The Interaction between the Terrestrial Isopod Porcellio Scaber Latreille and One of its Dipteran Parasites, Melanophora Roralis (L.) (Rhinophoridae)

Abstract

Melanophora roralis (L.), a rhinophorid fly, was reared from naturally infected sowbugs, Porcellionides pruinosus (Brandt) and Porcellio scaber Latreille, collected at several locations in the eastern United States, the majority from P. scaber. Aspects of the host-parasite interaction were studied in a large sample ofisopods from North Carolina. The flies were sexually dimorphic for body size, and varied in size as a function of individual host size. Differences between the fly sexes were due to their differential utilization of a common range of host sizes rather than to the use of different host sizes; this result was directly confirmed with laboratory rearings of full sibships. Two potential indicators of individual fitness, adult longevity, and female fecundity, were direct functions of parasite, and hence host, size. The size dimorphism in Melanophora roralis, and physiological features associated with it, may reflect adaptations in this parasite for population persistence at low population densities. The Rhinophoridae are dipteran flies largely, or exclusively, specialized as larval parasites of terrestrial isopod crustaceans (Crosskey, 1977). The family is small, with about 100 species currently recognized, and has had a long and confused history of affiliation with both the Calliphoridae and the Tachinidae (for review see Crosskey, 1977; Kugler, 1978). Despite the generalizations often made about rhinophorid natural history, relatively little is known.about these flies. Host records for most species are lacking, and very little information has been published on host-parasite interactions since Thompson's (1934) monograph on the British species. Only one rhinophorid, Melanophora roralis (L.), is widespread in North America. The species ranges from Michigan to Kansas and from New Hampshire to Florida and Louisiana (Sabrosky and Araud, 1965). Although this fly has been frequently collected, it is rarely very abundant (Brues, 1903). M. roralis is known to be a parasite of Porcellio scaber Latreille (Thompson, 1934) and Oniscus asellus L. (Jones, 1948), but details of the host-parasite interaction have received little attention due to the sporadic and rare occurrence of adequate samples. We have occasionally reared M. roralis from isopods collected for other experimental purposes. One of these collections, from North Carolina, had an incidence of parasitism high enough to prompt the present study.