Why are these springtails congregating on the darker patches of sycamore bark?

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Why are these springtails congregating on the darker patches of sycamore bark? Some possibilities: They are better concealed on the dark patches, the dark patches are warmer (thermoregulation), or the dark patches support a slightly different microbial community that provides more food to the springtails.

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Location: Massachusetts, United States,
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Keywords: animal, animals, arthropod, arthropods, collembola, ecology, entognatha, islc, mystery, physiology, puzzle, springtail, springtails, sycamore, thermoregulation