244 Annals Entomological Society of America [Vol. V, 
actual condition, though most of them indicate the media as 
coalesced at base with cubitus. An examination of the figures 
of the pupal wings of Hepzialus thule shows the trachee of media 
lying along side the tracheze of radius in the wing cavities of 
radius. A short distance from the base of the wing the median 
trachea diverges from the radial trachea into a broad vein 
cavity of its own, which passes obliquely across the wing for a 
short distance and then turns toward the margin of the wing 
between and parallel to the radial and cubital vein cavities and 
tracheee. The oblique part of the median vein cavity is the 
anterior arculus (aa). Near the point where the median trachea 
bifurcates, a broad vein cavity joins the median vein cavity and 
passes obliquely, posteriorly to the cubitus. This latter oblique 
vein cavity is not supplied with a trachea. It is the cross-vein 
part of the arculus, the posterior arculus (pa). The posterior 
arculus of the front wing is much longer than that of the hind 
wing. This explains why in adult wings, the media frequently 
appears to be joined to the radius in the front wing and to the 
cubitus in the hind wing. 
There is a great variation in the constituent parts of the 
arculus in the wings of different insects. This is especially 
true in the Diptera. A generalized condition is found in many 
wings, such as those of Tabanus or Leptis where the meaian vein 
joins the middle of the arculus. In such cases, the anterior 
arculus and the posterior arculus are subequal in length. Two 
lines of modification may be developed from this generalized 
condition. The media may migrate along the arculus nearer 
and nearer to the radius until it actually joins the radius. The 
anterior arculus through this migration becomes shorter and 
shorter, with a corresponding lengthening of the posterior 
arculus. When the media joins the radius, the anterior arculus 
is obliterated and the arculus is wholly cross-vein in structure 
or posterior arculus. If the posterior arculus atrophies in the 
descendants of such forms, the radius would appear to arise 
directly fron the radius without an abrupt bend. The second 
condition is found where the media migrates toward cubitus. 
There results a similar shortening of the posterior arculus and 
an elongation of the anterior arculus. This may proceed until 
the media is free from the arculus and coalesced with the cubitus 
as in the wings of Pantarbes or Erax. Whereas the cross-vein 
like structure was all cross-vein or posterior arculus in the first 
