Wildlife Photographer of the Year: the curious case of parasitic bat flies

Flies are a fabulous and incredibly diverse group of animals and bat flies are just one fantastic example.

A prominent parasite

As a scientist was taking measurements of a Mozambican long-fingered bat during a biodiversity survey in Gorongosa National Park, she spotted a spider-like fly clinging tightly to the small mammal's fur.

Dr Piotr Naskrecki, a wildlife photographer and entomologist, happily took a few photos of the large parasite and plucked it off the bat to identify it.

'At the time of taking the photo, I was already quite familiar with parasitic bat flies but was quite surprised at the sight of the giant individual right on the bat's face,' says Piotr.

Bat flies are external parasites that spend almost their entire lives on bats, feeding on their blood. There are hundreds of species of bat fly. Some occur on multiple species of bat, whereas others are much more host specific. The insects belong to two families – Streblidae and Nycteribiidae. The individual in Piotr's image is a member of the latter group and assigned to the genus Penicillidia.

Bats are incredibly dextrous so you might think that they would choose to rid themselves of the hitchhiking flies.

'Bats can reach any point on their body quite easily and are very fast but I have never seen them try to get rid of a bat fly. Maybe years of experiencing these flies has taught them that they have no chance of removing them, so they have just learned to live with them,' Piotr suggested in a Nature Live Online livestream.

The flies move around on their host but usually nestle into the fur on the lower back. This bat, however, didn't seem particularly perturbed by the parasite's decision to cling to its face instead.

'Since bats' main mode of gathering information about their surroundings is echolocation rather than vision, it is not critical for the bat to have its eyes unobscured, which may explain why the bat did not attempt to remove the parasite.'

Piotr's image, Big Bat Bloodsucker, was awarded Highly Commended in the Wildlife Photographer of the Year competition.

On entering this image, he says, 'I thought that it was the perfect combination of a visually interesting scene and an interesting biological relationship between the two organisms.'

Flying without wings

True flies, a group also known as Diptera, are thought to have originated around 260 million years ago, long before the first bats. It is likely that as mammals evolved, flies began to associate with this new niche, co-evolving over millennia and ultimately leading to parasitic bat flies.

'I find parasites immensely fascinating,' says Piotr. 'Their bodies are exquisitely adapted to life on their hosts, often becoming dramatically different from their nearest free-living relatives.

'In the case of the bat fly, its body has been modified by natural selection to resemble a hard-bodied spider, capable of running very fast on the fur of its bat host.'

A parasitic lifestyle has caused some bat fly species to permanently lose their wings through evolution. Other bat flies have wings but shed them when they find a host. There are some species that retain theirs, however, they don't have great aerial agility and rely on their bat hosts for most of their air travel.

'A lot of these sibling species have lost their wings, they've lost their halteres – they've lost what it means to be a fly. But they have become these hardened warriors,' Dr Erica McAlister, Senior Curator of Diptera at the Museum, explained in a livestream.

Bat flies can also either lack eyes or have vision that only enables them to sense light. Instead, they may find their way about using their sense of smell and with the hairs (setae) covering their bodies, which may allow them to sense air currents and vibrations.

Even so, the bat flies' ability to hold on tight to their host is critical, as they can't survive long without access to blood.

'They've got really amazing claws to cling onto the fur of the bats. For some, if they fell off a bat mid-flight, that's them basically ruined. They really need to hang on tight,' says Erica.

Live young

The only time a bat fly intentionally leaves its host is to give birth.

'While most flies lay hundreds or thousands of eggs, hoping that at least one will survive, bat flies give live birth to a single, fully developed larva,' explains Piotr.

While they don't produce many in comparison to other flies, bat flies can produce multiple young in their lifetime but invest in just one at a time. A single egg hatches inside the female fly's body, and the larva feeds, grows and moults, all while being nourished via a milk gland.

The adult fly leaves her host to find a suitable site near the bat roost, such as a cave wall or tree branch, to give birth to the larva just before it's ready to pupate. Once it is deposited the female bat fly hurries back to the safety of her host.

'Because this happens right inside a bat colony, the adult fly that the larva will eventually turn into is almost certain to find a host. Thus, there is no need for producing a very large number of larvae.'

While giving birth to live young may be something that is typically associated with mammals, a number of other animals also do so, including some reptiles and sharks. All members of the fly superfamily Hippoboscoidea, which includes bat flies, tsetse flies and louse flies, give birth to live young. 

Unloved and misunderstood animals

Piotr has been both a participant in Wildlife Photographer of the Year and a member of the jury, and finds that 'unloved' animals like parasites are often underrepresented in the competition.

'Yet they are often more interesting and beautiful than the bulk of the organisms that receive the spotlight,' he says.

But championing unloved and misunderstood animals, such as a spider-like fly, can present challenges.

'The response to the image ranged from very positive to very negative. This image was reported several times to Facebook for promoting violence and the image was censored by social media.'

'Nature is amoral – neither bad nor good. The fly doesn't want to hurt the bat, it has to feed on its blood to survive.'

Bat flies are a group of animals that many may have never come across before, and Piotr hopes that his image helps highlight just how fascinating and unexpected the natural world can be.

He says, 'I hope this image will make some people start paying attention to smaller, often neglected members of the natural world.

'We are surrounded by wonderful, often incredibly interesting organisms, and their only problem is that they are much smaller than us and thus go unnoticed.'

Book tickets to visit to the Wildlife Photographer of the Year exhibition or explore the online gallery and discover the breathtaking diversity of the natural world.