1912} Homologies in the Wing-veins of May-flies 93 
Peculiarities of May-fly tracheation. 
If the tracheation of May-flies (Fig. 1) be compared with 
the most generalized types of tracheation in other orders several 
striking peculiarities will be observed. 
The radial trachea instead of showing its typical five parts 
is usually destitute ofa sector (cf. Pl. V, Fig. 5 with Figs. 1, 3, 
7, etc). 
The medial trachea has its characteristic four parts 
(Pl. V, Fig. 1) but the M;, trachea bears a branch on the posterior 
_ side (labelled R, ? in the figures) in whichit appears to terminate. 
If the tracheation in the consecutive figures of the wing- 
pads in Pls. V, VI, VII, be now examined important differences 
will be seen. The series shows a continuous reduction of large 
tracheae and a replacement of them by small tracheal branches. 
A gradual evolution in the tracheation is thus suggested. An 
evolution by reduction, which has left some principle trachez 
so reduced as to be hardly recognizable but still holding their 
proper places. 
The Costal and Sub-costal Trachee. 
The usual course of the costal trachea has been already 
described. Whenever present in well developed wing-pads it 
lies without exception in the developing vein which forms the 
front margin of the wing-pad and which is universally con- 
sidered to be vein C. 
This trachea has been found present as a short, delicate 
branch in the wing-pads of all but four genera, (Hexagenia, 
Polymuitarcys, Ephemera, Ephemerella, Pl. V, Fig..138, Pl. VI, 
Figs. 19,.21, 27). In one, (Ephemerella) this absence may have 
been due to the rather poor material, but in the others, examin- 
ations of many specimens failed to show its presence. The 
wing-pads of Chirotonetes, Heptagenia, Epeorus, and Iron 
(Pl. V, Figs. 1, 3, 5, 7, 9,) show a continuous reduction of the 
costal trachea. In Chirotonetes (Fig. 1) its branches thor- 
oughly aerate the base of the costal region. In the succeeding 
wing-pads its diminished branching makes the costal trachea 
less and less important in the aeration of this region. Its work 
is carried on by branches which spring from the trachea behind it. 
The sub-costal trachea is a single usually strong trachea 
which is parallel to the margin of the wing-pad. It lies in the 
longitudinal vein posterior to vein C, (Pl. V, Fig. 1). In none 
