264 
Psyche 
[ September 
certain number of weak branches posteriorly. Next follows 
in the may-flies the concave A 2 , then the convex A 3 . But 
even if these veins did exist for a long time in the dragon- 
flies, they are fused now with A x at the base, and in the 
majority of cases this region has suffered great reduction. 
Usually we still find in the hind wings of the Libellulidse 
two longitudinal veins, still quite distinct, which arise from 
Ai. As we find the suggestion of a fusion of the two longi- 
tudinal veins with A x in the “Protodonata” also, the con- 
ception of the composite nature of A becomes possible. 
We did not finish the investigation of Cu and the triangle. 
The triangle represents a frame which presses apart and at 
the same time strengthens Cu and M. The perfection of 
tins frame in the Anisoptera evidently becomes an acquisi- 
tion of importance and use in the mechanism of the wing. 
We find the formation of the triangle in the Archithemidse 
and Heterophlebiidse in statu nascendi. Its forms here 
were rather diverse, sometimes different in the fore and 
hind wings, and in general these triangles did not corre- 
spond entirely to the triangles of the Anisoptera. These 
groups with the aberrant triangle died out, and only the 
groups with the normal triangle were developed and became 
preserved up to the present time. In the series of Aniso- 
zygoptera-Zygoptera no triangle was formed, but in con- 
nection with this the original form of the wing is also not 
preserved, the venation, changed greatly in the anal area, 
became reduced, Sc became shorter, etc. But originally, 
as we notice in the Lestinse and in the related Anisozygop- 
tera, the distinct projection of Cu (and Ai) was here also. 
The same kind of projection is also clearly expressed in the 
very conservative group Ephemeroidea Ulm. The triangu- 
lar area between the bases of Cu 2 and Cui in the families 
of this suborder also closely resemble the triangle in the 
dragon-flies, only its external side in the former is repre- 
sented not only by one straight vein, but by two cross-veins 
(between Cui and Cuad, Cu 2 and Cuad), which are sel- 
dom placed opposite each other. The “inserted sector” Cuad 
is also often found in the Anisoptera ; it is also well repre- 
