Mycena haematopus, commonly known as the bleeding fairy helmet, the burgundy drop bonnet, or the bleeding Mycena, is a species of fungus in the Mycenaceae family, of the order Agaricales. It is widespread and common in Europe and North America, and has also been collected in Japan and Venezuela. It is saprotrophic—meaning that it obtains nutrients by consuming decomposing organic matter—and the fruit bodies appear in small groups or clusters on the decaying logs, trunks, and stumps of deciduous trees, particularly beech. The fungus, first described scientifically in 1799, is classified in the section Lactipedes of the genus Mycena, along with other species that produce a milky or colored latex.
The fruit bodies of M. haematopus have caps that are up to 4 cm (1.6 in) wide, whitish gills, and a thin, fragile reddish-brown stem with thick coarse hairs at the base. They are characterized by their reddish color, the scalloped cap edges, and the dark red latex they "bleed" when cut or broken. Both the fruit bodies and the mycelia are weakly bioluminescent. M. haematopus produces various alkaloid pigments unique to this species. The edibility of the fruit bodies is not known definitively.
The species was initially named Agaricus haematopus by Christian Hendrik Persoon in 1799, and later sanctioned under this name by Elias Magnus Fries in his 1821 Systema Mycologicum. In the classification of Fries, only a few genera were named, and most agaric mushrooms were grouped in Agaricus, which was organized into a large number of tribes. Mycena haematopus gained its current name in 1871 when the German fungal taxonomist Paul Kummer raised many of Fries' Agaricus tribes to the level of genus, including Mycena. In 1909 Franklin Sumner Earle placed the species in Galactopus, a genus that is no longer considered separate from Mycena. Mycena haematopus is placed in the section Lactipedes, a grouping of Mycenas characterized by the presence of a milky or colored latex in the stem and flesh of the cap. The specific epithet is derived from Ancient Greek roots meaning "blood" (αἱματο-, haimato-) and "foot" (πους, pous). It is commonly known as the blood-foot mushroom, the bleeding fairy helmet, the burgundydrop bonnet, or the bleeding Mycena.
In 1914, Jakob Emanuel Lange described the variety M. haematopus var. marginata, characterized by the reddish color on the edge of the gills; Mycena specialist Rudolph Arnold Maas Geesteranus considered the coloration of the gill edge too variable to have taxonomical significance. Mycena haematopus var. cuspidata was initially found in Colorado in 1976, and described as a new variety by American mycologists Duane Mitchel and Alexander H. Smith two years later. The fruit bodies are characterized by a "beak" on the cap that often splits or collapses as the cap matures. It was treated as Mycena sanguinolenta var. cuspidata by Maas Geesteranus in 1988.