For a long time, biologists were puzzled by the curious appearance and behaviour of the New Zealand bat fly. Now they think they’ve figured it out. Nick Baker reports…
There’s a fly that doesn’t have wings and a bat that doesn’t use the ones it has as much as you might expect. Both these evolutionary oddballs live side by side in a weird marriage made in heaven… or rather New Zealand.
The New Zealand bat fly (Mystacinobia zelandica) is found hanging around and on the furry bodies of the country’s only endemic mammals, bats, and one species in particular: The New Zealand lesser short-tailed bat, with which it shares an intimate and unique relationship.
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The concept of a fly on a bat isn’t surprising, as bats worldwide are riddled with all kinds of ectoparasitic flies, as well as mites, lice and fleas. Most of these flies feed on bats’ blood, but not in New Zealand. It is thought that the ancestors of the country’s bats arrived before parasites had evolved to exploit them – about 79 million years ago, when New Zealand parted company with Gondwana. Consequently, living in isolation on these islands, they’ve developed a totally different relationship with a totally different fly.
The New Zealand bat fly was only discovered in 1973 when a kauri tree containing a roost of New Zealand lesser short-tailed bats fell over, bringing the rare community into view
What is a New Zealand bat fly?
The New Zealand bat fly is different in many ways. For a start, it doesn’t look like a fly at all, lacking a pair of wings. It has given up flight and appears to be more like a scuttling spider missing a pair of legs.
It doesn’t need eyes either. Instead, it is equipped with bristles to sense and feel, and each leg is adorned with large hooks – helpful if you need to cling onto a bat’s fur. But it’s the fly’s behaviour that is of particular interest to biologists.
“It is more like a scuttling spider missing a pair of legs”
What do New Zealand bat flies eat?
In nature, when you find two unrelated species living in such intimacy, there is often a transaction of some kind going on – most often a parasitism. But unlike so many of their kind, New Zealand bat flies don’t appear to be freeloading off their hosts. Analysis of their stomach contents showed they didn’t contain any bat blood, just fruit and pollen. All they were stealing was a little body heat, which may not be insignificant in New Zealand’s often cool and temperate climate.
Still, without wings and without a hematophagous habit, it was a puzzle that had scientists miffed. What were they feeding on? How did they get pollen if they couldn’t fly about? Conversely, the bats’ stomachs didn’t contain any bat flies – odd for an animal more than partial to munching on invertebrates. So what was going on?
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The relationship between fly and bat
It turns out that the bat colonies are a warm, secure home for the flies, with plenty of food provided in the form of bat droppings, and in consuming this rich, gloopy soup, the flies are, in turn, providing a clean-up service for the bats. The droppings might not look or smell that appetising to us, but they are delicious and nutritious to a bat fly. The omnivorous diet of the lesser short-tailed bats explains the pollen in the flies’ stomachs.
Living in this prosperous fly utopia has also given the flies the opportunity to evolve something akin to the ‘caste system’ of more famous social insects such as bees, a unique feature among flies.
Members vary considerably in size and the beefy mature males take on colony guard-duty. When disturbed by a bat, they move towards the intruder and buzz like a furious dentist’s drill. The high-pitched sound is produced by the muscles housed in their chunky thorax. These muscles would have powered the wings of their distant ancestors, but they haven’t atrophied as might be expected and have been repurposed as a kind of sonic gun. This acoustic assault appears to be an effective deterrent to stop the bat fly from being eaten by its host and the bat from losing its friend with benefits.
The lesser short-tailed bat
The lesser short-tailed bat is one of only three native species of terrestrial mammal found in New Zealand, the others being the short-tailed bat and the greater long-tailed bat (the latter is presumed extinct having last been seen in 1967).
The lesser short-tailed bat has filled a niche that is exploited by mice and shrews on the continents. It spends much of its time scurrying around on the forest floor feeding on flowers, fruits and insects.
Illustration credit: Peter David Scott/The Art Agency