38 



NORTH AMERICAN DIPTERA. 



the anterior, small or middle cross-vein fbj, which is very 

 constant in flies, and affords in most cases, a key to the 

 venation, no matter how intricate. It connects with the 

 third longitudinal vein in front (in a few cases the sec- 

 ond longitudinal vein); the cell behind it is the discal, 

 in front, between the second and third longitudinal veins, 

 the first sub 'marginal; on the outer side the first posterior; 

 on the inner side the first basal. Just back of the first 

 basal cell and separated from it by the fourth longitud- 

 inal vein, is the second basal cell; back of the second basal 

 and separated by the fifth longitudinal vein, is the third 

 basal or anal cell. Back of the anal cell and including 

 the free posterior proximal portion of the wing is the ax- 

 illary cell. In the horsefly the anal cell is seen to run back 

 obliquely to near the posterior margin of the wing, where 

 it terminates acutely, that is, the anal cell is closed near 

 the border of the wing; should the two veins that close 

 it run separately into the margin of the wing, then the 

 cell is said to be open. Counting from the third longi- 

 tudinal vein (posterior branch) backward along the pos- 

 terior border of the wing, to the vein that closes the anal 

 cell outwardly, the student will count five different cells, 

 the first of which, as already said, borders on the first 

 basal cell, the second and third on the discal cell, the 

 fourth on the discal and second basal cell, and the fifth 

 on the second basal and anal cells; these cells are called 

 the posterior cells, and are numbered from before back; 

 ih^ first is bounded by the third and fourth longitudinal 

 and the anterior cross- vein; the second posterior by the 

 anterior branch of the fourth vein in front, its posterior 

 branch internally and behind, often superfluously called 

 the anterior intercalary vein; the third posterior cell is 

 bounded in front by the posterior branch of the fourth 

 vein, internally by the posterior cross vein (d), posteriorly by 

 the anterior branch of the fifth vein; the fourth posterior 



