When male drone flies (Eristalis tenax (L.)) of the spring and summer generations stop dispersing, they settle within individual home ranges that provide them with sheltering, resting, basking, grooming, feeding, and mating sites. Away from its mating place, a resident male rarely responds to other insects. On its mating site, however, it is territorial, attacking alien species, such as bees, wasps, and butterflies, as well as conspecific intruders. Territorial duty is demanding, and resident males take rest periods outside their territories whenever they can. When prevented from doing so, either by sky conditions which confine them to the territory, or by crowding which eliminates many neutral sites, they become increasingly aggressive. Males on open, horizontal territories (e.g., in flowerbeds) are more likely to notice intruders, and therefore are more liable to attack them, than males on vertical territories (e.g., on broad-leaved shrubs). The aggressiveness of E. tenax has social and ecological ramifications beyond its own species, since bees may stop foraging and aphidophagous syrphids may not oviposit in places where drone flies are exceptionally active.