1912] Homologies in the Wing-veins of May-jlies 101 
the wing-pads examined and the second in three (Pl. V, Fig. 5; 
Pl. VI, Fig. 17, Pl. VII, 31). All of the anal tracheze have 
been nearly always found in recently emerged sub-imagoes. 
Replacement of main tracheze by small branches does not 
occur in the anal region. As might be expected, the burden 
of aeration does not fall here but in the middle region 
of the wing-pad. 
The Tracheal Stem. 
As already stated, the single’ tracheal stem of May-flies is 
similar to that of no other order, those of other insects as far as 
known having a dorsal and ventral root (Fig. 5, A. a, b). 
Fig. 5. Diagrams of Tracheal Stems showing shifting of the cubito-anal trachea. 
A. Tracheal bases in the hypothetical wing of insects (after Comstock and 
Needham). 
B. In the wing-pad of a hypothetical May-fly. 
C. In the generalized wing-pad of Epeorus. 
D. In the specialized wing-pad of Callibetis. 
In these wing-pads the base of the cubito-anal trachea 
makes a characteristic prominent downward loop (Fig. 5, B, C). 
This loop swings the trachea out of the route which it would 
seem naturally to take. It is more prominent in generalized 
than in specialized wing-pads (Fig. 5, of C and D). 
