Laboratory analyses of vision in tsetse flies (Dipt., Glossinidae)

Abstract

Histological examination of the eyes ofGlossina morsitansWestw. andG. austeniNewst. showed that visual acuity, as calculated from ommatidial size and interommatidial angle, varies in different zones and planes of the eye, being greatest in the forward-facing region. A study of optomotor responses to revolving black-and-white striped patterns indicated that the eyes could perceive both rapid and very slight movement: responses showed a unimodal distribution with frequency of ommatidial stimulation ranging from one flash/25 s to around 80 flashes/s, with an optimum response at 5 flashes/s. Movement perception was also enhanced by summation of ommatidia simultaneously stimulated, provided the stimulated ommatidia were not contiguous. Strong optomotor response was shown to the movement of black stripes so narrow as to cause merely a 7% reduction in the amount of light entering an ommatidium: from this it was inferred that the eye's contrast sensitivity is well developed. Visual acuity determinations from optomotor responses showed no species or sex differences, and the values corresponded closely with those obtained from anatomical measurements. Of the properties of selected figures examined for attractiveness to orienting flies, linearity of form and brightness contrast with background illumination were deduced to be the most important aspects.