Stripes and checked patterns repel horseflies

There’s nothing which ruins a picnic quicker than a downpour – or a swarm of uninvited flies joining the feast.

But now, experts have discovered a simple solution to this all-too-familiar plague: Sit on a checked picnic blanket.

According to the new study, chequered and striped patterns trigger an optical illusion in the eyes of a horsefly that dazzles it, preventing it from landing and biting.

The discovery came after scientists from the University of Bristol set out to discover the mechanism behind the apparent disorientating effect of zebra stripes on flies.  

Study authors say the exact mechanism remains unknown, but finding out chequered patterns also dazzle flies prove it isn’t the ‘aperture effect’.

This would only work if the flies were only distracted by stripes – it is similar to an effect in human sight where a striped barber-pole appears to move upwards.

The study only applied to horseflies and the researchers did not look at whether striped and chequered patterns could fend off other insects, such as wasps. 

The discovery came after scientists from the University of Bristol set out to discover the mechanism behind the apparent disorientating effect of zebra stripes on flies

Lead author Martin How from the University of Bristol told MailOnline: ‘These can be a pest to humans, so wearing patterned clothes would also protect us from bites.’ 

The team set out to rule out mechanisms that ‘can’t apply’ such as the aperture effect phenomenon that would require a striped regular pattern to work. 

The question of why zebras are striped has puzzled biologists for over 150 years and study author Tim Caro has worked to discredit many theories over the past decade.

Professor Caro has examined theories such as their use as camouflage from predators, a cooling mechanism and their a role in social interactions.

Stripes acting to confuse predators is another common explanation, but it too is flawed when looking at the scientific data, according to Caro. 

Instead, mounting evidence suggests that it is parasitic flies that are confounded by the zebra’s distinctive patterning and this new study set out to find out how.

The mystery of why zebras have their characteristic stripes has perplexed researchers for over a century

The mystery of why zebras have their characteristic stripes has perplexed researchers for over a century

‘Moving stripes, such as those on the rotating barber-pole signs outside barbershops, appear to move at right angles to the stripe, rather than in their true direction, so the pole appears to move upwards, rather than around its axle.

‘We set out to see if this illusion also takes place in the eyes of biting flies as they come to land on striped hosts,’ said Professor How.

‘As any fly approaches a landing surface, it will adjust its speed according to how quickly the surface expands across its vision, enabling a slowed and controlled landing.

‘Stripes however could disrupt this ‘optic flow’ through the aperture effect, leading the fly to believe the landing surface is further away than reality. Thus, the fly fails to slow down or land successfully.’

The findings published in Proceedings of the Royal Society B add to mounting evidence that parasitic pests are confounded by the zebra’s distinctive patterning. 

In Africa, blood sucking horse flies carry diseases that may seriously harm zebras, which led the beasts to evolve stripes which help to protect them.


One hypothesis for how stripes on a zebra keep insects at bay is that they interfere with optic flow patterns.

These are needed by flying insects to execute controlled landings. 

This could occur through disrupting the radial symmetry of optic flow via the aperture effect, or through misregistration of repeated features – the evenly spaces stripes. 

By recording and reconstructing fly behaviour around horses wearing differently patterned rugs, a study tested out different mechanisms. 

They found that flies avoided landing on, flew faster near, and did not approach as close to striped and checked rugs compared to grey. 

‘Our observations that flies avoided checked patterns in a similar way to stripes refutes the hypothesis that stripes disrupt optic flow via the aperture effect,’ the University of Bristol team explained.

They said this critically demands parallel striped patterns. 

This means another mechanism has to be at play between flies and horses. 

A series of experiments backed the theory by narrowing down the possible mechanism that lead to the confused landing among horseflies.

Previously, the same University of Bristol team showed the flies would approach horses in striped rugs as often as plain rugs. But they failed to land or slow down. 

Essentially, stripes disorientated the pests – making them collide with the skin or buzz off in another direction.  

The researchers covered domestic horses with different rugs and recorded the flight of flies attempting to land – 3D reconstructions found horizontal and vertical stripes as well as checked patterns were equally as effective at repelling flies. 

Checked rugs provide visual input free from the aperture effect, the team explained, saying in theory the flies would be expected to land without difficulty.

Yet flies had real problems – hardly landing on checked or striped rugs at all – suggesting that stripes are not unique when it comes to deterring horseflies.  

‘Not only do these exciting studies bring us closer to understanding one of the world’s most iconic and photogenic species, they will be of great interest to farmers attempting to reduce the damage caused by fly bites,’ said Caro.

He said that it would also be of interest to horse-wear companies who could create new clothing that is designed to distract horseflies. 

Protecting livestock from flies in the spring before they are seen on-farm is vital to prevent major economic production losses.

Zebras are quite good at not getting bitten by flies – not just through their stripes.

They use their tails to bat them away and, when flies do land, they don’t stay long because zebras move around a lot. 

The findings have been published in the journal Proceedings of the Royal Society B. 


There are several possible explanations as to why zebras have black and white stripes, but a definitive answer remains to be found. 

There are a number of theories which include small variations on the same central idea, and have been divided into the main categories below.

  1. Apparent size increase 
  2. Visibility in poor light 
  3. Moving stripes may dazzle predators
  4. Camouflage 
  5. Social benefits
  6. Fitness indication 
  7. Protection from tsetse flies. 

The areas of research involving camouflage and social benefits have many nuanced theories.

For example, social benefits covers many slight variations, including:

  • Zebras recognise each other on the basis of their stripes
  • This is especially important in the visual communication between mothers and their foals
  • Stripes might also be visual markers for group bonding or to direct companions to particular parts of the body for grooming.

Anti-predation is also a wide-ranging area, including camouflage and various aspects of visual confusion.   

These explanations have been thoroughly discussed and criticised by scientists, but they concluded that the majority of these hypotheses are experimentally unconfirmed.

As a result, the exact cause of stripes in zebra remain unknown.