The Ecology of a Sheltered Clay Bank 185 
eral days these mothers had been digging, carrying mud 
or bringing in caterpillars. On July 30 I returned after 
a few days absence and found the numbers of this wasp 
materially reduced, so I suspected that this was about 
the time for the passing away of the first generation. 
Thereafter, a careful watch up to August 10 revealed no 
trace of this citizen of the colony. On August 14, one 
wasp, probably the first of the second generation, was 
found at work. Up to ‘August 20, this one and another 
were the sole representatives of their tribe, and on Au- 
gust 30, three were out. Although September 3 and 4 
were almost sunless, yet 7 individuals were active; this 
was the maximum number of the second generation 
Seen at any one time; from September 6 to 23, only 
two or three at a time were out at work. On the last 
visit of the year, on October 3, the day was gray and 
chill, but it found two of these mothers still at work, 
one plugging her burrow with mud and the other bring- 
ing in caterpillars, both plodding faithfully on toward 
the completion of their work, oblivious to the fact that 
it Was now only a matter of days or perhaps hours until 
for them the sun would shine no more. 
In 1918 this wasp, like the other species described 
above, emerged and began its work much earlier than 
in 1917; on May 28, they were already out and busy. 
In 1919, no records were made of their emergence, but 
in 1920, when the bank was visited on May 27, they were 
again found at work. Their appearance is correlated 
- With temperature conditions in later pages. They ap- 
peared in slightly greater numbers in 1918 than in 1917. 
On July 17, about the same number were at work; on 
July 31, none at all; hence this is evidently approxi- 
mately the dividing line between the two generations. A 
