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.