THE ORDER DIPTERA 39 



towards the hind margin of the wing. Certain short trans- 

 verse veins connect some of the longitudinal veins : between 

 the 3rd and 4th longitudinal veins, in the middle of the wing, 

 lies the anterior cross-vein ; between the 4th and 5 th longi- 

 tudinal veins, in the outer half of the wing, runs the oblique 

 posterior cross-vein ; near the axil of the wing there are three 

 cross-veins — the humeral^ between the costal and subcostal ; 

 the anterior basal, between the 4th and Sth longitudinal; the 

 posterior basal, between the 5th and 6th longitudinal. The 

 spaces or " cells " included between the several veins are all 

 distinguished by special names, as may be followed in Fig. 

 5 : the discal" cell" and its boundaries should be particularly 

 noticed : it lies in the field of the wing, behind the anterior 

 cross-vein ; but in order to understand the classification of 

 flies the arrangement of all the veins and cells must be 

 studied. 



In the wing of the gadfly (Fig. 6), the venation appears 

 to be more complicated. The following are the chief 



Fig. C— Wing of Gadfly (Tdbanua). 



differences : — the costal vein runs all round the edge of the 

 wing from root to axil ; the 3rd longitudinal vein is forked ; 

 the 4th longitudinal forks twice ; the 5th longitudinal takes 

 a zigzag course ; both of the basal " cells " and the anal 

 " cell " are long and large ; owing to the bifurcations of the 

 longitudinal veins the number of "cells" in the outer and 

 posterior half of the wing is increased. 



In the wing of the mosquito (Fig. 7) the costal vein runs 

 all round the edge of the wing also; the ist longitudinal 

 vein is very long ; the 2nd longitudinal bifurcates ; the 3rd 



