Earthworms tend to surface in large numbers during heavy rains for several reasons, not simply because the oxygen levels in the soil changes. Earthworms need to remain moist to survive and consider rain a good thing.
Earthworms do not have lungs like humans. Instead, the surface of their skin, known as a cuticle, absorbs oxygen through a process called diffusion.
More Oxygen in Air
The earthworm's cuticle must remain moist to absorb oxygen. When they are in soil, they receive oxygen from air trapped underground. Rain-soaked dirt, however, fills that air space with water, and although earthworms can absorb oxygen from the water, air contains more oxygen, so they will surface for more efficient breathing.
Favorable Moist Conditions
Earthworms must remain moist to breathe and need moisture to move around. After a rain, the conditions above the ground are extremely favorable to them, so they surface to move around without fear of drying out.
Rain Mimics Evening Conditions
Worms come out at night when the air is moist and there is no hot sun to dry their skin. When it rains during the day, the conditions above ground resemble the cool, damp conditions at night, so worms will surface without waiting for nightfall.
Earthworms have a higher chance of bumping into another worm when they are on the surface after a heavy rain and can travel greater distances. Some species of earthworms seem to prefer mating in moist conditions on the surface because they have more room to maneuver.
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