THE REPRODUCTIVE BIOLOGY OF SPHAEROCERID KLEPTOPARASITES OF DUNG BEETLES
The first detailed behavioral account of a phoretic insect, the sphaerocerid kleptoparasite of dung feeding scarabs, Borborillus frigipennis, is presented. More briefly noted are the behaviors of three other dipteran symbionts of scarabs, two of which are newly discovered species.
Kleptoparasitism of beetle dung caches is probably a means of escaping competition in the "parent" feces; non-scarab competitors and predators are less abundant in beetle stored dung. B. frigipennis is one of the few spring survivors of an abundant and diverse winter sphaerocerid fauna. The decline of nonphoretic sphaerocerids parallels the increase in the numbers of larger calypterate dung flies.
Since oviposition in B. frigipennis occurs only underground, phoresy is an inexpensive and safe means for female flies to maintain contact with hosts. Males ride in order to mate. Last male advantages in sperm competition are thought to be the reason that males follow females underground into scarab burrows, from which there is no host-independent escape.
For the first time, sexual selection pressures--male-male competition and female choice--are shown to influence host choice in a parasitic animal. Female B. frigipennis have a preference for either vacant or densely populated beetles, presumably as a means of avoiding competitors of their larvae, on one hand, and choosing the best possible mate on the other. Unexpectedly, the gender of flies aboard beetles plays no role in the decision of flies joining groups. Copulatory success of males is dependent on male size relative to that of rivals. Larger males obtain more matings, have greater access to females, and are more likely to engage in copulation when they encounter a female. Smaller males in the laboratory are more apt than larger males to leave hosts. Differences in the numbers of males and their sizes relative to competitors between naive groups of flies (on walking beetles) and experienced groups (on flying beetles) suggest that smaller males in nature also abandon highly competitive environments. The density of flies influences the probability a beetle will attract a female phoretic, and significant correlation exists between single male ridership and density.