OSAKA — Every autumn, dead mantises can be seen lying on the asphalt of roads. A type of parasitic worm may be leading the creatures to their deaths there, researchers at Kyoto University and others have determined.

Horsehair worms are parasites that hatch in water and first attach themselves to water-borne insects. They grow inside mantises who devour their hosts, and manipulate the latter into jumping into water. From there, they exit the mantises’ abdomens and lay eggs in water grasses and similar places.

It is believed that infected mantises are attracted to water by the light reflecting off the surface, which contains a lot of polarized light with horizontally inclined electromagnetic waves.

A group including Kyoto University associate professor of ecology Takuya Sato and the same university’s then-graduate student Yuna Sawada hypothesized that the infected mantises could be mistaking asphalt for water. When they measured the amount of horizontally polarized light reflecting off of it, they found it was of a similar strength to water surfaces that infected mantises would jump into. In an indoor experiment using light sources, it was found that infected mantises were attracted to sources with stronger horizontally polarized light.

In order to confirm the creatures’ actual behavior, the group released infected mantises onto asphalt and cement roads painted three different colors. The infected mantises more frequently walked on the asphalt.

Furthermore, researchers caught over 100 specimens of Harabiro mantises, or Asian mantises, from on trees and on asphalt surfaces at four locations in Taiwan and Japan, and found that those taken from trees had a low rate of infection, compared to over 80% of those taken from asphalt.

“Horsehair worms have evolved to skillfully manipulate mantises, but conversely by human activity they are strangling their own necks,” Sato said.

The research results have been published in the U.S. scientific journal PNAS Nexus.

(Japanese original by Mai Suganuma, Osaka Lifestyle, Science & Environment News Department)