Two New Subspecies of Herpetomonas muscarum (Leidy, 1856) Kent, 1880

SYNOPSIS. Herpetomonas muscarum muscarum n. subsp. was isolated from Musca domestica L. In culture at 20 C it assumed the opisthomastigote (up to 15%), double‐flagellate and flagellate promastigote forms. At 30 C or with 4% urea added to cultures at 20 C, the proportion of opisthomastigotes was greater (up to 40%). In experimentally infected flies only transient infections, which included both opisthomastigotes and promastigotes, occurred. The promastigotes were 15–30 μ long and the kinetoplast was small and subspherical or transversely elongate. H. muscarum ingenoplastis n. subsp. was isolated from Phormia regina (Meigen). In culture at 20 C almost all individuals were double‐flagellate promastigotes 20–40 μ long and less than 1% were opisthomastigotes. At 30 C or with added urea there was no increase in the proportion of opisthomastigotes and the cultures were not vigorous. In experimentally infected flies opisthomastigotes were 5–39% of the population depending on the part of the gut sampled. In all stages the kinetoplast was large (1.5–2.5 μ long) and tear‐drop‐shaped with the point directed posteriorly. In artificially mixed cultures of H. m. muscarum and H. m. ingenoplastis the former predominated after a short time and eventually survived alone. A mixed culture that was about 98% H. m. muscarum was fed to Phormia regina and produced heavy pure infections of H. m. ingenoplastis , which lasted for 22 days with no indication of decline. No evidence of cyst formation was found in either subspecies.