EXTERNAL ANATOMY. 13 



though placed at various angles to the two veins it connects. 

 Occasionally (in Mongoma, for instance) the 5th vein bends down 

 at the tip, meeting the 6th vein and thus closing the anal cell, 

 instead of running to the margin of the wing as in most genera. 

 The 5th longitudinal vein, with the 4th, the 6th, and 7th all 

 spring from the base of the wing. The 6th longitudinal vein lies 

 posterior to the 5th and is normally straight or only gently 

 curved;* the 1th longitudinal, being the last vein, usually parallel, 

 or approximately so, to the length of the wing. 



In TiPtiLiDJE, the 5th, 6th, and 7th veins are never forked. 

 The 7th vein varies in length and direction ; in the PIYCIIOPTE- 

 RI:N^E it is comparatively short, taking a distinct downward curve 

 at the tip to meet the wing-margin. In Trichocera the same 

 thing occurs, only the vein is much shorter still, so short as to 

 be easily overlooked altogether. In other genera, Mongoma for 

 instance, the 7th vein, though much shortened, is only gently 

 curved. 



The term central cross-veins was sometimes used by Osten 

 Sackeri to designate the veins between the end of the praefurca 

 (transversely across the wing's length) and the posterior cross- 

 vein ; in this case the short basal section of the 3rd vein, the basal 

 part of the fork of the 4th and both sides of the discal cell would 

 be included in addition to the two cross-veins proper. t 



Not infrequently a genus or species is distinguished by the 

 presence of an extra vein which is constant in its occurrence. Such 

 veins are called supernumerary. When they occur abnormally in 

 individual specimens (often in one wing only) they are called 

 adventitious, and such occurrences are quite frequent, especially 

 in the ERIOPTERINI, in which their unexpected presence causes 

 much difficulty to the beginner. 



The Cells. The recognition of the cells, once the terminology 

 of the veins is mastered, is comparatively easy. 



The first, immediately below the costa, and bounded pos- 

 teriorly by the auxiliary vein, is the costal cell ; that between the 

 auxiliary vein and the 1st longitudinal vein (often difficult to 

 perceive on account of these two veins lying so close to one 

 another) is the subcostal cell. In cases where the auxiliary vein 

 and the 1st longitudinal vein are united (Toxorhina, Styringomyia} 

 the subcostal cell is of course absent. 



The 2nd longitudinal vein always has in front of it the marginal 

 cell. "When the 2nd vein is simple the cell immediately behind it 

 is the si&marginal, but when the 2nd vein is forked there are 

 two such cells, in which case they are called the 1st and 2nd 



* Absent altogether in PTYCHOPTERIN^:. 



t This is rather a vague definition, but so many species have clear wings 

 except for slight infuscations on all the veins or portions of veins that lie 

 transverse to the wing's length, that most authors have adopted it for descrip- 

 tions of certain species at some time or other, myself included. The term 

 should in any case be used with caution. 



