There is increasing interest in flies as potentially important pollinators. Flies are known to have acomplex visual system, including 4 spectral classes of photoreceptors that contribute to the per-ception of color. Our current understanding of how color signals are perceived by flies is based ondata for the blowflyLuciliasp., which after being conditioned to rewarded monochromatic lightstimuli, showed evidence of a categorical color visual system. The resulting opponent fly colorspace has 4 distinct categories, and has been used to interpret how some fly pollinators may per-ceive flower colors. However, formal proof that flower flies (Syrphidae) only use a simple, categor-ical color process remains outstanding. In free-flying experiments, we tested the hoverflyEristalistenax, a Batesian mimic of the honeybee, that receives its nutrition by visiting flowers. Using arange of broadband similar–dissimilar color stimuli previously used to test color perception in polli-nating hymenopteran species, we evaluated if there are steep changes in behavioral choices withcontinuously increasing color differences as might be expected by categorical color processing.Our data revealed that color choices by the hoverfly are mediated by a continuous monotonicfunction. Thus, these flies did not use a categorical processing, but showed evidence of a colordiscrimination function similar to that observed in several bee species. We therefore empiricallyprovide data for the minimum color distance that can be discriminated by hoverflies in fly colorspace, enabling an improved understanding of plant–pollinator interactions with a non-modelinsect species.
