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SYNOPSIS A mimicry complex is described in which numerous taxa of beetles resemble flies. A probable explanation for the convergence on a common pattern is suggested, and the evolution of the complex is discussed.

1. Ants, parasitoids and flies are about equally frequent at foliar nectaries of Bytmeriu uculeutu (Sterculiaceae) in lowland Costa Rica during the dry season, a pattern previously unreported but also observed at other plants in the area.2. Species of Ectutommu, Cremutogaster and Camponotus were the most frequent of twenty-four ant species on Byttneriu, eight of which nested in the hollow stems. Ants spent most time at nectaries and little in patrolling.3. Collections at nectaries yielded large numbers of species of parasitoid Hymenoptera with few individuals of each. Rearing studies of leaf-feeding herbivores yielded several species of parasitoids, including one species taken at a nectary and two others congeneric or closely re1 a ted.4. Flies appear to be nectar thieves, in the same sense as nonpollinating floral visitors, despite close association with Bytfneriu. 5 . Ant-plants may be poor models for the larger number of species of less specialized plants with extrafloral nectaries. Plants which have only extrafloral nectaries may better suit the needs of parasitoids than ants, and plants such as Byttneriu may benefit as much from parasitoids as from ants.