 

 tion. Planchon ('44), Osten Sacken ('70), Riley, Townsend, and 

 Comstock ('95) considered them as tracheal gills, Osten Sacken, 

 Riley, and Osborn even stating that they connect with large inter- 

 nal tracheae. Miall and Hammond, and Johannsen, on the other 

 hand, regarded them as blood gills and without tracheae. Such is 

 the condition of our knowledge concerning this mode of respira- 

 tion of Simulium larvae. No connected work on the finer anatomy 

 of their gills has been published. Such mention of the subject as 

 occurs is confined to the gross anatomy and is scattered through 

 accounts of life histories and descriptions of species. 



In undertaking a more detailed study of the subject as exempli- 

 fied in SI)iniUiim pirfiprs, both living and sectioned larvae have 

 been exainiiu'd. For the study of the trachea? found in the region 

 of the gills it is very hel])fiil to examine living larva-, as in fixed 

 material tin- lack of air in these passages makes it hard to see their 

 finer branches. Most of the histological details were, of course, 

 gained through a study of sections. To prepare these, larvie were 

 killed and fixed in Flemming's mixture, in hot and cold absolute 

 iilcohol, and in hot and cold Gilson's fluid. The hot alcohol and 



