130 
Psyche 
[March 
gence pattern is variable and freshly emerged adults have been col- 
lected throughout the day and even in early evening on cold, cloudy, 
or rainy days (Lutz and McMahan, 1973; Corbet, 1963; Lutz, 
1961; Taketo, i960; Kormondy, 1959; Robert, 1958; Lieftinck, 
1932). The larvae of S. semicircular is, under normal conditions, do 
not leave the water until after sunrise and the tenerals generally fly 
at noon. A similar shift of the emergence pattern to the middle of 
the day has been described for Aeschna interrupta at high altitudes 
in California (Kennedy, 1925) and for Aeschna subarctica on the 
moors near Kiel, Germany (Schmidt, 1968). 
The emergence curve of Somatochlora in Colorado is characterized 
by brevity (16 days) and a high degree of synchrony (Fig. 2, 
Table 1). Similar synchrony has been described for other dragonfly 
species which have been categorized by Corbet (1963) as “spring” 
species. Only one of these species, Oplonaeschna armata, exhibits a 
shorter time for the total period ( 10 days) and for the 50% emer- 
gence time (2 days). Oplonaeschna inhabits the water-pocket streams 
of New Mexico and Arizona and is subject to flash floods and severe 
dessication (Johnson, 1968). The lower altitude corduliid, Tetra- 
gone.uria cynosura, averages a 23-25 day emergence period with 50% 
of the adults emerging by the 6th or 7th day in Michigan (Kor- 
mondy, 1959) and North Carolina (Lutz and McMahan, 1973; 
Lutz, 1961 and 1962). Tetragoneuria in Michigan also exhibits a 
bimodal curve in which a second peak of emergence occurs 10 days 
after the end of the first peak. This bimodality has been described 
for other dragonfly species (see Corbet, 1963) but is missing in the 
Somatochlora curve (Figs. 2 and 3). 
Selective pressures contributing to the synchrony of the single peak 
of the Somatochlora emergence curve may well involve the brevity 
of the season. Both in 1968 and 1969, 10 days after the end of the 
initial emergence peak at a time when T. cynosura (Kormondy, 
1 : 959 ) and Anax imperator (Corbet, 1957 ) normally exhibit a small 
second peak of emergence, the shallow areas at Irwin Pond South 
and Pond Nine had largely dried up. Somatochlora larvae leave the 
water by climbing up the emergent sedges. They do not crawl out 
on the land. Lack of emergence sites or competition for the few 
remaining sites can be expected to be a factor in selecting for a tem- 
poral shift toward early emergence. 
Another contributing factor may involve intraspecific interference. 
The diurnal emergence pattern which is necessitated by the climatic 
character of the habitat creates problems which normally do not affect 
