1987] 
Williams & Banks — Pseudacteon obtusus 
11 
Discussion 
These observations represent the first, direct evidence that P 
obtusus is parasitic; although other investigators have assumed this 
to be the case (Williams, 1980). Our observations also document P. 
obtusus attacks upon naturally occurring S. invicta workers at a 
distance from the fire ant nest. Previous studies (for example, Wil- 
liams et al., 1973; Williams, 1980) have reported on attacks by phor- 
ids only on experimentally-disturbed fire ant mounds. Recently, 
however, Wojcik et al. (1986) discovered a single puparium of an 
unidentified phorid in a female alate. 
Feener (1981) noted that phorids of the genus Apocephalus attack 
major workers of the ant Pheidole dentata and “stampede” the ant, 
thus overcoming its defensive behavior. P. obtusus appeared in our 
study to promote a similar response from its host. Parasitism of P 
dentata also affected the competitive balance between it and S, 
texana (Feener, 1981). 
Foraging S. invicta major workers appear more susceptible to 
successful attack by the phorid fly P. obtusus than workers of other 
size castes. This may be the result of larger ants presenting proxi- 
mate, visual and/or chemical stimuli of greater magnitude than 
smaller ants. However, the fact that phorid females did not attack 
available sexuals which are as large or larger than majors suggests 
that other factors are involved. The preference for workers only is 
not unique to P. obtusus. Wasmann (1918) reported P.formicarum 
attacking only workers of Lasius spp. while ignoring winged males 
and females. 
Defensive ability may be another factor. S. invicta major workers 
are less aggressive than minor and media workers (Wilson, 1978) 
and Vander Meer (unpublished data) has determined that the 
venom sacs of major workers contain significantly less venom 
alkaloids per mg ant than the venom sacs of other worker castes. 
Observations presented here suggest that such behavioral and 
physiological differences may render major workers particularly 
susceptible to parasitization by P. obtusus. However, factors other 
than ant aggressiveness and deterrent capability may also contribute 
to caste-bias in host selection by P. obtusus. Perhaps most signifi- 
cant in this regard is the site of parasite oviposition. Compared to 
minor or media workers, major workers offer a larger area at the 
