1 8 DIPTERA OF NORTH AMERICA. [PART IV. 



in the groups in which they prevail ; but their importance arises 

 from their characterizing the great majority of the species : — 



1. In the T. longipalpi, the auxiliary vein ends in the first 

 longitudinal vein, being incurved towards it ; beyond the humeral 

 cross-vein there is no other cross-vein connecting the auxiliary 

 vein with the costa or with the first longitudinal vein. In the 

 T. brevipalpi the auxiliary vein, as a rule, ends in the costa, and 

 is connected by a cross-vein with the first longitudinal vein. 



2. The structure of the cells in the vicinity of the stigma is 

 totally different in the two divisions. The first longitudinal vein 

 in the T. longipalpi-is usually incurved towards the second vein 

 and attenuated in a peculiar manner before ending in it ; an 

 oblique cross-vein connects the first vein, a short distance back 

 of the tip, with the costa; this cross-vein, together with the 

 anterior branch of the second vein, form near the anterior 

 margin a small, trapezoidal cell, very characteristic of the T. 

 longipalpi (it is wanting, however, in Dolichopeza and some 

 related species). In the T. brevipalpi the first longitudinal 

 vein ends in the costa, and the cross-vein, at its tip or some 

 distance before it, connects it with the second longitudinal vein ; 

 no structure like the trapezoidal cell is apparent. 



3. The structure of the discal cell and the direction of the 

 veins surrounding it is different in the two divisions. In the T. 

 longipalpi, the vein separating the two last posterior cells (the 

 posterior intercalary vein of Mr. Loew ; compare Monogr. of 

 N. A. Diptera, I, p. xxiv, fig. 3, v) issues very near the inner 

 end of the discal cell, usually from the angle, between this cell 

 and the great cross-vein ; this, in most cases, gives the cell a 

 pentagonal shape, unless, as for instance in the genus Pacliyr- 

 rhina, the vein has no contact at all with the cell, and has the 

 appearance of the direct prolongation of the fourth longitudinal 

 vein ; in such cases the discal cell is a parallelogram. In the 

 T. brevipalpi the "posterior intercalary vein issues from the 

 latter end of the discal cell, and its origin is quite distant 

 from the great cross-vein, which is usually near the inner end of 

 the discal cell. The Amalopina show some approach to the T. 

 longipalpi in the position of the intercalary vein and in the shape 

 of the discal cell ; still the origin of the intercalary vein in the 

 Amalopina is usually rather distant from the great cross-vein. 

 Amalopis vernalis 0. S., is the only species which, in this 



