In December 2021, the South African dipterologists (entomologists specialising in flies) John Midgley and Burgert Muller set off on a long-anticipated collecting mission to Lesotho, the only country in the world that has its entire territory located at an altitude of 1,000 metres (3,280ft) and higher.
“Looking at the maps, we knew that the high-altitude plateau in north-eastern Lesotho would be interesting,” says Midgley. “Whenever you have an ‘island’ that’s cut off from its surroundings you get cool things.” Especially when you can count on the fingers of one hand the number of entomologists who have been there.
Their first collecting site was the Afriski mountain resort, at an altitude of 3,050 metres. Unfortunately, it rained the whole time they were there, rendering their flight-interception traps useless. With hardly any insects flying, they were forced to do a lot more sweeping – using strong nets to catch whatever was hiding in the bushes – than usual.
On the second day of collecting, Muller caught what he assumed was a type of wingless moth that he had previously seen at high-altitude sites. “I just sucked it up into the apparatus and popped it in a collecting bottle,” he recalls. On closer inspection that evening, however, he saw that it was clearly a fly. “She had what are called halteres, balancing organs ironically used during flight, and stubby little wings. The head was also clearly that of a fly.”
The two scientists, part of the Diversity of Pollinating Diptera in South African Biodiversity Hotspots project, had taken a microscope into the field, so they were able to pinpoint the mystery fly’s genus – Atherimorpha – that evening. Interestingly, they had caught 51 male specimens of Atherimorpha latipennis (a species discovered in 1956 but whose female had never been described) on the same day and the evidence pointed to their female find belonging to the same species.
“It’s not unheard of for only the female of a species to be flightless,” says Midgley. “But there were no examples in this fly’s family, let alone its genus.” The pair realised that to be sure, they would have to wait until Midgley returned to his base at the KwaZulu-Natal Museum in Pietermaritzburg, which holds the reference collection for the genus. With the specimen safely stashed, they visited six other sites in Lesotho over the next two weeks, identifying in the process at least one species new to science.
Back home, Midgley was able to confirm that the flightless fly with stunted wings was indeed a female A. latipennis. Despite the strange morphology of the female, the mouthparts and antennae were almost identical to the males they had collected. “The last experts to work on the genus emphasised these as distinguishing features, and we felt it was quite obvious that we had the right species,” he says. They decided against taking a sample for DNA analysis as it would have damaged their only specimen.
Martin Hauser, a senior dipterologist at the California Department of Food and Agriculture, who was not involved in the research, says: “Active flight has only originated four times in the last three billion years, so it’s always interesting when a species loses the ability to fly. It isn’t super surprising to find flightless species. But it is remarkable when the first case of flightlessness is reported in a family.”
Because nothing is known about the life cycle of A. latipennis, the scientists could only make educated guesses about why the female had lost the ability to fly.
There are many benefits to flying: it’s much faster than walking (especially if you’ve got 0.5cm-long legs) and it allows you to escape predators. “But flight is also costly,” says Midgley. “You have to grow wings, and it uses a lot more energy than walking.” In many species this cost-benefit analysis can be different for males and females, as you only need one male to mate with dozens of females.
“For the males it is worth flying around and being able to search a larger area for females,” says Hauser. “Even if, while flying, they are exposed to birds and other predators, and risk being blown off the mountain and ending up in a hot valley with no females.”
It would probably make sense for more species to have flightless females, adds Midgley, especially in places such as forests and caves, where flight is not that beneficial. “But that’s not how evolution works. It always has to start with a random genetic mutation and, from there, selection occurs … That’s why you don’t see it happening across the board.”
It’s not only insects that can lose the ability to fly. The general consensus is that ratites – flightless birds with flat breastbones such as ostriches, kiwi and emus – expanded into terrestrial habitats after the dinosaurs became extinct. There were no predators big enough to hunt them, so flight had fewer benefits.
Even more interesting are penguins, which still use their wings to “fly”, but only in water. Here the loss of flight probably came about because fully fledged wings were not efficient for moving in water. It was about there being an increased cost to retaining the capability to fly.
Midgley points out that puffins can “fly” on air and in water. “They are OK in both, but not as good at swimming as penguins and not super-agile flyers either.”
Midgley believes the discovery of the flightless female fly in Lesotho helps us understand the bigger picture. “Understanding the morphology of range-restricted species helps us predict how they will react to changes in the environment. Had we only known about the male, we might interpret this as a species that could easily move in response to climate change. Knowing that this isn’t the case can help us to protect an endangered ecosystem.”
Hauser agrees: “When we study larger questions in evolution, we need examples like this to come up with and test hypotheses. Like a house built of many bricks, each single brick is not very crucial. But without the bricks, you wouldn’t have a house.”
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