On a moonless night, light levels can be 100 million times lower than in bright daylight. And if we are practically blind and completely helpless in the dark, cats quite successfully track down prey, and butterflies nimbly flit between flowers on our balconies. While we sleep, millions of other animals rely on their visual systems to survive. The same can be said for the animals that live in the eternal darkness of the deep sea. What's more, the vast majority of animals in the world are mostly active in dim light. How do they manage to use such powerful visual characteristics, especially insects, with their tiny eyes and brains less than a grain of rice? What optical and neural strategies have they developed to see well in dim light?
To answer these questions, let's turn our attention to nocturnal insects. Despite their diminutive visual systems, nocturnal insects see perfectly in dim light. In recent years, we have found that nocturnal insects can avoid and lock onto obstacles during flight, distinguish colors, detect faint movements, learn visual cues, and use them for homing. They can even navigate using the faint picture of stellar polarization that the Moon creates and navigate using the constellations of stars in the sky.
In many cases, this visual performance seems to be completely disruptive to what is physically possible. For example, the Central American night bee, Megalopta genalis, absorbs only five photons with its tiny eyes when light levels are at their lowest levels - a completely elusive visual signal. And yet, in deep night, it can navigate the dense and tangled rainforest while foraging and safely return to its nest - an inconspicuous hollowed-out stick suspended in the underbrush.
To find out how this is even possible, scientists began to study hawk moths. These beautiful insects - the hummingbirds of the invertebrate world - are represented by graceful, fast-flying butterflies, which are constantly looking for flowers with nectar. Once the flower is found, the moth hovers in front of it, sucking the nectar through the proboscis, an oral tube.
The European night hawk moth Deilephila elpenor is a beautiful creature hiding in feathery pink and green scales, collecting nectar in the middle of the night. Several years ago, scientists discovered that this butterfly can distinguish colors at night, the first nocturnal animal known for this.
This butterfly recently revealed another of its secrets: the neural tricks it uses to see well in very dim light. These tricks are of course also used by other nocturnal insects such as the Megalopta. By studying the physiology of the nerve circuits in the visual centers of the brain, the scientists found that Deilephila can see well in dim light, effectively stacking the photons that it collects at different points in space and time.
This is a bit like increasing the shutter speed on a camera in low light. If you let the shutter stay open longer, more light will reach the image sensor and get a brighter image. The downside is that anything that moves quickly - like a passing car - won't get permission, so the insect won't see it.
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Neural summation
To combine photons in space, individual pixels of an image sensor can be pooled to create smaller but larger “superpixels”. Again, the disadvantage of this strategy is that even at high brightness, the image will be blurry and lacking in sharp detail. But for a nocturnal animal that tries to live in the dark, being able to see a bright, but devoid of detail and slow world will be better than not seeing anything at all (and this is the only alternative).
Physiologists have shown that neural summation of photons in time and space is extremely beneficial for nocturnal deylephila. At any intensity of night light, from twilight to the stars, the summation significantly increases the deilephil's ability to see well in dim light. In fact, thanks to these neural mechanisms, deilephilas can see in 100 times dimmer light than they would otherwise. The benefits of summation are so great that other nocturnal insects, too, very likely rely on it for good night vision.
The world seen by nocturnal insects may not be as poignant or well-resolved as that seen by their active diurnal relatives. But stacking ensures that it is bright enough to intercept prey, fly to the nest, and avoid obstacles. Without this ability, they would be as blind as the rest of us.
ILYA KHEL