256 
Psyche 
[September 
RS arises either from R or oftener from M. As well 
known, RS and M are tracheated in the may-flies in the 
majority of cases from one main trunk, which turns away 
from the side trachea (in the body), independently from R. 
This condition and perhaps also the fact that RS oftener 
“turns away” from M, gave Anne Morgan the motive to 
assert that the complicated vein which Comstock originally 
(4) indicated as RS, is actually M. We shall return below 
to the explanation of this moving off of RS from M, but 
now let us turn to the following veins. 
Cu arises near M and soon divides into two main 
branches; Cui and Cu 2 , with an inserted middle branch 
which joins at the base either to Cui or Cu 2 (“inserted 
sector”). In all Ephemeroidea (Palingeniidse, Ephem- 
eridse, Polymitarcidse, Potamantidse) , Cu 2 forms a down- 
ward curve similar to the curve of the cubitus in dragon- 
flies. By means of its projecting angle, this curve approaches 
and often completely fuses with A x . The cross-veins 
disappear between the very origins of Cui and Cu 2 ; on 
account of this, in my mind, they correspond morpho- 
logically as well as physiologically to the triangle of the 
dragon-flies (Anisoptera) . In Heptagenoidea and Bsetoidea 
this curve is absent (except in Bsetisca, Oniscigaster) , or 
it is expressed very faintly. The basal bifurcation and the 
general configuration of the origin of the cubitus in Ephe- 
meroidea are certainly secondary if compared to, e. g., their 
condition in Dictyoneuridse. The condition in Siphluridse 
(and perhaps in the Ephemerids of the Permian) is there- 
fore less changed. 6 The peculiar condition in Ephemeroidea 
originated in the receding of the furcation point towards 
the base (this is a very common condition among the Palseo- 
dictyoptera, and in Ephemerids it unquestionably took 
place in RS), and by forming the curve in Cu 2 . This con- 
nection with A 1? just as that in the dragon-flies, has unques- 
tionably mechanical advantages, although the method itself 
and the original causes of the curve in Cu 2 are not clear to 
us. It is difficult to say yet if one can see in our Cui and 
6 However, it is possible that in some Siphlonuridas the curve of 
Cu 2 was lost secondarily; the condition in Oniscigaster is especially 
suggestive of this. 
