New association between red wood ant species (Formica rufa group) and the myrmecophilic spiders Mastigusa arietina and Thyreosthenius biovatus

Ants belonging to the Formica rufa species group, counting 10 representatives in Europe, are often referred to as red wood ants (RWAs). These dominant, mound building species are known to host in their nests an extremely diverse fauna of associated myrmecophilic arthropods, among which are the two W-Palaearctic spider species Mastigusa arietina (Thorell 1871) and Thyreosthenius biovatus (O. Pickard-Cambridge 1875). The actual host range of these spiders within the Formica rufa group is little known, due to the taxonomic uncertainties that have characterized RWAs in the past. We conducted a large-scale survey for assessing the occurrence of both spider species in association with different RWAs, with a focus on an accurate identification of the ant species. We recorded co-occurrence data for 5 European representatives of the Formica rufa group, and we reported for the first time on the co-occurrence of M. arietina with Formica aquilonia Yarrow 1955, Formica lugubris Zetterstedt 1838 and Formica paralugubris Seifert 1996, and of T. biovatus with F. aquilonia. We found no association between the rate of presence/absence of the two spiders and host ant species or sampling localities, which suggests a non-selective exploitation of RWA hosts by the two myrmecophilic spiders.