How does a horsehair worm lead a mantis to water?

The horsehair worm's life cycle

Horsehair worms (phylum Nematomorpha) lay their eggs in water. The larvae that hatch enter the bodies of the larvae of aquatic insects such as mayflies, mosquitoes, and midges, form cysts, and wait. When this insect grows up and flies onto land and an insect such as a mantis, cricket, or grasshopper eats it, the horsehair worm grows inside the new host.

A grown horsehair worm is usually about 5–10 cm long. When the host enters the water, it emerges from the body and finds a mate in the water. Adult worms don't eat, because their mouth and gut no longer work.

Mantises are drawn to the light off water

Light reflected off a water surface is strongly polarized, vibrating in one (horizontal) direction. In 2021 a research team at Kobe University in Japan showed that giant Asian mantises infected with a horsehair worm (genus Chordodes) are attracted to this horizontally polarized light.

• In the lab: When mantises were put in a chamber lit with polarized light on one side and non-polarized light on the other, infected mantises chose the polarized side more often. This was especially clear in bright conditions.
• In the field: Mantises were released between a tank with strong polarization but dark and a tank that was bright but weakly polarized. Of 16 infected mantises that entered the water, 14 went into the strongly polarized tank.

The researchers said this tendency "can explain" the behavior of infected mantises entering the water. How the horsehair worm produces this tendency has not yet been worked out.

A worm carrying mantis genes

In 2023 a team including Japan's RIKEN analyzed the genes of a horsehair worm that parasitizes mantises. 1,420 worm genes were most similar to mantis genes, more than to those of any other organism, and 1,342 of them had sequences that were on average 94.5% identical. Other horsehair worms that don't use mantises as hosts lacked these genes or had very different ones.

The researchers interpreted this as the result of genes moving from the mantis to the worm many times over a long period (horizontal gene transfer). It is the largest number of horizontal gene transfers reported between two animals. Among worm genes whose expression changed around the time the mantis was led to water were ones related to nerve regulation, light detection, and daily rhythms.

A stream changed by horsehair worms

Horsehair worms lead not only mantises but also camel crickets and grasshoppers to the water. In a 2011 study of a stream in Japan, camel crickets and grasshoppers infected with horsehair worms were 20 times more likely to enter the water than uninfected ones, and the insects that jumped in accounted for about 60% of the energy the stream's fish (trout-like fish) got in a year. Note that the hosts in that study were camel crickets and grasshoppers, not mantises.

People don't need to worry

Horsehair worms do not parasitize people. Cases of horsehair worms coming out in a person's vomit or urine have been rarely reported, but they are considered cases of accidentally swallowed worms passing through the body (false parasitism).

The 2012 Korean film Deranged (Yeongasi, the Korean word for horsehair worm) was about a mutant horsehair worm that parasitizes people and makes them jump into water, and drew over 4.5 million viewers. The premise in the film is fiction.

In the past some regions called the insect mantis "yeongasi" or "yeonkkasi." Which came first, the insect's name or the parasite's, hasn't been confirmed from texts.

Research in this article