98 _Annals Entomological Soctety of America [Vol. V, 
In the dragon-flies (Anisoptera) all stages of this switching 
are shown. In very young nymphs of dragon-flies the trachea 
are all separate and in their usual position (Fig. 3A). 
AS 
Fig. 3. Two stages of nymphal wings of dragon-fly Gomphus descriptus, 
after Needham, showing change in position of sector trachea. 
In the next stage the radial sector trachea has migrated 
across the M, trachea and reaches the margin between the 
M, and M, trachee (Fig. 3B). This modification is carried 
still further in the mature stage where the radial sector is 
between the M. and M; trachea. In the adult wing the place 
where the radial sector crosses over to M;, is always marked 
by an oblique cross vein. 
In the damsel-flies (Zygoptera) the Rs trachea is always 
attached to M;. There is no connection thus far found between 
the radial trachea and its sector which is completely stranded 
upon the M, tracheae. In the adult wing an oblique cross vein 
marks the point of crossing over of the sector in only a very 
few genera. 
In May-flies this trachea is one of the most constant features 
of the tracheation. The vein which follows it is likewise con- 
stant in the adult wing. In one species of this series an actual 
crossing of a strong branch of radius across the M, trachea has 
been found (Fig. V, Pl. 5). A large number of the wing-pads 
of this species were examined. Half of the wing-pads showed 
the radial branching just described and half of them gave no 
sign of it (Pl. VII, Fig. 41). An actual connection between the 
R and the Rs trachea cannot be shown by constant structures. 
However, May-flies and dragon-flies are closely allied groups 
and their general tracheation is similar in many points. Further- 
more this condition of the radial sector trachea is exactly the 
same as that just described in the damsel-flies where there can 
be no doubt that such a crossing has taken place. It is, therefore, 
