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You know how moths like to fly into lamps or crawl all over your tv screen at night? Why do they do this?! The answer is more complicated than you might think...

Hosted by: Hank Green
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Hank: You know when you’re outside at night with some lights on, and insects start flying straight into the light, and swarming around it? It’s a thing! What is the deal with that? Turns out, nobody knows for sure, but there are a few theories that suggest that electric lights, fires, and other bright, glowing things might confuse the natural instincts of these insects.

Insects that are drawn to light are known as positively phototactic – and one of the theories behind their behavior is that these insects use the moon to navigate at night. These insect species tend to be nocturnal, and many are migratory, so in order to stay on track while travelling long distances, they want to stay at a specific angle with respect to the moon. So the theory is that these insects mistake a point of light they find in the dark – like an electric light – for the moon.

This could also explain why certain insects tend to fly wild circles around our lights: Once they reach the glowing object that they think is the the moon, they keep drastically changing their angles to the light source, which throws them for a loop, literally.

There are some problems with this theory, though: For one, not all of positively phototactic insects are migratory, and not all of those use the moon to navigate. Plus, this doesn’t really explain why they would head straight toward a light, since they don’t fly directly at the moon. So, there’s another theory that has more to do with short-term protection than long-distance navigation.

Light can also be a sign of an unobstructed path, which is a good thing if you’re trying to escape a predator, or just get to Point B. So if insects think a light is a sign of an obstacle-free area, you can see why they’d want to head straight for it. And it also explains why they might fly straight into a bug zapper or a fire, because they don’t expect the light source to be so close.

Another thing to consider: Many insects are really good at detecting ultraviolet light – an ability that lots of flowers exploit by reflecting UV light to attract them. So positively phototactic insects might aim for your lamp or campfire because they instinctively think it’s something they want, like a source of food! Which, of course, makes me feel quite sorry for them. But the next time you see an insect slamming into a lamp, keep in mind that not even entomologists are quite sure why it’s doing that. Maybe with some more research, we’ll figure it out.

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