1930] Wing Venation of the Odonata and Agnatha 277 
mechanical point of view (among the even the main 
branches of M and Cu) ; they have changed their form 
(may-flies) and have acquired (dragon-flies) such impor- 
tant mechanical structures as the triangle and the parts 
adjacent to it, that they have in general become greatly 
mechanized; but by this means we hardly can explain the 
persistence of these two groups, because the wings of many 
extinct groups had also attained that mechanical perfection, 
particularly the Megasecoptera, Protodonata, and many 
groups and families of the Palseodictyoptera. The Megase- 
coptera, as well as the other groups mentioned, advanced 
very far, but nevertheless died out early. Evidently all 
this explanation is insufficient and something else is needed. 
Comparing the wings of may-flies and dragon-flies with 
the wings of the most primitive forms, e. g., Dictyoneuridse 
from one side, and from the other side the Megasecoptera, 
Protodonata and more specialized Palseodictyoptera, we 
find, between the one and the other, distinct differences. 
In the Megasecoptera the venation is so much reduced that 
their wings resemble those of the Dictyoneuridse only 
slightly. 
In the Protodonata the number of longitudinal veins, on 
the contrary, has increased greatly, but the relative dimen- 
sions of the systems of Rs, M and Cu, and the distribution 
of their branches have greatly departed from that which 
we have seen in the Dictyoneuridse. Similar changes in 
dimension and distribution of branches are encountered in 
many Palseodictyoptera. Dragon-flies (especially Anisop- 
tera) and may-flies, on the contrary, differ in that, disre- 
garding the various specializations, they preserved the origi- 
nal relative dimensions of the systems of the main veins 
and the distribution of branches (especially Rs!). The 
Anisoptera preserved their primitive form of wing. Such 
preservation of the fundamental primitive features was the 
reason why the specialization of the wings of dragon-flies 
and may-flies did not become fatal for them, as in the case 
of the former groups. A too rapid specialization , with the 
loss of the original primitive features, as we have in 
the Megasecoptera, Protodonata, etc., inevitably narrows 
greatly the potentiality of further evolutionary modifica- 
tions, and leads such groups to an end of development and 
