Parasites that are said to account for nearly half of all living things on earth. Among them, there are mysterious creatures who manipulate the host (shukushu) by various methods and pursue their own livability. It approaches its slightly grotesque appearance.
From the butt of a mantis that has fallen into a river or pond, a blackish elongated object may come out. This is not a mantis internal organ, but a parasite called "Gordioidea".
Wireworms hatch in water and first parasitize aquatic insects such as midges. When the host emerges, jumps out of the water, and is eaten by terrestrial insects such as mantis, it grows in the body as the definitive host. When the definitive host falls into water, it escapes from the belly and lays eggs on aquatic plants.
So why does the parasitized mantis enter the water? It has been thought to be manipulated by wireworms, but the mechanism has been a mystery for over 100 years. A team of Takuya Sato and Associate Professor of Kyoto University has elucidated a part of the mechanism.
Previous studies have shown that the definitive host of wireworms is attracted to the reflected light from the surface of the water and enters the water. The reflected light on the water surface is characterized by "horizontal polarization" that vibrates only in the lateral direction. Polarized sunglasses used for fishing cut only horizontal polarization so that you can see in the water.
The team showed the parasitic praying mantis with normal light and horizontal polarization, and confirmed in experiments that it was attracted to the horizontal polarization. In the pond that strongly reflects horizontal polarization and the pond that hardly reflects it, parasitic mantis jumped into the pond that strongly reflects.
According to another experiment by Mr. Sato, 90% of the terrestrial insects that fell into the water were parasitized by wireworms, and the terrestrial insects accounted for 60% of the food that mountain stream fish eat annually. It was also found that when water entry was prevented, mountain stream fish instead eat more aquatic insects and more algae feed on aquatic insects.
“The season when insects parasitized by wireworms are concentrated is a feast time for fish. You can store nutrients before the breeding season,” says Sato. Small parasites play a major role in connecting the two ecosystems of forests and rivers.
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