320 
Psyche 
[Vol. 94 
The Behavior of the Adults 
Adults of both sexes sit facing outward in empty cells on the nest. 
It is not known if they likewise sit in cells containing immatures or 
prey, but seems likely that they do. There is no doubt that the female 
wasps defend the cells in which they sit. When I touched nest N1 
while preparing to collect it to my great surprise I was immediately 
stung by a female sitting in a cell. Close examination of the position 
assumed by such females revealed that they sit curved in the shape 
of a letter “C,” so that both head and abdomen point outward at the 
mouth of the cell. When approached, the head is slightly retracted 
and the abdomen thrust forward to sting. I know of no published 
records of defensive stinging by Eumeninae or other solitary acu- 
leate wasps. I have been stung by airborne females of Zethus minia- 
tus while tampering with their nests, but those females, which also 
sit facing out of brood cells, withdraw into the cells when threatened 
rather than coming forward to sting. One X. nitidulum female also 
stung me readily when accidentally touched in the collecting net. 
Another unusual feature is that these wasps either sometimes 
begin provisioning before ovipositing, or they store prey in empty 
cells, as evidenced by the broodless cells containing prey on Nl. All 
vespoid wasps oviposit in an empty cell and begin provisioning 
later, often after the egg hatches (Evans and West-Eberhard, 1970). 
However, Isely (1913) found that some Kansas eumenines deposit a 
few prey in the cell prior to oviposition. This point merits further 
study in the case of X. nitidulum , since I saw only one (eggless) cell 
being provisioned. 
The contents of Nl suggest that vacated cells are reused. The 
three cells of N 1 that were open when collected were among the 
older (uppermost) cells of the nest; one was being provisioned, and 
one contained brood. Females may reuse vacated cells when they 
are available, and build new cells only when they are not. On N 1 and 
N2 there were five adult females and only four vacated empty cells; 
in that situation one new cell had been initiated. 
The fact that a large larva was found in an open cell may indicate 
progressive provisioning of the young. However, one of the closed 
cells of the abandoned nest (N3) contained the remains of a large 
larva and three uneaten prey. Perhaps X. nitidulum, like the primi- 
tively social eumenine Zethus miniatus (personal observation), 
engages in what Evans (1966) calls “truncated progressive provision- 
