BACH TO BASICS
Nature has its own special symphony between birdsong and the buzzing of insects. But now researchers believe the musical mating calls of mosquitos could have practical use in the fight against malaria. Daniel A Gross describes how wingbeats are revealing
It’s a warm summer afternoon in the Tanzanian village of Lupiro, and Mikkel Brydegaard is crouching in a brick hut, trying to fix a broken laser. Next to him, on a tall tripod, three telescopes point through a window at a tree in the distance. A laptop rests on an upturned box, waiting to receive a signal.
With a working laser, this system is known as lidar – like radar, Brydegaard tells me, but using a laser instead of radio waves. The setup is supposed to gather precise data about the movement of malaria mosquitoes. But as the sun starts to set outside, Brydegaard is getting nervous. He and his colleagues have spent a week in Tanzania, and their device still hasn’t started collecting data. They’re almost out of time.
Tomorrow, a solar eclipse will blot out the sun over Tanzania – an event that occurs only once every few decades here, and that Brydegaard and his team from Lund University in Sweden have travelled thousands of miles to see. Their immediate aim is to see if the eclipse affects the behaviour of disease-carrying insects. Their larger mission, however, is to demonstrate that lasers can revolutionise how insects are studied.
Lidar involves shooting a laser beam between two points – in this case, between the hut and the tree. When insects fly through the beam, they’ll scatter and reflect light back to the telescopes, generating data from which the scientists hope to identify different species. At a time when pests destroy enough food to sustain entire countries – and when insect-borne diseases kill hundreds of thousands of people every year – this arrangement of beams and lenses could, just maybe, improve millions of lives. But without a working laser, the trip to Tanzania will count for nothing.
Already, the team have come close to giving up. A few days ago, their two high-powered lasers failed to work. “My first thought was, OK – pack everything, we head back,” Brydegaard tells me. “There’s nowhere in Tanzania we can find a spare part.” He thought bitterly about the tens of thousands of dollars they had spent on equipment and travel. But then he walked into town with Samuel Jansson, his graduate student, and over bottles of beer they scrolled through the contacts on their phones. Perhaps, they started to think, it was possible to salvage the trip after all.
Lasers may be a cutting-edge tool for identifying insects, but at the heart of the lidar method is an elegant and centuries-old principle of entomology. Almost every species of flying insect, from moth to midge to mosquito, has a unique wingbeat frequency. A female Culex stigmatosoma mosquito, for instance, might beat its wings at a frequency of 350 hertz, while a male Culex tarsalis might at 550 hertz. Because of these differences, an insect’s wingbeat is like a fingerprint. And in recent years, the study of wingbeat has undergone a renaissance, especially in the field of human health.
Long before lasers or computers, wingbeat was thought of in auditory – even musical – terms. A careful listener could match the buzz of a fly to a key on the piano. That is exactly what Robert Hooke, a natural philosopher, did in the 17th century: “He is able to tell how many strokes a fly makes with her wings (those flies that hum in their flying) by the note that it answers to in musique during their flying,” wrote Samuel Pepys, a British civil servant and friend of Hooke’s.
But the fact that Hooke relied on his ears must have made his findings difficult to communicate. Knowledge was traditionally shared through scientific papers, letters and specimen drawings, and so entomologists tended to rely on vision rather than hearing. “The field has had a very, very narrow focus for a long time,” says Laura Harrington, an entomologist and epidemiologist based at Cornell University, New York State.
In the 20th century, however, researchers began to break the mould. The main wingbeat detection method was visual: the chronophotographic method, which involved taking photographs in rapid succession. This had its limitations, and a few keen-eared researchers felt there was an advantage to Robert Hooke’s auditory approach – especially Olavi Sotavalta, an entomologist from Finland who had the rare gift of absolute pitch. Just as a composer with absolute pitch might transcribe a musical passage by ear, Sotavalta could identify the precise tone of a mosquito’s wings without the aid of a piano.
“The acoustic method makes it possible to observe insects in free flight,” Sotavalta wrote in a 1952 paper in Nature. In other words, because he had absolute pitch, Sotavalta was able to make wingbeat observations not only with cameras in the laboratory, but also in nature, with his ears. Scientists are informed and constrained by the senses they choose to use.
Sotavalta’s peculiar approach to research suggests that certain scientific insights emerge when separate disciplines collide: he used his canny ear not only to identify species during his research, but also for music. “He had a beautiful singing voice,” says Petter Portin, an emeritus professor of genetics who was once a student of Sotavalta’s. Portin remembers him as a tall, slender man who always wore a blue laboratory coat. Sotavalta’s papers in the National Library of Finland are a curious combination of letters, monographs on insect behaviour, and stacks of sheet music. Some of his compositions are named after birds and insects.
We discovered that males and females actually sing to each other. They harmonise just prior to mating”
becomes clear that he’s trying to apply music theory to birdsong. “The song of the two Sprosser nightingales (Luscinia luscinia L) occurring in two successive years was recorded acoustically and presented with conventional stave notation,” he wrote.
Following on from this are nearly 30 pages of notes, graphs and analysis of the rhythm and tonality of the birds. After highlighting the similarity between the two songs, he declares: “Because of the short distance between the places where they were singing, it was concluded that they were perhaps father and son.” It is as though his work is a search for some kind of pattern, some musical idea, shared by members of the same species.
However, his paper in Nature was rather more consequential. There, Sotavalta describes the uses of his “acoustic method” of identifying insects using his absolute pitch, and theorises about the subtleties of insect wingbeat: how much energy it consumes, and how it varies according to air pressure and body size. Even so, only decades later did scientists such as Brydegaard reaffirm the relevance of wingbeat in the study of insects – for example, malaria-carrying mosquitoes.
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In Tanzania, Brydegaard, Jansson and engineer Flemming Rasmussen do not have absolute pitch – and, even if they did, it wouldn’t help much. There are millions of insects in and around the village, and they drone on in a symphony that never ends. What these scientists have, in place of a keen ear, is a high-tech gadget and two broken lasers. And their phones.
When the lasers failed, it took a few false starts to find a solution. A researcher in Côte d’Ivoire had a working laser, but he was away in the USA. Brydegaard considered sending for a replacement by mail, but knew that – thanks to customs and the day-long drive from the airport in Dar es Salaam – it probably wouldn’t arrive in time for the eclipse.
Finally, they sent a text message to Frederik Taarnhøj, CEO of FaunaPhotonics, their commercial partner, and asked if he would consider sending a scientist from Sweden with some spare lasers. Taarnhøj said yes. So the trio made a few frantic calls and ultimately convinced another graduate student, Elin Malmqvist, to board a plane the very next day. When she did, she was carrying three small metal boxes in her suitcase. The saga was not over yet, however. Even after the huge expense of the last-minute flight, the first replacement failed: Brydegaard, in his hurry, confused the anode with the cathode, which short-circuited the laser diode. The second laser yielded a beam, but, inexplicably, it was so faint as to be unusable.
It’s the last laser that Brydegaard now unpacks, hoping that at least this one will work as expected. By the time he screws it onto the tripod, it is almost sunset, and his agitation is palpable. Within the hour, it will be too dark to calibrate even a working laser. Everything rides on this piece of equipment.
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Laura Harrington’s laboratory at Cornell looks a little like a restaurant kitchen. What resembles the door to a walk-in freezer actually leads to an incubation room. It’s humid and lit by fluorescent lights. The shelves are covered in carefully labelled boxes. Harrington shows me mosquito eggs inside the kinds of disposable
containers you’d carry soup in. Over the top of the containers, to prevent mosquitoes from escaping, there’s some kind of net – bridal veil, she tells me. The method is not quite foolproof. A few mosquitoes have escaped, and they buzz around our ears and ankles while we chat.
When we talk about Sotavalta’s approach, Harrington says that he was “definitely ahead of his time”. Even in recent years, researchers who thought to listen to mosquitoes didn’t realise how many insects are capable of listening, too. “For a long time, scientists thought that female mosquitoes were deaf – that they didn’t pay attention to sound at all,” Harrington says.
But in 2009, Harrington put that long-standing assumption to the test. In an unusual and intricate experiment, she and her colleagues tethered a female Aedes aegypti mosquito to a hair, installed a microphone nearby, and placed both inside an upside-down fish tank. Then they released male mosquitoes inside the tank and recorded the results.
The team’s findings astonished Harrington, and led to a breakthrough in the study of sound and entomology. Aedes aegypti conducted a sort of mid-air mating dance that had everything to do with sound. Not only did female mosquitoes respond to the sounds of males, they also seemed to communicate with sounds of their own. “We discovered that males and females actually sing to each other,” Harrington says. “They harmonise just prior to mating.”
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This “mating song” isn’t produced by vocal cords. It is produced by flapping wings. During normal flight, male and female mosquitoes have slightly different wingbeats. But Harrington found that during the mating process, males aligned their wingbeat frequency with that of females. “We think the female is testing the male,” Harrington explains. “How quickly he can converge harmonically.” If so, mosquito songs may function like auditory peacock features. They seem to help females identify the fittest mates.
With these results in mind, and with a recent grant from the Bill & Melinda Gates Foundation, Harrington’s lab has begun development of a novel mosquito trap for field research. Similar projects have been undertaken by teams at James Cook University in Australia and Columbia University in New York City, among others.
For a researcher, there are drawbacks to the mosquito traps that currently exist. Chemical traps have to be refilled, while electric traps tend to kill mosquitoes; Harrington wants her new trap to harness the power of sound to capture living specimens for monitoring and study. It would combine established methods for attracting mosquitoes, like chemicals and blood, with recorded mosquito sounds to mimic the mating song. Importantly, it could be used to capture mosquitoes of either sex.
Historically, scientists have focused on catching female mosquitoes, which twice each day go hunting for mammals to bite – and which may carry the malaria parasite (males do not). But scientists have recently started to consider male mosquitoes an important part of malaria control too. For instance, one current proposal for curbing the disease involves releasing genetically modified males that produce infertile offspring, to reduce the population of disease-carrying mosquitoes in a given area.
Harrington’s hope is that an acoustic trap – using the mating song that attracts males – would help make new strategies such as this possible. “What we’re trying to do is really think outside the box, and identify new and novel ways to control these mosquitoes,” she says.