268 
Psyche 
[September 
absence of the trachea in RS 4 of may-flies, and also of a ser- 
ies of distal branches chiefly convex, as in Agnatha, Gdon- 
ata, etc ? Let us try briefly to throw some light on the ques- 
tion; and let us start with the may-flies. 
A characteristic feature of the tracheation of the aniage 
of the nymphal wing 12 appears to be its exceptional 
diversity in different groups and its marked instability, 
which is shown in a very great individual variation By 
studying the tracheation in different groups and comparing 
it with the venation, we observe in the diversity a definite 
correlation with the peculiarities of venation. I base this 
statement upon the work of Morgan, who gave a series of 
illustrations of the trachese of nymphs of different i iay- 
flies, as well as on my observations. Only in some of ne 
more primitive forms and then only in the early stages b: 
the development of the wing aniage , do we encounter, as 
shown by Morgan, in Chiratonetes, for example, the normal 
full tracheation, with the penetration of the trachese into 
all longitudinal veins through their bases. In the later 
stages of development of the same Chiratonetes the part of 
the trachea which goes into RSi (=M 4 Morgan) disappeared 
and instead of this a series of small trachese was directed 
into the vein RSi from the neighboring, stronger trachea 
R. A quite full tracheation exists according to Morgan in 
Heptigenia sp. (P. 5, Fig. 3), but here the tracheation in 
RSi and also in some other small branches has already disap- 
peared. I repeat, we seldom encounter normal tracheation. In 
most of the may-flies we usually encounter a regular alternat- 
ing of veins in which the trachese normally penetrate through 
the bases with veins which have no such trachese, and which 
are tracheated by numerous small branches from trachese 
of the neighboring veins (see e. g., Fig. 11, which illus- 
trates the course of tracheation as in species of Heptigenia 
Ulmer) . Almost always the vein (with a few known excep- 
tions in Chiratonetes and Heptigenia) which is tracheated 
12 If the trafcneoles are distinguished with difficulty in the wings 
of the imagines of dragon-flies, then it is still more difficult in the 
may-flies. However, the characteristic features of the venation and 
tracheation of the adults are already indicated in the nymphs, and 
for that reason we shall investigate the imagines instead of the 
nymphs. 
