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Biodiversity Blog


All things creepy: parasitism pt 1, mermithids and earwigs

Mermithid from Asian Hornet Vespa velutina Body
This is a mermithid found in an Asian Hornet. (Wikicommons photo: PeerJ, 2015)

In the spirit of Halloween and all that is spooky, we are doing a series of short blogs on parasitism!

In biology, parasitism at its most basic level is where one species benefits at the expense of its host. The parasite does not always kill its host, but when it does, it is called a parasitoid. Examples of both in the natural world are plenty and truly are stranger than fiction.

Let’s start this series on the poor earwig. Earwigs make up the insect order Dermaptera, with about 2,000 species in 12 families. Their common name is based on a myth of parasitism that is pretty creepy on its own: these insects enter a person’s ear where they set up home and feed on one’s brain. That doesn’t happen by the way, but maybe being a brain-eating earwig for Halloween is not such a bad costume idea.

The earwig actually has its own parasitoid to deal with, very similar to the scenario above, but more horrifying. Earwigs sometimes have a juvenile mermithid worm (or sometimes several) that resides in their head cavities. When the worm is mature, it forces the earwig to enter a body of water and drown itself. This allows the worm to emerge and mate as it only can in water.

Mermithidae is a family of nematode worms with more than fifty genera. They mainly parasitize arthropods, but sometimes they also infest spiders, scorpions, and crustaceans. 

 Earwig (Photo: Alex Wild)

But how do these worms force the earwigs to essentially commit suicide? Nothing is terribly conclusive, but researchers think that the worm manipulates the production of the earwig’s proteins involved in generating energy and movement. The worms could be affecting neuronal connections in the earwig brain. This would affect the earwig’s memory and cause it to perhaps lose its fear of water.

From a biochemistry perspective, it’s hard for researchers to determine a host’s changes due to its immune system fighting off the parasite, or if these changes are actually the parasite “getting behind the wheel” of the host.

If thoughts of worms busting through insect heads gives you nightmares, try this for some cheer. There are about 25 species that parasitize mosquito larva which makes them of interest for biological control. And we humans, as hosts for parasitic mosquitos, can take some comfort in that.

 ant and worm
 While not an earwig, a mermithid worm has busted out of this ant, xenomorph style.  (Photo: Alex Wild)


Read our other Parasitism Halloween Blogs


Part 2: the corpse lily

Part 3: the tongue biters

Part 4: mite pockets

Part 5: the crypt keeper


Greenwood, Veronique “First the Worm Gets in the Bug’s Head. Then the Bug Drowns Itself.” New York Times, November 22, 2019 (https://www.nytimes.com/2019/11/22/science/parasites-insects-drown.html?action=click&module=RelatedLinks&pgtype=Article)

Martins, Mirian Francisca et al. “First record of a mermithid worm (Nematoda, Mermithidae) parasitizing a third instar nymph of Triatoma sordida (Stål, 1859) (Hemiptera, Reduviidae, Triatominae) from Mato Grosso, Brazil.” ZooKeys vol. 980 79-91. 28 Oct. 2020, doi:10.3897/zookeys.980.55865

Parasite of the Day: Mermis nigrescens. Daily Parasite. May 29, 2015. (accessed online

Platzer, E G. “Biological control of mosquitoes with mermithids.” Journal of nematology vol. 13,3 (1981): 257-62.


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