Photostimulus intensity, rather than colour or polarization, is the key driver for stable fly visual attraction in laboratory bioassays

Foraging hematophagous flies such as the stable fly, Stomoxys calcitrans, are shown or expected to sense multiple characteristics of visual host cues. These include relative intensity, spectral composition, and the angle and degree of polarization. Here, we investigated the role of each of these visual cue characteristics on stable fly attraction. In laboratory two-choice bioassays, we offered groups of female or male stable flies a choice between paired photostimuli that differed in their visual characteristics, recording the flies' landing responses on stimuli as the response criterion. Bioassay flies preferentially selected lower-intensity visual stimuli even when paired stimuli markedly differed in spectral composition and polarization characteristics (a high degree of horizontal or vertical polarization). When paired stimuli had identical relative intensity, flies failed to discriminate between the stimulus that presented ultraviolet, blue, green, and red wavebands and a corresponding paired stimulus lacking one of these wavebands. Our results support the hypothesis that foraging stable flies may respond to colour and polarization characteristics of visual stimuli as modulations of perceived intensity. Acquired knowledge on the mechanisms that underlie stable fly visual attraction may inform the design of vision-based traps so that they become optimally attractive to stable flies and other biting flies.