40 Annals Entomological Society of America [Vol. V, 
Specimen VIII which was seven mm. long in November 8, 
metamorphosed three months later. 
Specimen I, which was one of the smallest obtained in 
November, was apparently an offspring of the last adults of the 
season, which metamorphosed in the latter part of August. On 
further plausible supposition, that it hatched the first part of 
September, specimen I was about three months old on Decem- 
ber third, when it was one and one-half mm. long. Adding to 
this the time it required specimen II to become two and one-half 
mm. long, and specimen III to attain three mm. and so on, we 
have a total of nineteen months, an apparent gain of five months. 
This gain, however, is easily accounted for owing to the fact 
that the specimens had the advantage of wintering over in the 
aquaria. Such a gain actually took place in the larger speci- 
mens, which metamorphosed in January, February, and March, 
and no doubt would not have metamorphosed until June, July, 
or August of the following summer had they remained in their 
natural habitat. 
The members of this species do not all emerge in the same 
day or few days, as is true in many other Ephemeridz, but 
adult specimens may be collected near Lake Mendota any time 
from the latter part of June to the latter part of August. The 
emergings are most numerous in the afternoon. The nymphs 
crawl up on the rocks, a split appears in the median line of the 
mesothorax which quickly extends through the pro and meta- 
thorax. The head appears first and then the thorax, closely 
followed by the first pair of legs. A few jerks cause the extri- 
cation of the wings and a moment later they become erect. The 
other two pairs of legs are pulled out about the same time and 
soon the entire subimago is exposed. After a short rest the 
insect flutters upward and usually settles a short distance away. 
This whole process is completed within five or six minutes. On 
several occasions, when the lake was quiet, I observed the entire 
emerging process take place at the surface of the water. The 
sub-imago skin is usually shed within a few hours after the emer- 
gence, and the entire life of the adult is comparatively short, 
lasting about six days at the most and many of the specimens 
live a much shorter time. 
I wish to express my thanks to Prof. S. J. Holmes for his 
suggestions and kind criticisms. 
Zoological Laboratory, University of Wisconsin. 
