Saturday, February 25, 2023

Why do horseflies avoid stripes?


(c) Martin How
Researchers at the University of Bristol have discovered the reason why horseflies tend to avoid attacking individuals with stripes, as opposed to those with big, solid dark patches. 

Their findings show that stark black-and-white contrasts and small dark patches are particularly
effective in preventing horsefly attack. Specifically, these features eliminate the outline of large, single-colour dark patches, which horseflies find attractive at close range. 


The team suggest that the thin back stripes serve to minimise the size of local features on a zebra that are appealing to the biting flies.


The research was led by Professor Tim Caro and Dr Martin How both from the University of Bristol’s School of Biological Sciences. An open access report of the work is published in the Journal of Experimental Biology.


 Prof Caro explained: “We knew that horseflies are averse to landing on striped objects - a number of studies have now shown this, but it is not clear which aspects of stripes they find aversive.


“Is it the thinness of the stripes? The contrast of black and white? The polarized signal that can be given off objects? So, we set out to explore these issues using different patterned cloths draped over horses and filmed incoming horseflies.”


The team found that tabanid horseflies are attracted to large dark objects in their environment but less so to dark broken patterns. All-grey coats were associated with by far the most landings, followed by coats with large black triangles placed in different positions, then small checkerboard patterns in no particular order. In another experiment, they found contrasting stripes attracted few flies whereas more homogeneous stripes were more attractive.


Professor Caro added: “This suggests that any hoofed animal that reduces its overall dark outline against the sky will benefit in terms of reduced ectoparasite attack.”


The team found little evidence for other issues that they tested, namely polarization or optical illusions confusing accurate landings such as the so-called ‘wagon-wheel effect’ or ‘the barber-pole effect’.


They conclude: “Our working hypothesis now is that horseflies are attracted to equid hosts owing to a combination of odour at a distance, then size of the animal contrasted against the sky or vegetation at a middle distance. But at close range, where they can no longer see the body's outline, flies make a visual switch to local features. If these are small dark objects contrasted against a light or white background, the horsefly no longer recognizes this as a host target and veers away. The contrast of stripes and their relatively small size are therefore the key elements of how stripes operate to thwart fly landings.”


Now the team want to determine why natural selection has led to striping in equids - the horse family - but not other hoofed animals.

Professor Caro added: “We know that zebra pelage – fur - is short, enabling horsefly mouthparts to reach the skin and blood capillaries below, which may make them particularly susceptible to fly annoyance, but more important, perhaps, is that the diseases that they carry are fatal to the horse family but less so to ungulates. This needs investigation.”


For more details, see:


Why don't horseflies land on zebras?

Tim Caro, Eva Fogg, Tamasin Stephens-Collins, Matteo Santon, Martin J. How

J Exp Biol (2023) 226 (4): jeb244778.

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