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Four new species of Triceromyces (Laboulbeniales) parasitic on Hydrometra (Heteroptera: Hydrometridae) were described and aspects of their morphology and development were discussed and illustrated with line drawings. Two of the species, T. floridanus and T. elongatus, are monoecious; T. floridanus, known only from Florida, U.S.A., has features allying it with T. elongatus, which ranges from Madagascar and Kenya in Africa to India, Indonesia, and Sri Lanka in Asia. The other two undoubtedly closely related species, T. lithophilus and T. terrestris, are dioecious and parasitize two closely related endemic species of Hydrometra thriving in a small region in northwestern Madagascar.Hydrometra cavernicola, the host of T. lithophilus, is semiterrestrial, living on damp rock faces or in small caves near water. The host of T. terrestris, H. phytophila, lives on terrestrial vegetation often far removed from the nearest stream or other source of free water. The male of T. lithophilus and T. terrestris consists of a few superposed cells bearing a terminal antheridium whereas the female has aprimary appendage bearing only sterile, antheridialike branchlets instead of functional antheridia as in a presumed closely related monoecious species T. hydrometrae. New records for T. hydrometrae were presented showing that it is widely distributed in both the New World and Old World. Circumscription of Triceromyces was emended and a key to all known species of the genus was given. Dioecy and its occurrence throughout the Laboulbeniales was reviewed and its morphological diversity summarized. It was concluded that the phenomenon has arisen independently several times in the order.

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© 1998 Richard K. Benjamin

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