Throughout nature, there are many strange examples of parasites manipulating their hosts into changing their behavior for their own benefit. There are fungi species that make ants climb trees before exploding from their heads, others that hijack cicada mating season and make their butts fall off, and single-celled parasites that make rodents sexually attracted to cat pee. Researchers have now uncovered how a parasite called the hairworm manipulates their praying mantis hosts into leaping into bodies of water.

"I am originally an ecologist and had found that the infected hosts that jumped into water became very important seasonal energy source for aquatic predators (salmonid fishes) in Japanese forested streams," study author Takuya Sato, Associate Professor at Kobe University, Japan, told IFLScience. "It is one of the good examples to show how parasites (that are easily overlooked in ecological studies) are important in mediating ecosystem processes."

Hairworms (Chordodes sp.) start their life in rivers and ponds, where their larvae get into the bodies of aquatic insects. These insect hosts then grow wings and exit the water, where predators such as the praying mantis lie in wait. If an infected insect is eaten by a mantis, the hairworm will then proceed to grow and mature in its body.

However, to make their way back into the water to reproduce, the parasite needs to get creative, manipulating their hosts into taking a dip. How the parasite goes about this has not been found out by scientists for over a century, but new research published in Current Biology claims to have cracked it.

It was previously thought that the brightness of the light bouncing off the water is what lures in the mantids and other hairworm hosts – but this explanation does not account for other bright locations that the hosts are not attracted to. "Previous studies have suggested that the positive photo taxis found in crickets infected by hairworm is a key factor triggering the water-entry behavior. However, as a field biologist, I cannot understand why we have almost never found hairworms in shallow, bright water areas, such as puddles," Sato told IFLScience.

Instead, the researchers hypothesize that the attraction is down to the polarization of the reflected light.

Light is an electromagnetic wave that that oscillates up and down perpendicular to its direction of movement. Normally, the wave can oscillate in any orientation, but polarized light only wiggles in one plane. Light reflected off water is mostly horizontally polarized, and previous research has suggested that insects can use this property of light to detect water, either to avoid it or seek it out.

The researchers conducted two different experiments using the praying mantis species Hierodula patellifera. First, the insects were placed in the middle of a cylinder that produced polarized light at one end and non-polarized light at the other. It was observed that after 10 minutes, mantids infected with hairworm were more likely to gravitate towards the polarized light than their uninfected counterparts. Interestingly, this was not observed when the light was vertically polarized rather than horizontally.

The study then moved outdoors to a mesh enclosure containing two pools. One pool was deep and dimly reflected horizontally polarized light, and the other was shallow and had a stronger light reflection that was only weakly polarized. They then released 31 infected mantids and 19 uninfected mantids into a tree between the two pools and observed the aftermath via video. Of the 16 infected mantids that leapt into the water, 14 chose to enter the pool reflecting horizontally polarized light. Only one uninfected mantis decided to take a swim.

Some of the results of the study also suggest that the circadian cycle has a part to play in this parasitic behavior alteration. The insects were observed in the lab to walk around more at noon, and many of the insects that entered the water did so around midday. This indicates that not only do hairworms induce their hosts to take the plunge, they may also make them do so at a certain time of day.

Now the researchers know what makes the infected mantises dive into the water, the next step is understanding the neural mechanism behind this behavior. "The neurological pathways for animals to perceive the polarized light in ventral rim area (not dorsal rim area that is well known to perceive the celestial light) is currently a hot topic in the animal vision neuroscience," said Sato. "We hope that understanding the mechanisms will help to make interesting progress on the animal vision neuroscience in general."

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