Harpella melusinae is common and cosmopolitan in the Northern Hemisphere, as well as throughout New Zealand and in most parts of Australia where blackfly (Simuliidae) hosts occur. The species is found attached to the peritrophic membrane in the larval midgut. In our 1991 South Island collections we found H. melusinae in 10 stream sites, primarily in Austrosimulium australense (Schiner) and A. ungulatum Tonnoir, but also in other, unidentified larvae of Austrosimulium (the only simuliid genus known in New Zealand).

Harpella melusinae is unquestionably the most common species of Harpellales in simuliid larvae around the world, and this is true in New Zealand as well. Our collections on North Island were from sites 1, 3, 5, 9, 12, 13, 16, 21, 22, and 25. The host species was Austrosimulium australense (Schiner) at all sites, except for A. multicorne at site 21. Approximately 95% of the larvae we dissected were infested with H. melusinae. Three larvae sampled from site 24 were uninfested.
On South Island, we found H. melusinae at sites 31, 34, 37, 38, 43, 44, 56, 57, 58, 63, and 67. At most sites, the simuliids were Austrosimulium tillyardianum. We collected A. laticorne Tonnoir at a few sites. Of approximately 130 larvae dissected, 75% were infested with H. melusinae. At sites 32, 50, and 64, a sampling of one to four larvae yielded no Harpella.
The morphology of H. melusinae from New Zealand tended to vary somewhat from typical material we have collected in France (from where the type was described), the U.S.A., and some other countries. In typical specimens, thalli of H. melusinae may range up to 750 µm long, are usually 6-10 µm in diameter, and the trichospores are approximately 150 µm long and about as wide as the thallus (Lichtwardt 1986). The thalli of New Zealand specimens are commonly no longer than 400 µm, with a diameter of about 8-11 µm, and the trichospores typically measured 52-135 x 5-7 µm. Thus, the trichospores of the New Zealand Harpella species tend to be narrower and shorter than those of the more usual H. melusinae. However, we are currently treating the New Zealand species as a variant of H. melusinae, rather than a new species.