Santa Rosa Nat ional Park is an arbitrarily defined 108 km1 patch of dry fo rest in the northwestern Pacific coastal lowlands of Costa Rica. It has a saturniid moth fau na of 30 breeding species (and 5 waif species) and a sphingid moth fauna of 64 regularty breeding species, 10 occasional breeding species, and 9 waifs (83 species in total). There is one endemic saturniid and no endemic sph ingids. Furthermore, nearly all of the saturniids and sphingids of Santa Rosa have very broad geographic and ecological ranges. The saturniids are dormant during the six month dry season and all survive the dry season within the Park. More than half of the sphingids migrate out of the Park during the dry season or even the second half of the rainy season. This migration is an integral part of Santa Rosa's interdependency with the rainforest parts of Costa Rica (and vice versa) . What determines the ceiling to the number of species of saturniids and sphingids in Santa Rosa? The Park is not isolated. There are no in· hospitable barriers to potential colonizing species in the immediately adjacent rainforest (10·15 km to the east). This rainforest is occupied by at least 25 spec ies of saturniids and 52 species of sphingids that also breed in Santa Rosa. Why don 't the other rainforest saturniids and sphingids move into Santa Rosa, at least during the rainy season? There is no suggestion that direct competitive interactions prevent the 31st or 65th species of saturniid or sphingid from moving into Santa Rosa. likewise, there appears to be a sufficient array of food plant species for the potential invader to find food . However, Santa Rosa supports a formidable array of predators and parasitoids that eat saturniid and sphingid caterpillars. On the one hand, this array is sufficiently depressed by the dry season that the breed ing saturn iids and sphinQids