LAUSR

Simple Summary Ant nests contain a multitude of resources which some organisms have evolved to exploit, living in close association with ants in order to prey on them, feed on their stored food, or simply in order to make use of their closely guarded fortresses as safe shelters. These organisms, known as myrmecophiles, have never been studied in much detail in the Maltese Islands—a small Mediterranean archipelago which, despite being one of the most densely populated countries in the world and facing enormous human pressures, has been shown to harbour a great diversity of insects, arachnids and crustaceans. In this study, we aimed to catalogue Maltese myrmecophile diversity and provide notes on the biology of all species encountered. As a result, we found at least thirty different species of insect, arachnid and crustacean which live exclusively inside ant nests in the Maltese Islands, four of which had never been known to inhabit this archipelago. An aphid-ant and a spider-ant association also appear to be previously unknown. We also discuss how the nests of Messor harvester ants may be important biodiversity hotspots due to these ants’ nests being suitable homes for many different species when compared to the nests of other types of ant. Abstract The intranidal myrmecophilous arthropod fauna of the Maltese Islands is reviewed. Thirty species from nine orders are found to be obligate myrmecophiles, of which four species are recorded from the Maltese archipelago for the first time: Phrurolithus sp. (Araneae: Phrurolithidae), Pogonolaelaps canestrinii (Berlese, 1904), Gymnolaelaps messor Joharchi, Halliday, Saboori & Kamali, 2011 and G. myrmecophilus (Berlese, 1892) (Mesostigmata: Laelapidae). Phrurolithus also represents the first record of the family Phrurolithidae in Malta. Notes on the biology and local distribution of each species are provided, including ant-myrmecophile associations, of which two appear to be previously unknown: the occurrence of Smynthurodes betae Westwood, 1849 (Hemiptera: Aphididae) in the nest of Plagiolepis pygmaea (Latreille, 1798) and Phrurolithus in the nest of Pheidole pallidula (Nylander, 1849). Fourteen additional species are found to be either only occasionally myrmecophilic, accidental ant-guests or potentially myrmecophilous, the latter remaining ambiguous due to a lack of knowledge of their biology. Of these, the family Caeculidae (Arachnida: Trombidiformes) represents a new record for the Maltese Islands, on the basis of Microcaeculus sp. occurring in a nest of Camponotus barbaricus Emery, 1905. Preliminary results indicate that Messor nests may be repositories of considerable myrmecophile diversity, with the most unique symbionts.