MODEL FOR SYMPATRIC SPECIATION IN MEGARHYSSA (HYMENOPTERA, ICHNEUMONIDAE) - COMPETITIVE SPECIATION

M. atrata, M. macrurus and M. greenei are dependent on the same host, larvae of the wood-boring Tremex columba (Hymenoptera: Siricidae) in the same geographic area. Adults are extremely similar in their ecology and behavior but do not appear to compete for food or shelter. Three factors permit larvae of the 3 spp. to be segregated ecologically by parasitizing different segments of the host population: The adults select a host larvae at a depth in the wood so that complete insertion of the ovipositor at right angles to the surface of the wood will just reach it; the ovipositors of the different species are of different lengths and the host larvae maintain themselves at about the same depth in the wood for most of their larval period. A mechanism is proposed whereby the ancestral Megarhyssa spp. diversifies and divides by several stages to achieve its present condition. The model is unusual because no geographic isolation is involved, the evolutionary force is intraspecific competition and the final outcome is the formation of 3 spp. separated by premating isolation.