Accepted by T. Wesener: 20 Mar. 2018; published: 17 May 2018 ZOOTAXA ISSN 1175-5326 (print edition) ISSN 1175-5334 (online edition) Copyright © 2018 Magnolia Press Zootaxa 4420 (3): 415–429 http://www.mapress.com/j/zt/ Article 415 https://doi.org/10.11646/zootaxa.4420.3.7 http://zoobank.org/urn:lsid:zoobank.org:pub:5F0337E2-CBFE-413E-BDFF-3D6131BE1609 Out of the dark void: Ommatoiulus longicornis n. sp., a new julid from Spain (Diplopoda, Julida) with notes on some troglobiomorphic traits in millipedes NESRINE AKKARI 1 , JOSÉ D. GILGADO 2,3 , VICENTE M. ORTUÑO 3 & HENRIK ENGHOFF 4 1 Naturhistorisches Museum Wien, Burgring 7, 1010 Wien, Austria. E-mail: nesrine.akkari@nhm-wien.ac.at 2 Section of Conservation Biology, Department of Environmental Sciences, University of Basel. St. Johanns-Vorstadt 10, CH-4056, Basel, Switzerland. E-mail: josedomingo.gilgadohormaechea@unibas.ch 3 Grupo de Investigación de Biología del Suelo y de los Ecosistemas Subterráneos. Departamento de Ciencias de la Vida. Facultad de Biología, Ciencias Ambientales y Química. Universidad de Alcalá. A.P. 20 Campus Universitario. E-28805. – Alcalá de Henares, Madrid, Spain. E-mail: vicente.ortuno@uah.es 4 Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100 København Ø, Denmark. E-mail: henghoff@snm.ku.dk Abstract A new millipede species, Ommatoiulus longicornis n. sp. is described from the subterranean habitat in eastern Spain, and the elongation of its antennae is discussed in relation to the habitat where it dwells. We take the occasion to discuss the elongation of appendages as a troglobiomorphic character in millipedes and shed the light on the various, sometimes con- troversial, hypotheses that could have led to these modifications in millipedes and other subterranean arthropods. Key words: Schizophyllini, Spain, taxonomy, new species, troglomorphism Introduction The fauna of the subterranean environment is classified, according to its degree of dependence, as troglobionts (true hypogean), troglophiles (facultative hypogean) and trogloxenes (accidentally found in the hypogean environment). A detailed explanation of these terms can be found in Sket (2008). Here, we take the occasion to discuss the concept of 'troglomorphism' or more correctly 'troglobiomorphism', which refers to convergent morphological traits observed in many (but not all) troglobionts as well as in some troglophiles (Christiansen 1962; Juberthie & Decu 1994; Desutter-Grandcolas 1997; Hernando et al. 1999; Dethier & Hubart 2005; Pipan & Culver 2012; Culver & Pipan 2015). The term troglobiomorphism refers to the fact that “cave” animals have similar traits (troglobiomorphies) due to convergent selective pressures in the hypogean environment (Culver & Pipan 2015). However, this is not as simple, as not all troglobionts are troglobiomorphic and some troglobiomorphies are only observable in certain taxa (see list of troglobiomorphies in Christiansen 2012). Several troglobiomorphies might also be expressions of previous adaptations to other environments, i.e. exaptations to the hypogean environment (Hernando et al. 1999; Heads 2010; Gilgado et al. 2014). It was recently demonstrated that the subterranean realm is composed of different habitats, which leads to a divergent evolution in some characters (Pipan & Culver 2012, Trontelj et al. 2012; Culver & Pipan 2015), leading to the existence of specialized troglobionts with different sizes and degrees of appendage elongation. According to Culver & Pipan (2015), this leads to a contradiction within the troglobiomorphism hypothesis and makes it necessary to refer to these morphological traits as convergences, or divergences, rather than uncritically classifying them as troglobiomorphies. There are several types of subterranean environments with various characteristics (Culver & Pipan 2009, 2014; Ortuño et al. 2014). The deep hypogean environment is traditionally understood as constituted by bigger caves and the fissure network of the bedrock. The latter was reported by Racovitza (1907) as the true habitat of many