346 Trans. Acad. Sci. of St. Louis 
so small a room, was as clear as it was interesting. To 
recapitulate the story of the occupants, the original 
builders, Sceliphron, had made their nests and left them 
to the insect community; then the succeeding inhabitants 
followed in peaceful succession. First after the Sceli- 
phron came renters by the name of Trypoxylon clava- 
tum. These were abundant, and used the old mud nests 
with only slight modification. After T. clavatwm had 
been watched and studied here for two years, it was 
suddenly discovered, when the season of 1919 opened, 
that clavatum was no more; that a species never here- 
tofore seen in this building was present in fair numbers, 
and had taken possession of the situation for its abode. 
This new citizen in the community, that had the art of 
making dainty little nests out of old tumble-down, twice- 
used mud huts, was Pseudagenia mellipes. 
In previous descriptions and in the paragraphs preced- 
ing I have already shown that P. mellipes is a fairly 
versatile little creature. In a former work, when we re- 
corded that this insect was found in the old cells of Scelt- 
phron (fig. 38 shows the old nests broken to expose to 
view the mud cells of P. mellipes), and discovered heavy 
mandible marks on the outside of the nest, we registered 
the suspicion that this was the work of P. mellipes. This 
suspicion was confirmed by the observation of this lot 
of little creatures in action. The following extracts from 
my notebook will give the details. 
At 12:50, on September 6, I noticed a one-celled nest 
of Sceliphron with a P. mellipes aboard. The opening 
was half sealed in a manner which was new to me; @ 
wall was being built across the aperture from the two 
opposite sides, leaving a vertical crack across the center 
about 5 mm. wide. After five minutes the opening Was 
smaller. My chief attention was at that time concel- 
