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Strange Behaviour Exhibited by Amazonian Ants


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I recently spent several months volunteering in South America, and for a portion of that I was in the Amazon rainforest. One evening, whilst we were sitting around a single candle as it poured with rain outside, we noticed some odd behaviour - three large ants (two workers and one soldier) were running around and around the candle in very neat circles. We watched them do this unceasingly for at least an hour. Curious, I found that wherever I moved the candle they would follow it and continue then going in circles. It was only when I extinguished the candle at the end of the night that they stopped circling and went off in random directions.


Can anyone explain this behaviour?

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Many insects navigate by the light of the moon. By keeping the moon's reflected light at a constant angle, the insects can maintain a steady flight path and a straight course.


A point source of light like a candle or a light bulb radiates light on all sides, so if an insect confuses the light from the point source for moonlight, the insect can't keep the light at a constant angle, as it does with the moon. As it attempts to navigate a straight path, it ends up caught in an endless spiral dance around the point light source.




Edited by Arete
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Just a thought, but an animals behaviour, especially in insects, can be substantially altered by parasites. The bahviour alteration can be directed by the parasite in such a fashion that promotes the spread of the parasite, usually through a larger host. A theory for explaining this behaviour could be as follows: the ant picks up the parasite though the feces or corpse of the main host. The parasite hijacks the horomone system of the ant, making it prone to move towards light and move in an eye-catching pattern making it easier for the main host to find and eat the ant. When the main host eats the parasite filled ant the cycle is complete.

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By keeping the moon's reflected light at a constant angle, the insects can maintain a steady flight path and a straight course.
There's a complication, or subtlety, in there: many insects that are drawn to darkness, and avoid lighted areas, are trapped by point sources of light as well.


The current most likely explanation derives from a property of the image processing common to most organic visual systems including apparently those of most flying insects, which is that areas immediately abutting bright image areas appear very dark. It's a sort of optical illusion, or optical feature, or property of common image processing - human eyes work like that as well. By this hypothesis some of the insects apparently circling lights are not so much holding an angle on the light as aiming straight for what they perceive as the region of maximum darkness, immediately adjacent to it.


You can see this in the flight paths of some insects as they first approach a light - they do not spiral widely around it from a distance, but fly apparently almost directly toward it until quite close, at which point (and not before) things start to go haywire for them.


My guess is that would describe the ant behavior better than the constant angle spiral - that they head for the immediate neighborhood of the candle when it has been moved, rather than walking off at some angle to it initially.

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