Fig. 1. Optical characteristics of Eristalis tenax eyes showing the `bright zone' of males, in which facet diameter is maximal in the fronto-dorsal eye but is not associated with smallest interommatidial angle. Interommatidial angles are given as the average of the angles across horizontal and vertical sampling baselines. Data are from a single male and single female fly and in close agreement with data from the other fly tested of each sex. f=frontal, d=dorsal, l=lateral.