274 
Psyche 
[September 
in the Zygoptera is supplied by its own trachea as well as, 
to a greater or less extent, by the small branches of the 
neighboring veins. RS 2 , together with a series of other 
veins in the region of Rs and M, are supplied by small 
tracheoles (see fig. 9), but in Zygoptera the vein RS 4 is 
tracheated in this way at its basal part, while in its distal 
part usually (but with exceptions) by one fairly large 
branch which arises from RS 3 (fig. 9). In the Anisoptera, 
this same RS 4 receives its trachea from R 4 , and even the 
basal part of this vein (the “bridge”) is tracheated chiefly 
by the branch of the tracheae directed posteriorly. Comstock 
and Needham conclude from this fact that the vein RS 4 is 
actually RS, which takes root in the region of the media; 
and they extend this conclusion to the Zygoptera, in which 
this type of tracheation of RS 4 was never observed. From 
my historical point of view, such a tracheation of RS 4 , and 
in particular the difference in both suborders, becomes 
generally clear. The convex branches of RS and also A 4 
for the same reasons as in may-flies lost their original and 
normal mode of receiving trachese and acquired a secondary 
tracheation at the expense of the ever-increasing small 
branches from the neighboring trunks, which used to enter 
here before. Such a method of tracheation was preserved 
in the basal part of RS 4 in Zygoptera, but in the greater 
distal half one treachea which separated from RS 3 became 
predominant. This acquisition of predominance by one or 
several branches in dragon-flies as well as in may-flies, is 
often found in the distal longitudinal veins, and there is 
nothing surprising that one of the trachese strengthens at 
the expense of the other in RS 4 . Originally, when the 
present-day type of tracheation was only beginning to 
evolve, the greater diversity probably took place here, as 
in the may-flies. Having lost its tracheation, Rs 4 received 
at first the tracheoles from the neighboring trunks as well 
as from R, and in the same group some tracheal branches 
obtained predominance ; and in other groups,, other branches, 
etc. The tracheation of RS 4 in the Anisoptera from R was 
at last established (why, is another question) ; in Zygop- 
tera, partly from RS and partly by the mere preservation 
of an earlier means of supply by small tracheoles (in the 
basal half). It is difficult to say why the tracheation of 
