Spectral sensitivity of the horse fly Tabanus nigrovittatus (Diptera: Tabanidae)

Spectral sensitivity functions were calculated from electroretinograms recorded from dark-adapted compound eyes of male and female horse flies (Tabanus nigrovittatus Macquart). Females had a borad sensitivity in the violet to green area of the spectrum; their spectral sensitivity was fitted by a theoretical mixture containing 20% of 440-nm and 80% 520-nm rhodopsins. Older females (8-18 days) were 93 times more sensitive than 1-day-old females. Males showed a narrower sensitivity function with more blue and less green sensitivity. Older males (8-18 days) were the most blue-sensitive of all groups; their spectral sensitivity was best fitted by a mixture containing 10% 440-nm, 70% 480-nm, and 20% 520-nm rhodopsins. Older males that were light-adapted to red light showed an apparent decline in the contribution of the 520-nm rhodopsin to overall sensitivity, as expected if this pigment is present in a separate system. The sensitivity function of 1-day-old males was best fitted by a mixture of 55% 480-nm and 45% 520-nm rhodopsins. The absolute sensitivity of both groups of males was close to that of the older females. All flies had substantial ultraviolet sensitivity, averaging 67% of the sensitivity at the longer wavelength maximum. The role of the differing sensitivities in males and females, and in young and old females, is discussed in relation to the visual behavior and sexual dimorphism of horse flies.