the Anatomy of Gcenis macTura. 281 



antero-lateral angles of the foremost segment ; they are 

 erect, two-jointed (?.), subulate, acuminate, and have 

 long fringes. From their position, their minuteness, 

 and the dirtiness of the animal, they are liable to be 

 passed over ; or they might be mistaken for rudimentary 

 hind-wings. The second segment is ebranchiate, and 

 its upper posterior margin is slightly produced, espe- 

 cially in the middle. The next five segments bear 

 pairs of single branchial plates. The first pair is dis- 

 proportionally large, and formed of obtusely oval in- 

 crassated lamellae, which are truncate anteriorly, ciliate 

 at the sides and tip, and conceal the other four com- 

 pletely. The left plate slightly overlaps the other, its 

 truncate edge is applied to the second joining, fitting 

 under the produced edge of the second joint, which keeps 

 it steady during the act of respiration. Each of them 

 is traversed by a longitudinal lambda-shaped crease 

 whose prongs meet the anterior edge, and on the 

 undei'side is membranaceous and permeated by a trachea, 

 which runs at first obliquely inwards, and then length- 

 wise, giving off several rather straight branches in a 

 dendroid manner. The remaining plates are very dif- 

 ferent from the first, being delicately membranous, semi- 

 ovate, and deeply fringed. The trachea divides almost 

 at the base of the gill into about six fastigate, only 

 slightly divided, branches, whose branchlets, without 

 undergoing much diminution of calibre, run up each of 

 them into one of the filiformly dissected fringe-processes. 

 The gills are successively smaller backwards, each in its 

 turn is extensively covered by its immediate predecessor, 

 and the contiguovis fringes of adjacent lamellae are inter- 

 laced with one another in repose. In the action of 

 breathing, the protecting plates are raised slightly, and 

 remain motionless, whilst the other pairs are briskly 

 agitated forwards and backwards. The intrusion of mud 

 between them is efiectually guarded against by the 

 before-mentioned cilia and fringes. The tenth segment 

 bears the caudal setee, which have spreading hairs in- 

 serted upon their joinings in two opposite rows. The 

 ventral ganglionic chord, and the alimentary canal, ofi'er 

 no extraordinary peculiarities. 



Dimensions: — body 6'5 ; setae, med. 3"8, ext. 4. millim.. 



Although foreign to the title of the paper, I may men- 

 tion that the adult insect holds its wings horizontally 

 extended in repose, seldom erecting them ; and also that 

 the forceps of G. halterata, Fab., are apparently jointless. 



