266 
Psyche 
[September 
tion of Cu) begin to form themselves from the veins exist- 
ing there, no matter which ones. It seems to be not an 
impossible idea that the upper side in some groups could 
be the remains of the original part of Cui. Cui, as in the 
may-flies, had to be concave, and this top side is concave, 
changing at the end into a sharply convex M. The fact that 
its end does not reach M would therefore be comprehen- 
sible. On this theory I do not insist because I cannot offer 
any definite facts of proof. 
In the Zygoptera the true triangle does not exist, but in 
the Agrionidse — and they form the main part of the Zygop- 
tera — a structure was formed which in its function, to a 
certain degree, replaces the triangle. This is the “quadri- 
lateral,” which hereafter takes the form of a narrow tri- 
angle. It is very certain that such a structure as it is repre- 
sented in the Lestinse, for instance, does exist, and is indi- 
cated by the fact that from the Anisozygoptera up to the 
present time only such a form as ( Epiophlebia suprestes 
Selys) was preserved, where the “quadrilateral” has a form 
identical with that in the Lestinse. Where the projection 
of Cu was not fixed by the formation at this place of the 
frame of a triangle, or quadrilateral of the type of Lestinse, 
there the existence of this projection, and below the projec- 
tion of Ai, lost its mechanical significance. More than that, 
this projection because of the functioning of this vein would 
be entirely without purpose and even harmful. It is not 
surprising, therefore, that in the Calopterygidse, where the 
“quadrilateral” did not adopt the form as in the Lestinse and 
Epiophlebia, Cu tended to straighten itself out in diverse 
ways ; a tendency which reached its maximum development 
in Calopteryx and related forms. 
Preceding the above discussion, I compared the venation 
of the dragon-flies with the venation of the Carboniferous 
Protephemeroidea (Trilolsoba) and through them to the 
Dictyoneuridse. I referred little to the Protodonata because 
the evolution of their wings was along different lines from 
the dragon-flies. It is true that in part of them two 
branches of M are represented as well as the two branches 
of Cu, which do not exist in the Odonata; but in other 
respects they are more specialized and go further than the 
dragon-flies from such groups as the Dictyoneuridse. One 
