Anacamptodon splachnoides is an uncommon moss almost entirely restricted to water-filled treeholes in deciduous trees in eastern North America and Europe. There has been uncertainty regarding taxonomic placement of the genus because of conflicts between gametophytic characters, in which it resembles Amblystegiaceae, and sporophytic characters, which seem to ally it with Fabroniaceae, Campyliaceae, or other families. Recent evidence from DNA sequencing clearly places Anacamptodon in Amblystegiaceae despite features of the sporophyte such as an erect capsule, rostrate lid, reflexed peristome teeth, and low endostome membrane. All these unusual features, including sticky spores (which seem to have been overlooked), are characteristic of species of Splachnaceae, until now the only group of mosses whose spores are known to be dispersed by flies. Field observations of A. splachnoides over a period of 16 months revealed that the moss mat and sporophytes are regularly visited by many species of flies that are also treehole specialists. Of 12 species of flies captured, nine carried spores of the moss. Many of these are strong fliers with hairy legs and bodies that inadvertently pick up the sticky spores, dispersing them in a directed fashion to other treeholes, where the females lay eggs that develop into aquatic larvae that later emerge as adults. Though differing in some respects from the adaptations seen in Splachnaceae, the parallel evolution of sporophytic characters related to entomophily is remarkable. In addition, we consider other aspects of the ecology of this moss that may help explain its rarity, such as treehole location, pH of rainfall versus stemflow and treehole water, and a possible beneficial relationship with certain wood-rotting fungi.