Morph determination in Melittobia , a eulophid wasp

. 1. The Caribbean and Central American eulophid wasp, Melittobia sp. 1 ( hawaiiensis complex), (Hymenoptera: Chalcidoidea), whose larvae are exophagous on the prepupae of solitary wasps and bees, has three adult female and two adult male morphs. 2. The males, which constituted only 5% of the progeny, had morphs with, and without, ocelli. 3. In the females, brachypterous, negatively phototactic ‘crawlers’ developed mainly at low densities (<8.9 eggs cm ‐2 ), while above this density an increasing number and percentage of macropterous, positively phototactic ‘fliers’ were produced. Intermediate ‘jumpers’ were positively phototactic but could not fly. 4. In a typical progeny twenty to thirty ‘crawlers’, twenty to forty ‘jumpers' and up to 400 ‘fliers’ emerged in an overlapping sequence. 5. Experiments demonstrated that the proportions of the three morphs emerging were unrelated to the sequence in which the eggs were laid, to the species of the host and to the quality and absolute amount of food given, but related only to the density experienced by the young larvae. 6. We relate this trimorphism of dispersive female morphs to the three levels of ecological space (food item, patch and distribution of patches) postulated by Hassell & Southwood (1978) and suggest the general ecological conditions necessary for it to evolve.