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Desert ecosystems are more complex than expected and, in this type of habitat, interspecific interactions can be an important force structuring the community. In this paper we described an interaction appearing between an harvester ant species (Messor bouvieri) and several arthropod taxa feeding on ant-nest debris in a southern European arid land. We have found 23 arthropod taxa associated with the nests of Messor bouvieri. During the daily period of maximum activity, the abundance of beetles inside mounds of ant-nests can be 660 times greater than the beetle abundance on the soil surface. Five beetle species were significantly more abundant inside the ant mounds than outside. We believe that four of these five species-Morica hybrida, Pimelia integra P. monticola, and Tentyria incerta-are highly specialized in the use of ant-nest debris as an important source of food. Finally, we think that the remaining species, Aphodius baeticus, uses the ant nests principally as an arena for mating. In conclusion, we believe that, during the hot season, this ’unusual’ interaction could be an essential factor regulating the food web and trophic structure in these arid lands. (C) 1995 Academic Press Limited