1915] Interesting Western Odonata 301 
ney Lakes. Five other species of Aeshna are found in Cali- 
fornia. These are scattered from the sea level up to 5,000 feet 
altitude, but nevadensis is the only form which flourishes at 
the extreme upper limit of 7,000 feet. The adaption, which 
permits this one species to occupy territory so far beyond the 
range of the genus in general, is a change in the time of 
emergence. As far as is known the species of Aeshna emerge 
in the nighttime, an adaption to preserve them from the birds, 
but nevadensis emerges in the day time. At this high altitude 
there are few species of birds that inhabit the shores of the 
lakes, so night emergence is not necessary. Day emergence, 
however, is a necessity as the nightly temperature at this 
altitude is very near freezing, if not actually at times below. 
It is interesting to note here that night emergence in Aeshna 
is a highly specialized habit, as most Odonata emerge in the 
day time, and that nevadensis belongs to one of the more 
generalized or primitive groups of the genus. Perhaps it 
has merely retained the primitive manner of emergence. 
For two western species an interesting form of migration 
was observed. These are Cordulegaster dorsalis and Octo- 
gomphus specularis. The coast mountains of California and 
the western slope of the Sierras contain many perennial torrents, 
which do not rise high enough to contain snow water, yet occur 
in such steep gulches that they are a succession of rushing 
rapids and roaring cascades. These are inhabited by but three 
species of dragonflies, Cordulegaster, Octogomphus and an 
undescribed species of Aeshna. The nymphs of this Aeshna 
are agile, active creatures entirely able to stem the swift currents 
of these torrents, but the nymphs of Cordulegaster and 
Octogomphus are slow and clumsy. Moreover they do not 
live in the tree roots as do the Aeshna nymphs, but in the 
case of Cordulegaster, crawl over the bottom in the quieter 
parts of the pools, and, in the case of Octogomphus, burrow 
thru the organic trash in the deeper holes. Being, as it were, 
loose in the stream these two, during their three-year life, are 
washed farther and farther down stream by each succeeding 
freshet, so that when they come to emerge they. may find them- 
selves several miles below the point at which they hatched. 
This washing down is compensated by a migration upstream 
of the imagoes. On Stevens Creek, south of Stanford’ Uni- 
