Why are insects attracted to light?

Moths have positive phototaxis, which means they are naturally attracted to light.

On any given night, hundreds of moths, flies and other insects can be seen making endless circles around lamp posts and porch lamps. This might seem like an exercise in futility or more proof that insects aren’t very smart, but there are actually several theories as to why insects attracted to light make these nightly pilgrimages. However, there is no single scientific explanation for this behavior.

Earthworms have negative phototaxis, which means they are repelled by exposure to light.

Not all insects attracted to light act on these impulses. For some, a bright light source is seen as an emergency beacon, and when in doubt, they instinctively head towards the light, which is often higher than their current danger-filled position. Moving towards darkness would be seen as moving downwards, which can be even worse than not moving at all. Light, for some insects, can be seen in the same way as air bubbles pointing to the surface of water for other creatures.

Certain insects, such as cockroaches, are repelled by exposure to light.

Another popular theory is that insects use light as an aid to navigation. An insect flying north, for example, could judge its direction by keeping a natural source of light, such as the sun or moon, on its right side. This method works well as long as the light source remains constant and at a distance. If an insect encounters a glowing round porch light, however, it is confused by the light source. A moth will continue to circle a light because it instinctively wants to keep the light on a certain side of its body while navigating.

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A popular theory is that insects use light as an aid to navigation.

The difference between insects that are attracted to light and those that are not is a phenomenon known as phototaxis. Certain insects, such as cockroaches or earthworms, have negative phototaxis, meaning they are repelled by exposure to light. Moths, flies and many other flying insects have positive phototaxis, meaning they are naturally attracted to it.

Cockroaches are nocturnal and repelled by light.

There is some debate in the scientific community as to why a positively phototactic insect will continue to hover around an artificial light source even if natural light is available. Some believe that the insect is not really attracted to the light itself, but to the darker areas that surround it. Others suggest that the insect’s eyes, which often contain multiple lenses, cannot easily adjust from light to dark, leaving the insect vulnerable to predators when blinded at night. It may be safer for the insect to remain in the light than to fly away and be too blind to react to threats and obstacles.

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