1912] Homologies in the Wing-veins of May-flies . 97 
M;; trachea has accessory branches, (Pl. V, Figs. 1,3). These 
are lost and the M; and M, trachea course toward the margin 
as simple parallel trachee (Pl. V, Fig. 7). Later the M, trachea 
becomes reduced (Pl. V, Fig..9) and ultimately disappears. In 
Hexagenia (Pl. V, Fig. 13) both the M; and M, trachee have 
disappeared and a secondary trachea has been developed which 
lies in the accessory vein between vein M; and My. A variation 
of this occurs in Ephemera where the secondary trachea springs 
from the M; trachea (Pl. VI, Fig. 17). In the wing-pads 
which follow, the M;+, trachea has either nearly disappeared 
(Leptophlebia and Siphlurus, Pl. VI, Fig. 23, Pl. VII, Fig. 29), 
orit has become entirely obliterated (Leptophlebia and Callibetis, 
Figs. 23, 31). In the former cases it is visible in very clear 
preparations as a small but very distinct trachea lying in the 
base of vein M3-++s. The veins deserted by this trachea are 
thoroughly aerated by a network of small branches from the 
trachez ibetore and behind. .(Pl: VI, Figs, 23,25; Pl: VII; 
Figs. 29, 31).. Sometimes variable secondary tracheze. from 
either side (see Blasturus Pl. VI, Fig. 27, and Blasturus PI. 
VII, Fig. 40, another. specimen) aerate the region between vein 
M; and M4. 
The climax of the changes in the aeration of the medial 
region is illustrated by the wing-pads of Siphlurus and Calli- 
betis, (Pl. VII, Figs. 29, 31). In the former but two strong 
trachez remain, the M, trachea with its apparent continuation 
and the M, trachea. In Callibetis only the My, trachea is 
still strong. All the other trachee in the wing-pad are weakened. 
The Radial Sector. 
In the preceding description no mention of a radial sector 
has been made, the radial trachea being described as an un- 
branched trachea and the vein R as an unbranched vein. It is 
strange that so important an element should be lacking in a 
wing where the venation is not greatly reduced. 
Between M, and M:z there is an undetermined vein which 
may be an accessory vein or may be the radial sector in an 
unusual position. This second possibility becomes a very 
strong probability when we consider what has happened in the 
Odonata. As has been conclusively shown (Comstock and 
Needham ’98-’99) and (Needham ’03) an actual switching of 
the sector trachea there takes place. 
