The miltogrammine fly, Miltogramma rectangularis. is a common and widesprea
d brood parasite of the large Australian native bee, Amegilla dawsoni. Rece
ntly eclosed flies emerged from the underground brood cells of the bee and
walked rapidly across bare ground to reach cover in the vegetated fringe of
the nesting area. While walking, the flightless flies were vulnerable to b
ird and ant predators. but they may have minimized their exposure by mainta
ining a constant compass orientation until a perch was found. Once perched
inconspicuously, the dies spent about an hour inflating and hardening their
wings before they were able to Ay. At some sites, males located, courted,
and mated teneral females in a scramble competition mating system. Although
hundreds of flies emerged from parts of some nesting areas over several we
eks, adult females seeking to larviposit on passing bees were generally sca
rce. The proportion of brood cells lost to the parasite was low (under 5%).
As a result, it seems unlikely that parasite pressure by the fly contribut
es greatly to the male size dimorphism that characterizes A. dawsoni, becau
se even if brood provisions intended for large sons were at greater risk of
exploitation by the parasite, the percentage lost would still be too small
to outweigh the great mating advantage experienced by the larger males.