and Charles W. Myers for their suggestions and assistance in the field. I especially thank the latter for the photographs for Figs. 9 and 11. Field and laboratory studies were made possible by support from the National Science Foundation (Grants GB-1441 and GB-5818), and field studies in Panama were carried out under the auspices of a grant from the National Institutes of Health (NIH GM-12020) in co-operation with the Gorgas Memorial Laboratory in Panama.
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Tilley et al. (1968) studied this question.