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A robot flower experiment reveals how hawkmoths see at dusk

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The curious incident of the moth in the nighttime

Armin Hinterwirth

Moths are impressive insects. They can hover over swaying flowers and gather nectar in the dark — conditions that would make most diurnal animals, like us, essentially blind. That's why researchers have taken a special interest in figuring out how these insects see at night. The finding, published today in Science, suggests that scientists have figured it out — and the new knowledge could one day help researchers build tiny robots that can fly in varying light conditions, without crash-landing in a tree.

The moths slowed their visual system

To make the discovery, researchers put hawkmoths in front of robotic flowers in low light. The moths slowed their visual system down to collect more light over long periods of time, the researchers discovered. The change in moths' brains resembles what a photographer does when she slows a camera's shutter speed. It's a crazy adaptation that comes with some drawbacks, but it helps the moths see far more clearly than you and I ever could.

Credit: Rob Felt, Georgia Tech

Hawkmoths are equipped with "superposition compound eyes" that allow them to see a thousand times more sensitively than insects with regular compound eyes in low light. It’s a powerful adaptation, but to see at night, they need to see a million times more sensitively than insects with more conventional eyes. Clearly, moths have another trick up their sleeves. So, biologists came up with an experiment designed to test the hawkmoth's vision under various lighting conditions.

Moths need to see a million times more sensitively than insects with conventional eyes

The researchers built a flower-shaped robot filled with an aromatic mixture known to initiate foraging behaviors in moths. The robotic flower was programmed to sway and jitter at different speeds. Then, the scientists released female hawkmoths in a lab under light conditions that matched those seen in the late afternoon and mid-dusk. The researchers also filmed the moths as they tried to keep up with their scented targets.

Thanks to the moving flowers, the researchers were able to determine that hawkmoths lose some of their precision and begin to lag behind swaying flowers when they move above a certain speed, in very low light. "[The moth] sees more poorly following motions that are above 2 hertz," says Simon Sponberg, at biologist at Georgia Tech and a co-author of the study — a frequency that's equivalent to the robot flower swaying back and forth twice in a single second.

But slowing their visual systems would affect their flight

Sponberg and his team created a computer model of the moths' behavior; it suggested that if the insects slowed their visual systems to catch more light, it would affect their flight. The model predicted that at certain frequencies of flower movement, the moths would overshoot the flower, moving past where the plant ended up — and that’s exactly what happened when they replicated the conditions in the lab with live moths.

Sponberg lab, Georgia tech

"We found that at exactly those frequencies, the moth overshot the flower by exactly the amount we predicted it would," Sponberg explains. Their researchers concluded that the moth's visual system acts kind of like a camera does at lower shutter speeds. When the moth slows its visual processing, it sees better in dim light — but if an object moves too quickly, the moth sees a blur.

So what happens when a flower sways too quickly in nature? As it turns out, that doesn't happen too often. "Flowers move at all kinds of frequencies, but 95 percent of their motions are below 2 hertz," Sponberg says. By observing five species of flowers commonly targeted by hawkmoths in their natural setting, the researchers were able to conclude that the moths had figured out a way to slow down their visual system just enough to let light in for longer periods, without compromising their ability to see swaying flowers in dim light.

Flowers rarely sway above 2 hertz

Either that, or the flowers figured out a way to avoid swaying so fast that moths can’t keep up with them. Moths are pollinators after all, so letting them gather nectar benefits the flowers, too. In truth, the answer might be a combination of both — but it’s too early to tell.

A moth's visual system is matched to its task, says Jessica Fox, a neurobiologist at Case Western University who didn’t participate in the study. The moth can slow it down to forage in the dim light. "But if it slowed any further, it wouldn't be able to keep up with the flowers it needs to be able to see."

Credit: Rob Felt, Georgia Tech

"Nocturnal insects have exquisite visual abilities at night."

"Nocturnal insects have exquisite visual abilities at night," says Eric Warrant, a zoologist at Lund University in Sweden who didn’t participate in the study. Over the last 20 years, Warrant’s lab has helped show that nocturnal insects can see color, learn visual landmarks and use them for homing and navigation, and navigate using the celestial polarization pattern formed around the Moon or by the Milky Way. For him, the results of the Sponberg study adds "yet another piece to the growing body of evidence indicating that nocturnal insects see extremely well."

The finding is intriguing, both from a neuroscience and an evolutionary perspective. But studying how hawkmoths see at night isn’t just about advancing our understanding of evolutionary theory. Researchers who wish to build flying robots for rescue missions may want to take a page from the hawkmoths’ book.

"Insects are capable of doing many of the things that we desire autonomous vehicles and robots to achieve, and also in a miniature package," Warrant says. Understanding the principles by which a flying nocturnal insect negotiates the world, or tracks wind-tossed flowers, could therefore provide clues to achieving the same precision in a flying robot. "By understanding how animals see in low light, we can get some better senses of how we might be able to do that as well," Sponberg says.