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24 Introduction Herpomycetales and Laboulbeniales are two orders within the class Laboulbeniomycetes (Ascomycota, Pezizomycotina), consisting of arthropod-associated biotrophs. Both orders are unique among related fungi in that they do not form hyphae; instead, thalli are produced by mitotic divisions from a two-celled ascospore. Herpomycetales was recently described and includes a single genus, Herpomyces Thaxt., with 27 described species–all associated with cockroaches (Blattodea) (Haelewaters et al. 2019b; Gutierrez et al. 2020). The Laboulbeniales order, on the other hand, successfully radiated on a wide range of hosts. Representatives of this order can be found in three arthropod subphyla, including mites and harvestmen (in subphylum Chelicerata), millipedes (in subphylum Myriapoda), and many orders of true insects (in subphylum Hexapoda). The vast majority of about 2,325 described species (Kirk 2019) are known from beetles (order Coleoptera), hence the common name once introduced for the group, “beetle hangers” (Cooke 1892). The early taxonomic history of these fungi is fraught with confusion (Blackwell et al. 2020), but the incorporation of sequence data has led to a conclusive placement of these fungi within Ascomycota (Blackwell 1994; Weir and Blackwell 2001; Schoch et al. 2009).

Early studies on Laboulbeniales (including Herpomyces at that time) in Belgium and the Netherlands are scarce. In Belgium, Collart (1945, 1947) and Rammeloo (1986) made noteworthy contributions, followed by multiple publications by De Kesel and colleagues (1989–present). The Laboulbeniomycetes from Belgium were for the first time summarized by De Kesel and Rammeloo (1992), who reported 1 species of Herpomyces and 47 species of Laboulbeniales. De Kesel et al. (2020) provided an updated – and illustrated – Catalogue of the Laboulbeniomycetes of Belgium, with a total of 115 species (3 Herpomycetales, 112 Laboulbeniales) from 222 host species. For more details regarding the study of Herpomycetales and Laboulbeniales in Belgium, we refer to De Kesel and Rammeloo (1992) and De Kesel et al. (2020). In the Netherlands, thus far, no effort has been made to publish a checklist.

The study of Laboulbeniales in the Netherlands started during a meeting of the Dutch Entomological Society in 1906, triggered by a question from Dr. Johannes P. Lotsy, then director of the “Rijksherbarium” (Leiden). In response, Prof. Dr. De Meijere remembered that he once observed an infected Drosophila funebris (Fabricius, 1787) fly, collected at the ARTIS Amsterdam Royal Zoo in 1904, but had not thought it worthy of mention at the time. Recent infected material of D. funebris from nature reserve De Kaaistoep has thus far always been associated with Stigmatomyces entomophilus (Peck) Thaxt. (Haelewaters et al. 2015b) and hence it is likely that S. entomophilus represents the very first report of Laboulbeniales from the Netherlands. The first published account was a developmental study of Stigmatomyces baeri H. Karst. by Boedijn (1923). The fungus was found on an atypical host – Fannia canicularis (Linnaeus, 1761); this fly is the only reported host for Fanniomyces ceratophorus (Whisler) T. Majewski, which is morphologically different from Boedijn’s (1923) drawings. We agree with Thaxter (1931) that the fungus was probably correctly identified by Boedijn, but perhaps the host was not.