"Cricket Pupa" Earthworm: Who says only those who stand in the light are heroes?

Produced by: Science Popularization China

Author: Ji Qiaoqiao (Doctor of Agronomy)

Producer: China Science Expo

Since the beginning of summer in 2024, many parts of our country have been continuously hit by heavy rains. Fortunately, emergency rescue measures were implemented in a timely manner to minimize the losses and ensure the safety of the people.

Careful observers will find that many earthworms often crawl out when it rains. After the rain stops, drowned and sun-dried bodies can be seen everywhere, and birds and other animals that feed on earthworms can also have a good meal. So, will the earthworm population decrease as a result?

earthworm

(Image source: pixabay)

Why do “crispy” earthworms seek their own death?

It is obvious that darkness is the paradise of earthworms, so by coming out on their own initiative, are they seeking death?

The answer is no. There are three main reasons why earthworms actively drill out: to find resources, to reproduce, and to escape from adverse environments.

Looking for resources: When an area where earthworms depend for survival becomes no longer suitable for survival due to excessive populations, fewer survival resources, etc., some earthworms will drill out of the ground at night while the ground is relatively safe and humid (earthworms breathe through their skin, and the environment needs to reach a high humidity to support their breathing) to look for a new suitable place to live.

Reproduction activities: Earthworms are hermaphrodites and cross-fertilize, and they mainly reproduce in spring and autumn. In order to find high-quality gene sources for their offspring, a large number of earthworms will come out of the ground during the breeding season for a "blind date."

Escape from adverse environment: Many pesticides such as carbendazim have a strong killing effect on earthworms. Long-term drought will cause the soil to dry and compact, and flooding will cause the habitat to become oxygen-deficient. Physical or chemical factors such as these will cause the habitat to no longer be habitable and will force earthworms to drill out and escape.

Will earthworms be drowned by heavy rain?

Earthworms often live in soil and organic matter piles rich in organic waste. They are very sensitive to light and afraid of it. They will only come out under specific conditions. For example, heavy rainfall can cause the humidity of the environment on which earthworms depend for survival to increase sharply, and even be submerged by stagnant water, causing the earthworms' living environment to quickly become unsuitable.

There are at least two strategies that earthworms have for coping with waterlogging stress. The first strategy is escape. The earthworms that adopt this strategy are mainly those with low tolerance to waterlogging. They will crawl out of the waterlogged environment and look for a relatively dry place.

The second strategy is "endurance". Earthworms that adopt this strategy are mainly species with high tolerance to waterlogging. They consume low oxygen and can survive in low oxygen concentrations and hypoxic conditions for a long time, and will never come out after a heavy rain. At the same time, related studies have shown that rainfall enhancement has no significant effect on the number and biomass of earthworms.

(Photo source: veer photo gallery)

How do earthworms maintain strong populations?

a. Special magic of rebirth after death

We have known since childhood that earthworms have the unique ability to regenerate after being cut off. When an earthworm is cut in two, the wound heals and grows into two earthworms.

Some scholars have conducted relevant experiments, using 18 different cutting methods to observe the earthworms. The results show that no matter whether the head is cut off or the tail is removed, the earthworm has a chance to survive, and the survival rate is mainly affected by the number of remaining segments. The more segments there are, the higher the probability of survival. However, it is difficult for the test individuals to recover to a completely healthy and complete state.

Earthworm amputation experiment: (i) amputation of the front end of the earthworm (a), (ii) amputation of the front and back ends of the earthworm (b), and (iii) amputation of the back end of the earthworm (c). A total of 18 different types of amputations were performed to examine the effect of the number of remaining segments on the regeneration rate of earthworms.

(Image source: Reference 9)

b. Strong reproductive capacity

The reproductive capacity of earthworms should not be underestimated. At the same time, the reproduction method of earthworms is also very peculiar. Earthworms are hermaphrodites, but they need cross-fertilization. When two adult earthworms meet, they will move closely to each other in opposite directions, put their female holes close to each other's spermatophore holes, exchange sperm with each other, and complete the fertilization process. When the reproductive activity is over, both earthworms will become pregnant and lay earthworm cocoons/eggs.

Earthworm reproduction process

(Image credit: Follow the worms, by Christopher Cudworth)

The results show that when conditions are right, earthworms can lay about 5.5 eggs in 28 days, and each egg can hatch 1-7 small earthworms. In other words, each earthworm has the potential to reproduce 35 small earthworms in one month, which is also a high-yield species.

Conclusion

Earthworms are considered the most important ecosystem engineers in the soil, playing important roles in loosening the soil, decomposing organic waste, and promoting nutrient cycling. Darwin was also impressed by the powerful force and ecological role of earthworms and evaluated earthworms as the most important species in the world.

The tiny earthworms, harmless to humans and animals, hide in the dark. They usually stay in their pupae quietly, but occasionally they emerge to peek into the outside world. They are much more tenacious than we think. As successful people who are well versed in the R strategy (winning by quantity), their prosperity has also contributed a lot to the world. Next time you see them walking on the roadside, remember to try not to step on them!

References:

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2. Shen Zhifeng. Response of earthworm communities to simulated nitrogen deposition and rainfall enhancement in Jigong Mountain and its influencing factors[D]. Henan University, 2019.

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9. Xiao N, Ge F, Edwards C A. The regeneration capacity of an earthworm, Eisenia fetida, in relation to the site of amputation along the body[J]. Acta Ecologica Sinica, 2011, 31(4): 197-204.