Species Richness of Parasites on Hosts: Agromyzid Flies on the British Umbelliferae

(1) The number of species of larval agromyzids (Diptera: Agromyzidae) mining plants in the family-Umbelliferae within the British Isles is reviewed. (2) A regression of the number of agromyzid species (S) on the geographical range of each plant (measured in 10 km squares, A) (leS + 1 vs leA) is highly significant, but explains only 32% of the variance. Several other 'species-area' curves are similar in this respect. (3) Geometric mean size of the plants, leaf-form (both measures of plant 'architecture'), and whether or not they grow in aquatic habitats explains some of the residual variation, but approximately 50% remains unexplained. (4) A wide variety of other characteristics of the plants, and their associated fauna (potential competitors and natural enemies) had no detectable influence on the number of Agromyzidae. Factors examined include taxonomic isolation, whether the plants were annual, biennial or perennial, the number of species of potentially competing microlepidoptera, and the number of species of parasitoids. (5) We suggest that the number of agromyzid species on many common and widespread umbellifers is considerably below the number that could finally evolve to exploit these plants, and that guilds of specialized leaf-miners are not 'equilibrium assemblages'.