'The photo was reported for depicting 'graphic violence', and of course there were accusations of photoshopping the whole thing,' he says. The flies ( Penicillidia sp), however, are both very real and very interesting. The ancestor of Penicillidia flies probably looked something like modern tsetse flies, which also feed on blood. But after millions of years of co-evolution with their furry hosts, bat flies have become highly specialised parasites. For starters, their bodies became flattened and very hard, making it almost impossible for their hosts to squash them.

Special 'claws' at the end of each foot and hairs on the legs make it extremely difficult to dislodge them from fur. 'These insects know the value of a good host and once they land on the furry back of a bat, they never leave it again,' explains Naskrecki. 'Female bat flies, like their relatives tsetse flies, are remarkably good mothers,' he adds. 'The great majority of insects lay hundreds or thousands of eggs, betting on one or two of them making it to adulthood. Instead of laying eggs the female gives birth to a single, fully developed larva, which immediately turns into a pupa.' Bat flies, on the other hand – like humans – prefer to invest a lot in a much smaller number of offspring, hoping that they will all make it to the reproductive age.

In fact, because the parasites can't survive for very long on their own, the only time a female bat fly will leave its host is when the time comes to drop her larva off in a safe place – usually the wall of the bat's cave roost. Then, she'll quickly rush back, guided by the smell and warmth of her host.

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