270 
Psyche 
[September 
small branches or neighboring trunks nevertheless remain) . 
The methods of tracheation, as it was noted above, are very 
diverse in general and subject in particular to a strong 
individual variation. The one or the other vein is tracheated 
sometimes only by small branches, sometimes also by larger 
ones. The latter penetrate either from the nearer or from 
the further lying trunks, etc. Evidently a series of causes 
not considered here (that is, certain circumstances), influ- 
ence the distribution of smaller branches. Sometimes, how- 
ever, the one or the other method of tracheation is under- 
standable. In a number of forms, as noted, the trachea RS 
does not continue into the distal branch RSi, but turns off 
into RS 3 , and RSi is tracheated by small branches. This 
we will understand if we remember that alongside RS there 
runs the trachea trunk R, which can supply RSi with 
branches. However, when R disappears, the trachea Rsl 
is usually preserved. Morgan, seeking for an analogy with 
the dragon-flies, noted that in one species of Heptigenia 
and only in part of these specimens, the trachese which 
supply RS 3 (=RS? Morgan) separate not only from the 
trunk RS, as usual, but also from R. Desiring to see in 
this the analogy with the Odonata, Morgan concluded that 
this vein is Rs, and that here we have a preservation of 
the crossing of the trachese and veins, which sometimes 
took place in the may-flies and now in dragon-flies. 13 It is 
not necessary to say that such a conclusion, which is now 
supported by Comstock (3), has not enough foundation. 
In all may-flies the concave RS 3 is tracheated in a normal 
way from a concave trunk RS (=M! Morgan and Com- 
stock), as it should be, and only in part of the specimens 
of Heptigenia studied the strongest trachea appears to be 
a branch of R. Taking into consideration the unusually 
great amount of variation in general, and the capricious- 
ness of the tracheae, can one attach such great significance 
to this exception, and conclude that this condition once 
existed in all may-flies, i. e., that in all may-flies RS takes 
root in the region of the media? Evidently such a con- 
13 Only in the Anisoptera. Morgan says that the crossing was 
unquestionably present in the Zygoptera, but that is pure hypothesis 
which has no supporting facts and is more than doubtful. 
