Why Do Earthworms Come Out in the Rain?

Why Do Earthworms Come Out in the Rain? Here are Some Speculations

Earthworms crawling above the ground is a common sight after a heavy rain. Read on to find out more about earthworms and why they come out to the surface after rain.
Earthworms belong to the subclass oligochaete of the phylum Annelida. They are commonly known as dew worms, rainworms, and angleworms. They are the largest organisms of the subclass oligochaete, and are also known as megadriles or big worms. They are generally 8 to 10 inches in length, though some species can reach a length of 12 feet. On the other hand, some grow to a length of just 1 inch. Till now, almost 2,500 species of earthworms have been discovered. They are generally reddish, dark-gray, or black in color.
Anatomy of Earthworms
Earthworms have a long, tubular body, covered with slimy and moist skin. They have many bristles on their body, also known as 'chaete' or 'setae', which facilitate their movement. The head and tail look identical with pointed ends. The circulatory system of earthworms is simple and mainly consists of two blood vessels, dorsal and ventral. These blood vessels are quite long and run through the entire body of the earthworm. The ventral blood vessel carries blood to the posterior end, while the contractile dorsal vessel pumps blood to the anterior end or front side of the body.
Numerous capillaries are also present in the body that collect blood from the ventral vessel and then distribute it to different parts of the body. The number of hearts varies among earthworms of different taxonomic groups. However, usually they have five hearts. They exchange gases and nutrients with the surrounding environment through a sinus known as the vascular sinus.
One interesting fact about earthworms is that they are hermaphrodites, i.e., both male and female reproductive organs are present in their body. A thick, glandular segment known as clitellum is also present in the body wall, which secretes the viscid sac for containing the eggs. They do not have teeth but strong muscles are present in the mouth. They do not have a well-developed respiratory system and breathe through their skin. They also possess the power of regenerating lost or damaged parts of the body. However, the extent of this ability varies among different species.
The Relation Between Earthworms and Rain
Earthworms need a moist environment for their survival. They mainly live underground to avoid sunlight, as sunlight removes the moisture from their skin. They would die quickly if exposed to sunlight for a long time. They need a moist environment for diffusion of gases through their skin. They are often seen above the ground after a heavy rain. There are many theories about this behavior. One opinion is that the amount of oxygen dissolved in water is less than that in the air. So, earthworms come above ground when their burrows get filled with rainwater. However, if sufficient amount of oxygen is dissolved in the water, they can live underwater for many days.
Another supposition is that the humidity after rain is high, and therefore the earthworms come out as the high humidity protects their skin from dehydration. Besides, they can also move faster on the moist surface, and therefore reach new places for populating. Another reason for coming above ground may be the fact that due to respiratory activities of organisms living underground, the level of carbon dioxide increases. This carbon dioxide forms carbonic acid with rainwater. So, to avoid such a harmful acidic environment, they come out to the surface.
To sum up, the shortage of oxygen in the burrows due to heavy rain, formation of carbonic acid, and mating are some of the possible reasons for which earthworms come out from the burrows after rain. Whatever may be the exact reason, they loosen the soil structure by their activities of coming up and down the burrows, which facilitates aeration of soil. Besides, they are also loved, especially by farmers, for increasing soil fertility by decomposing organic matter and converting it into essential nutrients required by plants.