Hosefly sitting on blade of grass

©IanRedding/ShutterStock

Are horsefly bites on the rise?

As the weather warms up the UK is often swarmed with reports of biting horseflies. A Museum fly expert explains what is going on.

Often large, persistent and painful, a bite from a horsefly is an experience unlikely to be forgotten. But why do horseflies seem to appear more in the summer months?

The insects tend to be more active during heatwaves. Yet it might not be that there are more horseflies than usual out for blood, but simply that there are more people outside, enjoying the summer.

More people out and about in the warm weather, exposing their skin, will increase the likelihood of people being bitten by the flies (some of which are also called clegs). Coupled with the ubiquity of reports from newspapers to social media, this can make it look like it is a bumper year for the insects. 

Why horseflies bite

Not all adult horseflies bite - only the females have mouthparts able to break the skin and feed on blood. This is because only the females need a blood meal.

Dr Daniel Whitmore, Senior Curator of Diptera and Siphonaptera at the Museum, explains, 'They need a high protein input to help develop their eggs after fertilisation.'

The males don't make eggs, so they don't need blood.

Horsefly biting skin

Horsefly saliva prevents the blood from clotting. ©IanRedding/ShutterStock

 

The way that horseflies feed on blood can seem brutal when compared to the precision of a mosquito. A pair of serrated mandibles saw into the skin, cutting until they break small vessels and the blood begins to flow. An anticoagulant in the fly's saliva then prevents the blood from clotting as the insect sucks up its meal.

While mosquitoes release a mild anaesthetic, horseflies don't - which is one of the reasons their bites are so painful. The fact that they cut into the flesh rather crudely only adds to this pain.

'The horsefly bite is much less sophisticated, likely because usually they have to pierce through much tougher skins than ours when they are feeding on a cow or a horse,' says Daniel. These are horseflies' more usual food source, but they will home in on any large mammal they detect.

In most cases, a bite will result in a raised, red area of skin, which might be painful and sensitive to touch. In extreme cases, bites can cause an allergic reaction that can result in dizziness, swelling and fluid-filled blisters that become infected. If that happens, it is recommended that you seek medical advice.  

How to identify a horsefly

The easiest way to spot a horsefly is by looking at its overall size. The insect tends to be large compared to other biting flies, often with colourful and patterned eyes. In males the eyes are so big they touch at the top of the head.

Not all horseflies are dependent on water, but many species lay their eggs on plants growing near ponds, rivers and streams. The larvae of some species are aquatic while others live in damp soil. All feed on other invertebrates until ready to pupate and emerge as adults. 

This means you're more likely to come across the larvae around bodies of water, although the adults will disperse. Farms are frequently a hotspot for these flies, as they are attracted by cattle and horses. 

Male horsefly sitting on wood

Male horsefly, with eyes touching at the top of its head. ©Mark Robinson/Flickr CC BY-NC 2.0

 

How to avoid being bitten

One of the easiest ways to avoid being bitten is to cover up. But according to Daniel, even this might not be 100% effective.

'Horsefly females have such strong, powerful mouthparts that they can sometimes bite you through your clothes,' he says. 'But obviously they are more likely to go for bare skin.' It's probably best to opt for loose-fitting clothing.

There is also evidence that the flies home in on their next victim by sniffing out exhaled carbon dioxide. This means that those doing more strenuous outdoor activities could be at greater risk of being bitten. 

Sweet secrets of horseflies

'Obviously, bloodsucking species are annoying,' says Daniel, but he emphasises that they play a vital role in the ecosystem.

In the UK there are just 30 species of horsefly, but globally there are over 4,500 species whirring around every part of the world except the polar regions.

As the female horseflies seek out their next blood meal, the males are searching for something much sweeter. Buzzing from flower to flower, the male flies actually feed on nectar.

In some cases, this makes the flies crucial pollinators, coevolving with the plants on which they feed. 

Horsefly sat on skin

Horseflies can be identified by their colourful eyes. ©Magre Flåten/Wikimedia Commons

 

'There are some species of horsefly in which both sexes have extremely elongated mouthparts to suck nectar out of long-tubed flowers,' explains Daniel.

So entwined is this relationship that the flies are thought to be involved in driving new species of flowers to evolve, and vice versa.

Along with many other flying insects, horseflies are also a key food source for many other animals higher up the food chain. They help underpin other, more charismatic species such as bats and birds, while the aquatic larvae of the insects feed fish.

So even though horseflies might plague us humans during the summer months, they're an important aspect of the much wider ecosystem.

More information