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[Vol. 87 
offspring of the cleptoparasite (Wheeler 1919, Evans 1966, Bohart 
1970, Eickwort 1975). Facultative cleptoparasitism, in which females 
occasionally usurp the nests of conspecifics rather that females of 
other species, is thought to be the initial step in the evolution of 
obligate cleptoparasitism. Examples of facultative parasitism have 
been reported for several species (Alcock 1975, Eickwort 1975) but 
not for A. aberti. 
During our observations at GF, one female. Silver, exhibited 
cleptoparasitic behavior. At 1250 hrs on 23 July, she opened a nest 
that was being provisioned by another female, Yellow Legs, while the 
latter was out foraging, removed a single larva, stung it several times, 
dropped it outside the nest entrance, and re-entered the nest. At this 
point the nest owner returned, forcibly evicted Silver and soon 
permanently sealed the nest; the larva removed by Silver was left 
outside the nest. Curiously, immediately after the nest was perma¬ 
nently sealed, Silver returned and performed a similar sequence of 
sealing behaviors. 
Soon after this sequence was completed, Silver located another 
nest that had just been finished by White. Silver opened this nest, 
removed four larvae, one at a time, and deposited them adjacent to 
the burrow entrance (Fig. 1). At this point. White returned and Silver 
was again forcibly evicted, and the burrow then sealed by White 
without returning the larvae to the nest (Fig. 2). As soon as White had 
departed, Silver returned, reopened the nest, and removed two more 
prey. Silver then grasped each larva in turn, flew off a distance of 
about 3 m, landed either on the ground or a fence post (Fig. 3), stung 
the prey, and then returned to White’s burrow and took the larva 
inside (Fig. 4). After all six larvae were treated in this manner, the nest 
was sealed and never reopened. It is possible that the egg originally 
deposited by White was dislodged during this sequence, although we 
have no evidence of this, nor do we know if Silver ever deposited an 
egg of her own. 
Prey stealing was observed on several occasions during our 
excavations at the Cornish site. In several instances, prey-laden 
females were attacked as they returned to their nests from foraging 
trips. Typically, several other females would pounce upon the prey- 
owner; in the ensuing struggle, one of the attackers would fly off with 
the prey which was dropped as the female attempted to defend 
herself. The stolen prey was typically carried off for several meters, 
