The latest Paper of the Month for Parasitology is Let’s go swimming: mermithid-infected earwigs exhibit positive hydrotaxis

The term ‘back seat driver’ is used colloquially to describe a passenger in a car who is dissatisfied with the driver’s actions and therefore tries to control or influence them. Consider for the sake of this analogy, your body as the car, the steering wheel as your brain. Now imagine that a ‘back seat driver’ in your car is constantly whispering to you, urging you to drive into a lake. Or they may just grab the steering wheel and yank it towards a passing river.

A micro CT scan of an earwig infected with an adult mermithid. The head of the earwig is bottom right and the tail is top right. All the rope/spaghetti-like structure in the image is a single adult worm. Pretty amazing this earwig was still running around! Credit: Sara Ferreira, at Otago university, in the anatomy department, under Neil Gemmels lab group.

For insects infected with hairworm and (as this new paper demonstrates) mermithid parasites, this hydrophilic ‘back seat driver’ nightmare is a reality. These unfortunate insects naturally avoid open water, yet for the aquatic parasites living inside them, water is a critical component for their survival once they leave the insect. Somehow these parasites control the wheel, and drive the insect into water. Often the insect drowns and the parasite emerges into an environment perfect for its survival.

Life cycle picture comparing the hair worm and mermithid life cycle. Credit: Ryan Herbison

This paper establishes that earwigs infected with adult mermithid worms, from a different phylum relative to hairworms, will enter and stay in open water far more frequently than non-infected earwigs. This reveals a new host-parasite system in which manipulation of behaviour may occur. Given hairworms from a different phylum manipulate behaviour in a very similar way, comparing the mechanisms both the mermithid and hairworm use to change behaviour would provide very valuable evidence for host manipulation.

An earwig (Forficula auricularia) and its recent mermithid parasitic worm (Mermis nigrescens). Credit: Steven Evans, from Otago university.

Studying the mechanisms by which parasites change and create behaviour provides access to the molecular mechanisms behind said behaviour. It lets us see how these ingenious creatures (and by extension natural selection itself) interact with and bring about changes in incredibly complex physiological systems. One day it is my dream that research into this area of science may inform or possibly revolutionise approaches in pharmacology, developing more effective, targeted medication based on the designs natural selection has produced for these manipulative parasites.


The full article is available to download for free until the end of the month.


Headline image credit: Haseeb Randhawa and Kim Miller.

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