BURROWING MAYFLIES. 273 
absence of adults; and that belated reappearances occurred on the 2d and rsth of 
September. It is not likely that the first great wave arose on the 12th of July with such 
suddenness as Mr. Stringham’s notes indicate; it is more likely that a few adults appeared 
earlier but were unnoticed. 
Mr. Stringham noted the difference in behavior of the adult mayflies accompanying 
changes of temperature, adults when it is warm being able to cling to vertical surfaces, 
when it is cool (as on Aug. 23, minimum temperature 58° F., mean 70° F., some 15° lower 
than during preceding waves of emergence) lying flat upon horizontal surfacesonly. That 
the adults adjust themselves, also, in relation to light (Krecker, 1915) and to wind is 
well known. Desiring to know whether waves of emergence are influenced by local 
meteorological conditions, I requested data from Fred Z. Gosewisch, in charge of the 
Keokuk station of the U. S$. Weather Bureau. He very courteously sent me rather 
full data sheets covering temperature, precipitation, sunshine, winds, etc., and I have 
studied these carefully in relation to the facts furnished by Mr. Stringham, but I have 
not been able to trace any relation between emergence and meteorological conditions 
other than that which goes with the progress of the season. The mean daily tempera- 
ture at Keokuk for the month of June is 72.5° F.; for July, 77° F.; for August, 74.6° F.; 
for September, 66.4° F. Emergence mainly occurs at the hottest part of the season, 
but belated transformations trail along into the comparatively cool weather of early 
autumn. 
PENTAGENIA QUADRIPUNCTATA.4 
JUNE 25.—They are transforming among the timbers of the boom above the lock. At 7.30 p.m. 
saw one leave cast (skin), and I took the insect and the cast. Took another just before it left the cast, 
but it struggled nearly free in the formaldehyde.® Saw more of these this evening than I have here- 
tofore seen this year. 
JUNE 27.—These are becoming more common. On a window sill over the water outside the com- 
pressor room, eastward of the lock, within an area of 114 square meters, I counted 31 of them at 6.15 a. m. 
Other windows similarly covered. At 9.30 a. m., on the entrance door of the power house (area about 
3 Square meters), there were 53 of them. They were much thicker on this black iron door than on the 
dirty white concrete all about. 
JUNE 29.—Less abundant. 
JuLy 1.—To-night large numbers about the power house; also about the dam. 
JuLy 4.—On door of power house and on adjoining walls. 
Jury 11.—[No notes, but there is a vial containing a single nymph.] 
AucusT 12.—In Burlington, Iowa, yesterday and to-day, large, yellow mayflies; not very abun- 
dant. 
AuGusT 23.—Scattering on dam and on power house. 
AucusT 31.—Collected a solitary live one at 7 p. m. 
SEPTEMBER 2.—A few present. 
SEPTEMBER 7.—One on Fisheries launch No. 27 (ran from Fairport to Fort Madison, Iowa). Did 
not see any live ones about Keokuk yesterday or to-day. 
These less continuous observations on Pentagenia seem to show that it appears 
commonly about a fortnight earlier in the season than Hexagenia, and in smaller swarms, 
and that it continues to appear in dwindling numbers through the season. On July 4, 
Mr. Stringham wrote: 
Entrance door to power house had about 200 mayflies on it this morning. The adjoining walls 
were likewise covered. There appeared to be seven or eight different species, but the most common 
@ Again from Mr. Stringham’s field notes. 
> This specimen served for certain determination of the nymph, and for the life history which appears on a subsequent 
Page. 
