1912] Homologies in the Wing-veins of May-flies 95 
Medial Trachea. 
The fullest development of tracheal branches is found in 
Chirotonetes (Fig. 2). The four typical branches of media are 
all present and well developed and there is a large accessory 
trachea attached posteriorly to the M, trachea and smaller ones 
attached posteriorly to M,. These accessories are usually 
wanting and need no further consideration. There is however, 
one peculiarity of the tip of M, which is of great importance 
since it involves the interpretation of the veins in the area 
between veins M, and Me. Here lies the most difficult problem 
in the interpretation of May-fly venation. 
Fig. 2. Tracheation in nymphal wings of Chirotonetes. 
A, B, C—Three early stages. 
D—Late stage showing venation also. 
Trachea M, continues through only half its course (Fig. 2, D). 
An apparent continuation of it turns abruptly rearward and 
lies in the strong oblique vein Rs?. This condition exists in 
mature wing-pads of Chirotonetes and in all the most general- 
ized wing-pads, (Pl. V, Figs. 1, 3, 5, 7, 9, 11). In very young 
wing-pads of Chirotonetes however the M, trachea extends 
through its whole course to the tip of vein M,, (Fig. 2, A, B, C). 
The Mz trachea lies in the corresponding vein. The M3+, 
trachea continues for some distance before dividing. It then 
separates into the two branches M; and My, which diverge 
slightly and proceed to the margin in a parallel course. 
In Chirotonetes small branches are almost completely 
absent from the medial trachez. In the series of wing-pads 
