Laboulbeniomycetes (Fungi, Ascomycota) of Denmark Author Santamaria, Sergi Unitat de Botànica, Departament de Biologia Animal, de Biologia Vegetal i d’Ecologia, Facultat de Biociències, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès (Barcelona), Spain. sergi.santamaria@uab.cat Author Pedersen, Jan Natural History Museum of Denmark (Zoological Museum), University of Copenhagen, Universitetsparken 15, 2100 København Ø, Denmark. japedersen@snm.ku.dk text European Journal of Taxonomy 2021 2021-11-26 781 1 425 http://dx.doi.org/10.5852/ejt.2021.781.1583 journal article 10.5852/ejt.2021.781.1583 2118-9773 5828924 Genus Rhachomyces Thaxt. MB#4686 Proceedings of the American Academy of Arts and Sciences 30: 468 ( Thaxter 1895 ) . – Type species: R. lasiophorus (Thaxt.) Thaxt. Acanthomyces Thaxt., Proceedings of the American Academy of Arts and Sciences 27: 36 ( Thaxter 1892 ) [MB#11] Brief description Monoecious. Thallus consisting of a primary two-celled receptacle (cells I + II), a secondary receptacle or main axis formed by cell II derivatives, and a tertiary receptacle or tertiary axis consisting of a row of cells, lateral alongside the main axis and giving rise to appendages. The main axis bears up to three different types of appendages, named “a” (in the lower receptacle), “b” (in the upper receptacle, below perithecia), and “c” (supporting the antheridia) (see Santamaria & Faille 2007 ). Remarks The genus consists of 90 species, of which 32 in Europe, six in Denmark .The most recently described species are R. insolitus ( Rossi & Christian 2020 ) , R. spinosus ( Santamaria et al. 2020a ) , and R. neoproliferans ( Buyck et al. 2021 ) . This genus has attracted the attention of several authors, especially for the species parasitizing cave-dwelling beetles (Rossi 2006; Santamaria & Faille 2007 ). Key to the Danish species 1. Primary appendage very different from the other appendages. Secondary receptacle or main axis consists of three superposed cells. On Syntomus Hope, 1838 (Col. Carabidae ).............................................................................. R. spinosus Santam. & Cuesta-Segura – Primary appendage similar to the other appendages. Secondary receptacle or main axis consists of more than five superposed cells........................................................................................................ 2 2. Some appendages are very long, far exceeding in length the perithecial apex. Main axis, under the perithecium, much shorter than the perithecium length. On Lathrobium (Col. Staphylinidae Paederinae ) ..................................................................................... R. pilosellus (C.P. Robin) Thaxt. – Appendages usually not exceeding the perithecial apex. More than five cells under the perithecium in the main axis; longer than the perithecium length........................................................................ 3 3. Appendages of type “c” very abundant among those of the type “a”, bearing a terminal falcate and pointed antheridium or its derived sterile cell; thus giving the thallus a comb-like appearance. On Philonthus and allied genera (Col. Staphylinidae Staphylininae ) ................. R. philonthinus Thaxt. – Appendages of type “c” uncommon, typically on the upper region of the main axis, under or near the perithecium. Neither falcate antheridia nor with a comb-like appearance ....................................... 4 4. Main axis of the receptacle continuing as a ± elongated branch (up to 250 µm in length), as long or longer than half of the length of the main axis below the perithecium. On Othius Stephens, 1829 (Col. Staphylinidae Staphylininae ) ................................................................... R. furcatus (Thaxt.) Thaxt. – This branch is much shorter. On Col. Carabidae .............................................................................. 5 5. Septa separating perithecial outer wall cell tiers w 1 from w 2 , and w 2 from w 3 marked with elevations. Perithecium uniformly coloured. On Trechus Clairville, 1806 and related genera (Col. Carabidae )......................................................................................................... R. canariensis Thaxt. – Septa separating perithecial outer wall cell tiers not clearly marked. Perithecium rounded in outline, subapically darkened. On Acupalpus Latreille, 1829 , Badister Clairville, 1806 or related genera (Col. Carabidae ).......................................................................................... R. lasiophorus (Thaxt.) Thaxt.