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Trichospores of Harpella melusinaein larval black flies from four streams in northwestern South Carolina were measured. The longest fresh trichospore was 242 μm, 38% longer than the reported maximum length of trichospores for this species. After staining with lactophenol cotton blue, trichospore length was significantly reduced by up to 24.4% in fresh larvae and up to 4.9% in fixed larvae. Mean trichospore length differed significantly between seasons and between sampling sites but did not differ significantly between middle and final instars. Trichospore length correlated significantly with host size. We suggest that taxonomic descriptions of trichomycetes should include measurements of fresh, fixed and stained trichospores.

A total of 2063 mid- to late-instar larval black flies were collected from 64 stream sites in South Carolina and screened for the presence of the trichomycete fungus Harpella melusinae. Sixteen of 18 host species were colonized by H. melusinaeon at least one occasion. Prevalence of H. melusinaein larvae of Simulium tuberosumcytospecies “A” was highest in acidic streams with low conductivity, whereas H. melusinaecolonized larvae of Simulium verecundummost frequently in slower-moving streams. Ecological conditions, therefore, can serve as predictors of the prevalence of H. melusinae. Prevalence in host larvae was significantly lower in the Piedmont ecoregion than in the Mountain ecoregion. Prevalence did not differ in the host species S. verecundumacross ecoregions, suggesting that different prevalences among host species might indicate some host preference. The prevalence of H. melusinaediffered significantly between two univoltine host species (Simulium venustumand Prosimulium magnum) at the same site but not between two multivoltine host species (S. tuberosumcytospecies “FG” and S. tuberosumcytospecies “CDE”), suggesting that host life history could be important in determining fungal prevalence.

The midgut-inhabiting fungi (Harpellaceae) Harpella melusinaeand Stachylina pediferwere induced to form zygospores, using an application of a pH 10 potassium hydroxide solution with culture media. The previously unknown zygospores of S. pediferare borne perpendicular to the zygosporophore, as in Harpella melusinae. The zygospores of the hindgut-inhabiting species Smittium coloradense, borne obliquely to the zygosporophore (in vivo), are described for the first time.