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This article shows pictures of earthworms during floods in Texas. The worms ball up, supposedly in order to survive the floods.

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Photo Credit: Texas Parks And Wildlife Department

Can anyone explain the phenomenon a little? I can't seem to find anything about this online. I know that earthworms surface when it rains for oxygen, but it seems weird that they meet in the middle of the street.

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    $\begingroup$ Interesting...It is almost as if earthworm herds perform gradient descent (or ascent in this case) like an intelligent robot to find the driest place in the surrounding area and moves to that location - but I wonder if the earthworms were moved in between the yellow lines intentionally to induce some journalistic shock value $\endgroup$
    – Fraïssé
    Commented May 30, 2015 at 18:52
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    $\begingroup$ Agree, we know they've been poked with a stick. Perhaps the reason is not biological but hydrological. Roads are crowned to improve drainage, so in a flood the center is most likely to be exposed as the water recedes. There's also something dark in the mass that I can't make out but it doesn't look like worm; maybe it's important. $\endgroup$
    – Gossar
    Commented May 31, 2015 at 11:55

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The first question: what the worms do on the ground during the flood? One of the answers to this question is -

"It gives them an opportunity to move greater distances across the soil surface than they could do through soil," said Dr. Lowe. "They cannot do this when it is dry because of their moisture requirements."

The second question: why they form ball after moving to the ground. There are two answers to this question:

First, as a part of surviving mechanism to prevent drying. As you can see, they cannot come back to the soil across the asphalt, thus they form balls and wait the weather to improve.

Second reason to form the balls - communication:

"The earthworms use touch to communicate and influence each other's behaviour, according to research published in the journal Ethology. By doing so the worms collectively decide to travel in the same direction as part of a single herd."

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I think another aspect to this has actually nothing to do with worm biology and instead relates to the roads and cars:

  1. Roads are typically graded so that water runoff moves off the roads -- this usually means that the center line(s) are a high point in the road.

    • See here for example

    • As a result, the worms would experience the least amount of water and runoff force if they congregated in the middle.

  2. Two "forces" could prevent formation of balls in the middle of either lane.

    • Water runoff might be stronger there preventing a stable "footing" for the worms to remain.

    • Even if balls formed in the lanes, car traffic would almost certainly destroy them leaving little evidence of them behind (especially if rain was washing them away).

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I'm sure the other answers play a part in this phenomenon aswell, but I was taught that worms do this because the worms dont have eyes, yet they still can sense light from dark, because staying out in the sun too long would dry them out, So they are between 2 yellow lines (to them they think that must be light) and the middle line is dark (the asphalt itself) so ultimately the worms are very confused and think they are somewhere safe and dark (like underground) They pass the first line trying to get through the light in search of darkness, and get stuck in the middle because they think the next line is more light. They sadly believe they have found a safe haven.

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    $\begingroup$ Thank you for your answer but it would be great if you could cite some authoritative references. References are important so that one can verify the information and can learn the details of the topic. Good references include peer-reviewed articles, books and websites of reputed academic/scientific organizations. Wikipedia is also good if it in turn is properly referenced. $\endgroup$
    – WYSIWYG
    Commented May 14, 2019 at 9:22

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