David Hu might seem like a typical mathematics student. He wears glasses and polo shirts, creates Web pages about his work, and even has a spoof of the Google search engine, a page he calls Hoogle, welcoming visitors to his Web site.

But on September 28th, the doctoral candidate successfully defended his thesis to a committee of professors at the Massachusetts Institute of Technology – a dissertation titled "The hydrodynamics of water-walking insects."

In other words, Hu has spent the last four years trying to figure out how bugs walk on water.

Insect locomotion may seem an unusual area of interest for a mathematician, but Hu’s success is certainly not the first instance of an individual from a seemingly unrelated field making significant contributions to another area of research. In fact, Hu and his advisor, associate professor John Bush, relied largely on technology pioneered by another surprising contributor to the study of animal locomotion – 130 years earlier.

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In 1872, Eadweard Muybridge was a successful photographer living in San Francisco, California. Already famous for his photographs of landscapes, he would become even more renowned over the course of the next eight years as a trailblazer in the use of high-speed photography.

Popular legend has it that Muybridge’s first high-speed photographic work was the result of a bet with businessman and former governor Leland Stanford, but in fact Muybridge was not a betting man. Stanford was (among other things) a racehorse owner, and commissioned Muybridge to determine whether all four hooves of one of his best horses were suspended in the air simultaneously as he trotted. (That the horse, Occident, was galloping in the first photographs taken by Muybridge is another incorrect element of the popular version of events.) Muybridge took on Stanford’s request in the interest of advancing his skills as a photographer, not for money, and certainly not for the advancement of science. Little did he know that the latter was where his work would arguably become most important.

Muybridge began photographing horses in motion almost immediately after Stanford’s request, but six years went by before he was able to produce a satisfactory series of images showing a horse running at a full gallop. (It should be noted that his photographic endeavors were interrupted in 1874, when he took enough time off from the cameras to shoot and kill his wife’s lover, be acquitted of the man’s murder, and leave the U.S. for a period of time while things cooled down.)

Muybridge eventually returned to California and began using a line of cameras to photograph a horse as it galloped past them, but his first attempts were unsatisfactory. The photographic technology of the time was not suited for capturing moving subjects, let alone accommodating the use of multiple cameras requiring extremely precise timing to produce the desired series of images. Muybridge, after many attempts to mechanically open and close the shutters on his cameras faster than they were designed to move, decided to use a new approach: electricity.

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One hundred and thirty years into the future, David Hu had a different problem: Denny’s Paradox.

In 2001, Hu was working with an advisor, John Bush, who was an expert in surface tension. Bush suggested that Hu examine the paradox raised by biophysicist Mark Denny, which stated that infant water striders should not be able to move. The problem was based on the idea that water-walking insects move by creating capillary waves (for all intents and purposes, ripples) on the surface of the water. Denny demonstrated that such a method of motion would require an infant water strider to move its legs much faster than it is actually capable of doing. Based on that theory of water strider motion, infant water striders should not be able to move – hence the paradox. (Adult water striders can move their legs fast enough to achieve forward motion using this method.)

"It was a problem readily examined with traditional techniques in fluid mechanics," said Hu, such as the evaluation of motion using mathematics and physics. But these techniques did not explain how infant water striders defied the paradox by achieving motion.

Fortunately for the field of fluid dynamics, Hu wasn’t afraid to try more difficult and untraditional methods; he even developed a robotic water strider. But it would take quite some time before he found an unconventional approach that yielded new insight into the infant insects’ theoretically impossible motion.

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Back in the 19th century, Muybridge was busy with his plans to use electricity in triggering cameras. Before hitting upon this idea, he had tried repeatedly to fire his cameras using traditional mechanical methods – as a horse galloped past, it would trip wires strung across the track and attached to the camera’s shutters. But this method caused two problems: first, it triggered the shutters too slowly, so that the horse had generally moved past the range of a camera by the time it fired. And more problematic was the motion of the shutters themselves: once opened, they closed too slowly, allowing too much light to enter the camera and creating an image of a blur where a horse should have appeared.

To combat these problems, Muybridge turned to electricity. He devised a method wherein electrical current interacted with a magnet to trip shutters’ catches, allowing the cameras to fire almost instantly after a horse’s hooves touched the trip wires. The shutters also opened and closed faster, creating images that depicted their subject much more clearly than in Muybridge’s previous attempts. The now-famous 1878 series of images that became "The Horse In Motion" still showed nowhere near the level of detail expected from today’s photographic equipment, but they were clear enough to show the horse’s hooves in each image – and in the most famous image of the series, to show all four of them off the ground. Muybridge went on to photograph lions, elephants, birds, and humans – from boxers to gymnasts performing somersaults to himself, walking. Nearly any animal that moved captured his attention.

The creation of these sequences and the technique used to produce them were a huge advance for photography, but also for science. The images made it possible to break down motion that was generally too fast for the human eye to dissect completely.

As one filmmaker says, the type of photography Muybridge used to capture "The Horse In Motion" is essentially the opposite of what’s done when making a movie. A movie produces the illusion of motion by stringing together many static photographs and projecting them in order, upwards of 20 per second. But reverse that process by slowing the motion down and looking at each image, and you get Muybridge’s work: frozen time in fraction-of-a-second increments. Ironically, in the very next year, he would design a device to do just the opposite: to take his own static images and put them back in motion.

Muybridge presented his "zoogyroscope," which eventually became known as the zoˆpraxiscope, to Stanford in 1879. The device was what might be called a precursor to the modern movie projector, taking a series of images like the ones Muybridge was so talented at producing, and rapidly projecting the sequence to create something that looked like motion.

Together, Muybridge’s motion disassembly and reassembly techniques would play an important role in David Hu’s study of water striders. What would happen if a series of photographs actually consisted of copies of essentially the same image? The zoˆpraxiscope would project a static image. And as it turns out, a similar situation would solve Hu’s problem.

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As David Hu began to study insect locomotion and, specifically, Denny’s Paradox, he used a combination of pure fluid dynamics logic and high-speed photography to come to an unexpected conclusion.

Fluid dynamics equations seemed to support Denny’s Paradox; according to calculations, infant water striders shouldn’t be able to move, at least not by the method scientists assumed the insects were using.

Hu was familiar with both Muybridge’s work and the high-speed photographic work of fluid dynamicists, including that of Ludwig Prandtl, who authored a paper that included 12 photographs of water flowing past spheres in 1904. With Muybridge’s original strobe photography concepts and the fluid dynamics-specific work of Prandtl in mind, Hu set out to photograph water striders in a similar fashion.

In order to accomplish this, he needed sophisticated high-speed photography equipment. "The insects are exceedingly small, further making them difficult to see," said Hu, "and they move too fast to be seen with the naked eye." If it was difficult for Muybridge to break down the motion of a horse, imagine the amplified difficulty of deconstructing the motion of an insect smaller than a horse’s eye.

Fortunately, the technology behind Muybridge’s ideas has continued to advance over the past century and a half, and high-speed photographic equipment is sophisticated enough to do just what Hu wanted. And in a stroke of good luck, the equipment he needed was available at MIT – at a center named for the most famous heir to Muybridge’s photographic throne.

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Harold Eugene "Doc" Edgerton was born in 1903, one year before Muybridge’s death. He studied electrical engineering at the University of Nebraska before enrolling at MIT, where he eventually became a professor.

Having been exposed to the art of photography as a young child, Edgerton had a lifelong interest in the field, beginning his photographic adventures by using strobe lights to study objects in motion. In 1932, he began to use strobe photography to create sequential-image series like the ones Muybridge had produced over 50 years earlier, albeit much faster, and at a much higher resolution. Some of his more famous works include a sequence of photographs of a splashing milkdrop and another of a bullet passing through an apple.

Edgerton’s work, like Muybridge’s in the 19th century, became widely known and highly acclaimed. He continued to develop his talents and won numerous awards, revolutionized sports photography, and was commissioned by the U.S. Army for work in nighttime aerial reconnaissance photography, for which he eventually received a medal from the U.S. War Department. By 1940, it seemed that the 37-year-old Edgerton had accomplished more than most people do in a full lifetime. But he was far from finished.

Later that year, Edgerton took his talents to Hollywood, helping to establish the role of high-speed photography in movie making. In the mid 40s, his Army work was used just before the D-Day invasion. And in the next three decades, Edgerton would work with Jacques Cousteau, develop a technique used to capture images of human blood flow, locate a warship that had been resting on the sea floor since the 1500s, locate a U.S. battleship that had been missing since the 1800s, and receive the U.S. National Medal of Science, among other honors. Eventually, MIT would establish an educational center in his name: the Edgerton Center, home to a number of programs for MIT undergraduates, as well as a state-of-the-art high-speed imaging facility.

MIT students are encouraged to use the photographic and video equipment available at the Edgerton Center in the course of their research, and David Hu took advantage of it. Eventually he focused on a specific aspect of insect motion: a water strider’s ability to ascend a meniscus, the point at the "end" of a liquid surface where the liquid meets a solid. You can see an example of this if you look through a clear drinking glass with water inside – the water curves upward slightly at the point where it meets the glass. This curvature is barely perceptible to a human, but to a small insect, it’s a considerable obstacle.

Some water striders can leap small distances, and can transition from water to land using that ability. Others lack this talent, and so must use another method when they want to move to dry land. Hu set out to study this method, and used the Edgerton Center equipment to capture the motion of a land-bound insect.

Water striders have six legs, and a natural assumption would be that in order to climb a meniscus and leave the water, the insect would have to move those legs. But high-speed photographs captured by Hu showed no evidence of the insect’s legs moving. An observer could almost envision Muybridge scratching his head while feeding a sequence of photos supposedly documenting motion through his zoˆpraxiscope and getting an apparently static projection as a result.

But as it turned out, this hypothetical zoˆpraxiscope wasn’t broken. The legs of the water strider weren’t moving – at least not in a way that human eyes could see. But by examining the images in a different light, Hu made a remarkable discovery.

By photographing a water strider climbing a meniscus while light was shined on the insect from above, Hu was able to distinguish small areas of the water below the insect where the photograph was brighter or darker than in the rest of the image. After studying the images carefully, Hu realized that the bright areas were created by light passing through raised sections of the water below the insects’ legs and being focused into one spot. Similarly, the dark areas were created by "indented" sections of the water, which diffused the light passing through them.

Hu determined that a water-walker pulls at the surface with claws on its front and back legs while simultaneously pushing on the surface with its middle legs. The resulting deformation of the water’s surface is what allows the water strider to climb an otherwise insurmountable meniscus.

"The camera made visible the various postures assumed by the insects," said Hu, "and drew our attention to the unique wetting properties at their leg tips."

The raised and indented sections of water in Hu’s images were in fact menisci – but not the type the insects were trying to climb. Instead, the water strider climbs a curved liquid surface by creating its own meniscus – in fact, several of them – using the pulling and pushing motions generated with its tarsi, the equivalent of a human’s ankle and foot. And just like humans, water striders can move their versions of ankles and feet without moving their legs. Goodbye, Denny’s Paradox.

Now a post-doc at New York University, Hu continues to study insect locomotion. His name already appears in biophysics and fluid dynamics literature as the mathematician who discovered the source of some water striders’ ability to ascend a meniscus. But he has no plans to stop there, citing the diversity and massive size of the insect population as reason enough to keep studying it. He calls the use of high-speed photography "crucial" to his research, and even cites a resemblance between one of his more striking published photos and some of Prandtl’s first work. From horses to milkdrops to water striders, the aesthetic significance and scientific applications of photography continue to coincide.

According to Hu, Thoreau said it best: "Nature invites us to lay our eye level with the smallest leaf, and take an insect view of its plain."

Michelle Sipics is Dragonfire's Science and Medicine editor.