EXTERNAL AKATOMT. 



15 



the great majority of genera in Tipulid-si, but occasionally closed 

 (Mongoma) by the turning down of the 5th vein at its tip, 

 meeting the 6th vein before tlie border of the wing. When open, 

 therefore, it runs the whole length of the wing from the base to 

 the wing-margin. Posterior to the 6th vein is the axillary cell. 

 In TiPULiD^, in which the 7th vein is nearly always complete, 

 that is to say, attains the margin of the wing, there is of course 

 yet another cell — the last — in the hind angle of the wing. lu 

 those families of Diptera which have the 7th vein incomplete, all 

 the space between the 6th vein and the hinder angle of the wing 

 is considered the axillary cell (called by some of the older authors 

 the " spurious '^ cell, presumably on account of its ill-defined 

 nature). Some authors speak of a 1st and 2nd anal cell, plus an 

 axillary cell. Where there has been occasion to mention this 

 ultimate cell specially as such, it is herein called the 2nd axillary 

 cell, thus retaining only one anal cell in the wing.* 



Wing pubescence. — A note may be made here regarding the 

 terms " wings bare " or wings pubescent " in this family. It is 

 probable that under very high microscopic power every wing will 

 be seen to possess extremely minute stiff hairs, but when such 

 are wholly invisible to the naked eye or to an ordinary entomo- 

 logical hand-lens, the wing is considered bare, or, as some authors 

 have termed it, glabrous ; and generally it is more or less 

 iridescent. These microscopic setae are therefore never regarded 

 as pubescence. When the naked eye or a hand-lens reveals 

 distinct hairs on the veins or on the surface of the wing itself, 

 the wing is considered pubescent. Practically all the veins bear 

 microscopic short stiff hairs, but if these are not clearly seen 

 without a microscope, the veins are not termed pubescent or 

 bristly. 



Fig. 4. — The basal portion of a fly's wing. 

 a, axillary lobe ; h, alula ; c, antitegula ; d, tegula. 



The alulce, tegulce (or squama;), and halteres. — The basal corner 

 of the hind margin of the wing is often well developed, at times 

 forming quite a projecting angle, and this part is known as the 



* In the case of Vtychoptcra, the 6th longitudinal vein being absent, there 

 are only two cells altogether between the 5th vein and the hind angle of the 

 wing, namely the anal and axillary cells. 



