94 Annals Entomological Society of America [Vol. V, 
of the wing-pads examined has there been any indication of a 
splitting of the sub-costal trachea into its two branches Sc; and 
SCo. 
In the first eleven wing pads in Plates V, VI, (Figs. 1, 3, 5, 7, 
9,11, 13,15, 17,7 49)'21) and in Siphlarus ‘(PR Vil; Bis, 29 )ei nie 
sub-costal trachea extends to the tips of the wing-pads. It not 
only aerates its own area, but in some wing-pads it supplies 
the region before (Pl. VI, Fig. 15), and in others the region 
before and behind it (Pl. VI, Fig. 17). The sub-costal trachea - 
of Choroterpes, Leptophlebia, and Blasturus (PI. VI, Figs. 23, 
25, 27) has nearly disappeared from its vein and is replaced by 
branches from the trachea behind it. This condition is similar 
to that which has already been noted in the costal trachea. It 
is a further step in the cutting down of main trachee. 
Radial Trachea. 
The remaining branch of the anterior trunk is a simple 
trachea parallel to the sub-costal trachea. It never deviates 
from its pathway in the radial vein. This is the radial trachea. 
It has been found to be unbranched except in one species of 
Heptagenia (PI. V, Fig. 5) and in only half of the specimens 
of this. 
In all cases except in Heptagenia (Pl. V, Figs. 3, 5) it is dis- 
tinct from the medial trachea throughout its course. In 
Heptagenia both divisions of the main trunk have coalesced at 
the base so that the radial and medial trachez appear to arise 
from the same stem (Pl. V, Fig. 3, 5). 
The development of the radial trachea is variable and its 
length has important effects upon the aeration of the region 
behind it. In the more generalized wings (Pl. V, Figs. 1, 3, 5, 
7, 9, 11) it extends to the apex of the wing-pad. Ample aeration 
of the wing-tip is thus provided. In the more specialized 
wing-pads the radial trachea extends only through the basal 
trd (Pl. V, Fig) 13; "Plo Vi, Pigs: 5,717; 19), 21,27), ronnie 
almost disappeared (Pl. VI, Fig. 25). 
A progressive development of fine tracheal branches follows 
the weakening of the radial trachea. When the trachea is 
reduced its area is aerated by fine branches from the trachee 
before and behind it (Pl. V, Fig. 18, Pl. V1, Figs: 15; 17, 25, 27, 
Pl. VII, Figs. 29, 31). Thus the place of 4 main trachea is 
again taken by secondary branches. 
