Mating Status and Choice of Group Size in the Phoretic Fly Norrbomia frigipennis(Spuler) (Diptera: Sphaeroceridae)

The phoretic sphaerocerid fly Norrbomia frigipennismates on scarabid dung beetles, follows them underground, and lays eggs in the collected dung. We found that male and female flies distribute themselves nonrandomly on beetles with respect to fly group size. The proportion of virgin females increased with group size, and females with mature eggs were most frequently found on beetles carrying fewer flies. Laboratory experiments revealed that mated females avoided larger groups, virgin females seemed to prefer large groups and mated females, small groups. Males showed no such pattern. These results suggest that N. frigipenniscan estimate group size, presumably an adaptation with a reproductive function. However, flies did not join hosts on the basis of the sex of already mounted flies.