Unrecognized for centuries: distribution and sexual caste descriptions of the West European Aphaenogaster species of the subterranea group (Hymenoptera, Formicidae)

There are only two Aphaenogaster species from the subterranea group in the western Mediterranean: A.ichnusa Santschi, 1925, from south-western Europe, and A.subterranea (Latreille, 1798), also occurring in central and eastern Europe. Historically, the two species have been widely misunderstood: A.ichnusa was long considered a Sardinian endemic subspecies of A.subterranea, while its continental populations were misidentified as A.subterranea s. str. Recently, A.ichnusa was elevated to species rank and its worker caste was redescribed with that of A.subterranea, allowing for their correct identification. Yet their distribution was documented in detail only for France and Sardinia. Furthermore, no morphological characters were described to distinguish the males and queens of the two species. By investigating private and museum collections, 276 new records of A.ichnusa are provided here and 154 of A.subterranea from the western Mediterranean. Additionally, qualitative and quantitative morphological characters were combined to identify their males and queens. We present the new southernmost, easternmost, and westernmost distribution limits for A.ichnusa. Based on our results, this species is widely distributed in Italy and Catalonia (Spain), also occurring on several Mediterranean islands, avoiding areas with continental climate and high altitudes. Sicily is the only island to host the less thermophilous A.subterranea, which otherwise extends westward to Galicia (Spain). Sympatric occurrence is not rare along the contact zone. Additional natural history observations are reported regarding foraging habits, associated myrmecophiles, habitat preferences, and colony structure in the two species.