Collections of forest mosquitoes totaling 72,211 individuals were made on the ground and in the canopy during the rainy season of 1950 at 18 localities throughout the length of Panama and into Costa Rica, in seeking an explanation for the failure of sylvan yellow fever to appear west of the Canal Zone prior to that time.
It was found that the known South American vectors of the disease, Haemagogus spegazzinii falco and Aedes leucocelaenus clarki occurred sparingly west into Costa Rica, but the only station at which they were present in substantial numbers was one located in the well developed tropical rain forest of the Atlantic slope.
The vertical stratification of the mosquitoes taken in the forest is summarized for each species of which more than 15 individuals were taken. These data are of importance in assessing the possible role of each species in terms of its contact with the arboreal mammals which are the intermediate hosts of the yellow fever virus.
It is suggested that physiographic and meteorologic conditions in central Panama and the Canal Zone area produce a forest cover which is intermediate between tropical rain forest favorable for the production of substantial numbers of tree hole breeding vector mosquitoes, and tropical deciduous forest in which conditions are less favorable. It is further suggested that since conditions are marginal in this critical area, it would take a particularly favorable rainfall pattern to produce the mosquito population necessary to carry the virus across into the Atlantic side rain forest of western Panama as happened in 1949 and 1950.