THE OPTICS OF TSETSE-FLY EYES IN RELATION TO THEIR BEHAVIOR AND ECOLOGY

The visual acuity of two species of tsetse flies, Glossina morsitans morsitans Westw. and Glossina pallidipes Aust., was investigated. Male G. morsitans eyes have an acute zone in the forward region, with large hexagonal lenses (mean minimum diameter, D = 33, SE +/- 0.7-mu-m), relatively small interommatidial angle (DELTA-phi = 1.08-degrees) and angular receptive field of individual ommatidia (DELTA-rho) of not less than 1.14-degrees. A narrow band of square lenses, with intermediate diameter and DELTA-phi, merges with smaller hexagonal lenses in the periphery (24 +/- 0.7-mu-m), with relatively larger interommatidial angle (DELTA-phi = 3.7-degrees) and small angular receptive field (DELTA-rho = c. 1.6-degrees). G. pallidipes eyes are similar, except that the lenses in the acute zone are larger than those of G. morsitans, in proportion to their larger body size. Female eyes are not significantly different from male eyes, except that they have a narrower region of binocular overlap (maximum for males = 24-degrees, for females = 18-degrees). The eye parameter (p = D-DELTA-phi) in the acute zone of male G. morsitans = 0.62, and in the peripheral zone = 1.56. These relatively high values are consistent with fast flight, visual detection of drift due to low wind speeds, mating chases and discrimination of cryptic host animals at high light intensities. The extended region of binocular overlap in males may serve as an early warning system of the approach of potential females. From our estimates, tsetse flies ought to be able to detect small objects against the sky c. 30 min before sunrise and after sunset, and to use their peripheral vision perhaps 15 min earlier and later than this.