10 
Psyche 
[Vol. 94 
third escaped by dispersing venom in the air by raising and vibrating 
its gaster (i.e., “gaster-flagging”, Obin and Vander Meer, 1985). The 
attacking phorid would then fly in search of another potential host 
worker, and the minor worker would resume foraging. In contrast, 
no major workers displayed any defensive behavior such as gaster- 
flagging, mandible gaping or lunging, although all of them tried to 
escape from attacking phorids. The major workers appeared “con- 
fused” by hovering phorids, moving off the foraging trail or away 
from the bait station into an open area (usually away from other 
workers). This action apparently benefited the parasite because the 
phorids closely pursued and attempted to oviposit only on major 
workers during this study (n = 10 attempts), even though minor 
workers clearly outnumbered major workers at the bait station and 
along the foraging trails. 
In one particular case, we observed a major worker of S. invicta 
under continuous attack by a lone phorid. We collected both host 
and parasite, placed them in a 20 ml vial. Once in the vial, the 
phorid attempted repeatedly to land on the agitated worker which 
was running back and forth in the vial trying to elude the fly. After 
approximately 5 min, the phorid landed successfully on the poste- 
rior end of the hosts’ head capsule, deposited one oval egg, and then 
began searching for an exit from the vial, paying no attention to the 
ant. The ant, however, attempted to remove the egg with its anten- 
nae and forelegs. However, the placement of the egg at the base of 
the head capsule near its junction with the thorax made it impossi- 
ble for the ant to do so. After several minutes of persistent attempts 
to dislodge the egg, the parasitized ant ceased grooming activities. 
We do not know whether or not the egg would have been removed 
by nestmates had the worker returned to its nest. Replicates (n = 5) 
of the above procedure were conducted using newly-mated female 
sexuals and other P. obtusus. These P. obtusus displayed no visible 
attack or oviposition behaviors while enclosed in a vial with indi- 
vidual S. invicta queens. 
During this study, five other species of ants were observed forag- 
ing for baits in the study area. These included Mycocepurus sp., 
Conomyrma sp. A, Conomyrma sp. B, Paratrechina longicornis, 
and Acromyrmex sp.. However, only the Solenopsis workers were 
attacked by P. obtusus , 
