1930] Wing Venation of the Odonata and Agnatha 273 
lost its base. This condition was carried over into the 
nymph, where in general there was at once laid down the 
foundation of imaginal venation. Such convex veins had to 
acquire a small tracheal supply by some other means. The 
small tracheoles in may-flies and dragon-flies (in nymphs 
and young imagines) frequently separate from the longi- 
tudinal trunks, and by means of the cross-veins usually 
reach the neighboring trunks. When the basal tracheal 
trunks began to weaken the small side branches of the 
neighboring trunks began to strengthen in their place, and 
finally to replace them completely in function. Because of 
causes which we cannot consider further, one or the other 
branches strengthened to the disadvantage of the others; 
and sometimes the tracheation, chiefly in one trachea which 
arose either from a neighboring or a remote trunk, ap- 
peared to be the most preferable. Since the strengthening 
of some of the small tracheae depends upon a thousand small 
causes, because of their multitude and original similarity, 
even in the development of one individual, it is natural 
that the secondary tracheation had to be a very diverse 
and variable one. Under such conditions in the interpre- 
tation of the venation one should not attach any impor- 
tance to the fact that in a certain species a vein receives 
its secondary tracheae from neighboring trunks and not 
from the side where it should be. Such appears to me 
the origin of the secondary tracheation of may-flies; and 
if this idea is correct for the may-flies, then it is also 
correct and entirely applicable to the dragon-flies. 
In dragon-flies the whole character of the tracheation of 
the wing in its relation to the venation maintains in general 
the same character as in the may-flies, since the dragon- 
flies have much less diversity and much more stability in 
tracheation than the may-flies. The tracheation of R is pre- 
served everywhere, which is comprehensible to a certain 
degree if we consider the shortening of the subcosta in 
the dragon-flies. The tracheation of M is also preserved, 
although in the Zygoptera it is sometimes greatly weak- 
ened. The convex RS 2 , RS 4 and A 4 obtain a secondary 
tracheation by small branches from the neighboring 
tracheae; the cross-tracheae are of course supplied by the 
small supplementary distal, longitudinal veins. The media 
