MadSci Network: Zoology

Re: My son asked me how to night crawlers know when it is night and time?

Date: Mon Mar 25 15:27:58 2002
Posted By: Aydin Orstan, Staff, Office of Food Additive Safety, Food and Drug Administration
Area of science: Zoology
ID: 1016387352.Zo

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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