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The Laboulbeniomycetes are a unique group of that are apparent external parasites of insects and other arthropods, both terrestrial and aquatic. These fungi are minute; their fruiting bodies commonly measure less than one millimeter. They live on the antennae, the mouthparts or other body regions of their arthropod hosts. Although several species of Laboulbeniomycetes have more or less extensive, root-like hyphal systems (haustoria) inside their hosts, as a group these fungi are apparently harmless to the animals they live on. These fungi are usually apparent only on adult hosts; apparently immature arthropods eliminate them during ecdysis (adult arthropods no longer molt). Some fungi in the Laboulbeniomycetes have separate female and male individuals, like Herpomyces.

In the micrograph to the right, two flask-shaped, about 0.3 mm long female fruting bodies of Herpomyces periplanetae can be seen. They are full of ascospores. Between them, there are two short, finger-like male individuals. Below, a part of a striated foot cell is visible. In nature, the fruiting bodies sit on the foot cell, which lays on the insect's antennae. Herpomyces periplanetae is a very widespread parasite on the antennae of adult cockroaches in the genus Periplaneta, like the common American cockroach, Periplaneta americana.