Blood-seeking horseflies prefer vessel-imitating temperature gradients on host-mimicking targets: Experimental corroboration of a new explanation of the visual unattractiveness of zebras to tabanids.

Several hypotheses tried to explain the advantages of zebra stripes. According to the most recent explanation, since the borderlines of sunlit white and black stripes can hamper thermal vessel detection by blood-seeking female horseflies, striped host animals are unattractive to these parasites which prefer hosts with a homogeneous coat, on which the temperature gradients above blood vessels can be detected more easily. This hypothesis has been tested in a field experiment with horseflies walking on a grey barrel with thin black stripes which were slightly warmer than their grey surroundings in sunshine, while in shade both areas had practically the same temperature. To eliminate the multiple (optical and thermal) cues of this test target, we repeated this experiment with improved test surfaces: we attracted horseflies by water- or host-imitating homogeneous black test surfaces, beneath which a heatable wire ran. When heated, this invisible and mechanically impalpable wire imitated thermally the slightly warmer subsurface blood vessels, otherwise it was thermally imperceptible. We measured the times spent by landed and walking horseflies on the test surface parts with and without underlying heated or unheated wire. We found that walking female and male horseflies had no preference for any (wired or wireless) area of the water-imitating horizontal plane test surface on the ground, independent of the temperature (heated or unheated) of the underlying wire. These horseflies looked for water, rather than a host. On the other hand, in the case of host-imitating test surfaces, female horseflies preferred the thin surface regions above the wire only if it was heated and thus warmer than its surroundings. This behaviour can be explained exclusively with the higher temperature of the wire given the lack of other sensorial cues. Our results prove the thermal vessel recognition of female horseflies and support the idea that sunlit zebra stripes impede the thermal detection of a host's vessels by blood-seeking horseflies, the consequence of which is the visual (non-thermal) unattractiveness of zebras to horseflies.