|MadSci Network: Zoology|
Dear Ed, You may have noticed that the night crawlers, the large earthworms that come out of the soil at night, and the other smaller earthworms don't come out on cold and dry nights. Yet, when it is raining lightly on warm days, you can sometimes find them crawling on sidewalks even during the day. So, it appears that it is not just the night time, but also the temperature and humidity that bring them out. The earthworms are more likely to survive outside the soil if the air humidity is high, if the temperature is above freezing, but not so high that it will dry them up and if their predators (for example, birds) are not around. There are several ways the earthworms can sense that it is suitable to come out of the soil. First, although they don't have eyes on the outside of their skin, they have photoreceptors (light sensitive nerve cells) below it. These photoreceptors tell the earthworm if it is light or dark outside. Second, the earthworms can probably feel the humidity of the air, which usually goes up at night. Third, they can probably feel the air temperature, which tends to go down at night. So, from spring thru fall, the nighttime is the best time for an earthworm to come out of the soil because the humidity is high, the temperature is neither too high nor too low and the birds are sleeping.
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