1930 ] Wing Venation of the Odonata and Agnatha 259 
themselves weaken at their origin and appear to be tied to 
the neighboring veins only through common cross-veins; 
and already have no real basal part of their own, nor any 
origin. Such absence of the origin is unquestionably a sec- 
ondary condition. In Palingenia and in some Ephemera, 
RS 4 arises normally from the trunk RS, but in others its 
origin disappears. The same seems to be the case with 
other sectors. In the specialized Bsetoidea this process went 
still further, and in Bsetidse all the longitudinal veins except 
the main ones (Sc, R,* RS,* RS 5 , Mi, Cui, A lf A 2 , A 3 ), lost 
their origins and became “independents,” Woodworth (16) 
and the distal ones became shorter and shorter and deteri- 
orated into a series of small veins along the edge, like the 
measurements on a ruler. How can one explain such a 
change ? It can be satisfactorily explained through 
mechanical and practical means. The longitudinal veins 
lose their origins where they enter by means of their bases 
into the space between the branches of the forking vein, 
which forms a sharp angle, for example, RS or M. Such a 
vein is of course always “convex” if the dichtomating one 
is concave, and vice versa. Because the part of support 
rests always upon the branching vein, the mechanical mean- 
ing of the basal part of the intervening vein diminishes and 
becomes insignificant; consequently the basal part of such 
a vein weakens, becomes thinner and disappears entirely. 
The economy of material requires the disappearance of the 
unnecessary part of the vein. Everyone knows that in the 
more specialized and (so to speak) “mechanized” wings, as 
those of the Bsetidse, all superfluous veins disappear, and 
the remaining ones tend to keep the intervals between 
themselves equal. If the origins of the “inserted sectors” 
remained, this would be an injurious accumulation of veins 
in different places. The disappearance of the base also 
requires the alternative distribution of veins in different 
planes. If the vein is “concave,” and the “inserted” vein, 
e. g., RS 4 , is “convex,” then near the union with RS the short 
and weak part of the base would have a broken-up aspect 
(from the convex side to the concave). The mechanical 
meaning of this broken-up part is insignificant, therefore it 
Misprinted Ri, RSc in original. 
