96 Annals Entomological Society of America [Vol. V, 
which follow it the progress of the medial trachea from a gen- 
eralized to a specialized condition is marked by a general devel- 
opment of small tracheoles which aereate this region. This 
has resulted from the reduction of the main trachez. Speciali- 
zation is also marked by an increasing importance of the fore- 
part of the medial trachea. In this the accessory trachea takes 
no part, but disappears entirely. The reduction of the tip of 
the M, trachea has taken place because that region is so well 
aerated by. the radial trachea, (PIV) Wigs: soo, (non le 
When the radial trachea is greatly reduced (PI. V, Fig. 13, Pl. VI, 
Figs. 15, 17, 19, 21, 238, 25, 27) the vein M, contains a trachea 
throughout, although in more generalized forms the terminal 
portion is wanting, or its area is supplied by tracheoles, (Pl. VII, 
Pigs.<29 730), 
When the costal and sub-costal trachea are also reduced as 
they are in Choroterpes, Leptophlebia, Blasturus, and Calli- 
beetis (Pl. VI; Figs. 23, 25, 27, Pl. VII, Fig. 31) the whole front 
of the wing is dependent upon branches from the M, trachea. 
With this increase in function the M, trachea usually becomes 
proportionately larger (Pl. VI, Figs. 23, 25, 27) or-it gives 
place to a mesh work of tracheoles (Pl. VII, Fig. 31). | 
The Accessory, (1) disappears early in this series. In 
Chirotonetes it extends to the margin; in Heptagenia (Pl. V, 
Fig. 3) it sends a branch over into the tip of vein Ms. In 
Epeorus (Pl. V, Fig. 7) it has become greatly shortened, and 
only. its stump is left in Iron (Pl. V, Fig. 9). The vein which 
succeeds it 1s one of the most prominent accessories in the 
May-fly wing. In most of the wing-pads this vein is aerated by 
branches from the trachee before and behind it (Pl. V, Fig. 
£3; Pl VA, Bigs ola, iia 
The M, trachea maintains its full length and gains import- 
ance as the trachee near it become reduced. In Chirotonetes 
there is no need for the short posterior branch which it bears, 
but in Heptagenia (Pl. V, Fig. 5) a branch in approximately the 
same position aerates vein M3; and an accessory. This function 
is similarly performed in Choroterpes and Blasturus, (Pl. VI, 
Figs. 25, 27) and by means of tracheoles in other wing-pads 
CPI Wi tiossils, 17) 
If the course of the M3;-+, trachea be followed through this 
series it will be seen that there is a continuous reduction of this 
trachea which ends in its complete obliteration. At first the 
