CHAETOTAXY. 15 



course of the fourth longitudinal vein. The anterior basal trans- 

 verse vein, at the base of the discal cell, is very short, and the 

 portion of the fourth longitudinal vein before the anterior (small) 

 transverse vein is bent downwards in such a way as greatly to 

 narrow the basal half of the discal cell, the width of the distal 

 portion of the anterior basal cell being correspondingly increased ; 

 the fourth vein bends abruptly upwards to the point of contact 

 with the anterior transverse vein, where it forms with itself 

 what is practically a right angle ; from this point it again runs 

 obliquely downwards, and is once more bent upwards at the 

 exact point of contact with the posterior transverse vein, 

 ultimately reaching the margin of the wing some distance before 

 the apex. The first posterior cell is open, but its distal por- 

 tion is narrow and much drawn out. The third longitudinal 

 vein is very close to the second, and shows a marked approxi- 

 mation to the costa, the sub-marginal cell being correspondingly 

 narrow ; the third costal cell is considerably elongated ; the 

 second, third, and fourth longitudinal veins all turn upwards at 

 the tips, and the anterior transverse vein is very oblique ; the sixth 

 longitudinal vein becomes obsolete soon after the anal cell, 

 though in clear, unchitinised form it can still be traced nearly 

 to the margin of the wing. Posterior basal tran verse vein, 

 closing the anal cell, angulate in the middle, so that the distal 

 angles of the cell are acute. Posterior transverse vein slightly 

 curved, bent up somewhat abruptly to meet the fourth longi- 

 tudinal. Wings varying in colour according to the species from 

 light drab to sepia-brown ; membrane of the wing rilled.* 



Chaetotaxy of G LOS sin a. 



Cephalic Bristles. — One pair of vertical bristles of large size 

 (the largest and most conspicuous bristles on the head) ; on the 

 occipital region no trace of the pair of bristles termed by Hough 

 (Proc. Acad. Nat. Sc. Philad. 1898, p. 166) the occipito- central ; 

 greater ocellar bristles (ocellar pair of Osten Sacken) small ; 

 post-vertical pair not differentiated in size from the remainder of 

 the lesser ocellar bristles ; frontal bristles largest below (trans- 



* Cf. Garry de N. Hough, M.D., " Some Muscinae of North America," 

 "Biological Bulletin," Vol. I (1899), p. 20, note 1 :—" These rills are very 

 fine grooves in the surface of the wing, which run in a sort of radiate 

 manner towards the border. They are very numerous. A rilled wing 

 denotes a higher stage of development, a more recent form, than an 

 unrilled wing." 



