Sarah Becker, of Cromwell Primary School, asks :-
Why do earth worms come to the surface when it rains?
Patricia Fraser, a soil scientist with Crop and Food Research at Lincoln, responded.
I don't know of any experimental evidence that demonstrates why earthworms do come to the surface when it rains. It is an observation made by unknown numbers of people from all walks of life, but it still seems to be unclear as to the real reason why they come to the surface. There are many hypotheses. Some suggest that the earthworms do it to avoid 'drowning'. Others suggest that it assists them in the finding of a mate to enhance reproduction. Another theory is that that they make use of the moist conditions to ease their dispersal at a time when there is a lack of deadly UV rays. Still another poses the idea that it is due to elevated levels of CO2 in the soil.
Many gases are dissolved into raindrops as they fall. One of the most soluble gases is carbon dioxide, because it chemically combines with water to form H2CO3, or carbonic acid (the fizz in soda pop). In fact, natural rainwater is generally between pH 5.5 and 6.0 because of the dissolved CO2.
Because CO2 is a waste product, most animals have a negative chemotactic response to elevated levels of CO2 (they move away from the source), much like you tend to want to leave a room filled with ammonia or bleach fumes. Rainwater with a high concentration of CO2 filling the earthworms' burrows could very easily elicit a similar response.