Prehibernating Musca autumnalis (Diptera: Muscidae)--an overwintering host for parasitic nematodes.

Diapausing face flies, Musca autumnalis (De Geer), are characterized by the presence of hypertrophied fat bodies (HFB) and arrested ovarian development. A total of 1752 face flies were caught around cattle in south-central Sweden during the last 5 weeks of the grazing season, in August-September. Females in this material (1139) were examined in search of HFB and to determine their reproductive status. It was assumed a priori that gonoactive flies would not undergo diapause. Flies were also dissected in search of the nematodes Parafilaria bovicola (Tubangui), Thelazia spp. and Heterotylenchus autumnalis (Nickle). Only females were found to be infected, of which 0.6% contained P. bovicola, 0.6% Thelazia spp. and 1.4% H. autumnalis. Both gonoactive and nongonoactive flies were infected with P. bovicola and Thelazia spp. Only nongonoactive Thelazia-infected flies had HFB, which is suggestive of flies able to overwinter. In the Heterotylenchus-infected flies with HFBs, arrested development (hypobiosis) of the nematode was recorded.