Invertebrates
Horsehair Worm
Nematomorpha · also called Gordian worm, hairworm
Find what looks like a length of coarse hair swimming in a puddle or a dog’s water bowl and you have found a horsehair worm, an adult parasite that has just made a cricket walk into the water and die so it could get out.
Overview
The horsehair worms are a phylum, Nematomorpha, of about 350 species. The adults are living threads, 1 to 3 mm thick and 10 to 100 cm long, that live briefly in fresh water only to breed. The larvae are internal parasites of insects — crickets, grasshoppers, beetles, and mantises — absorbing nutrients through the skin.
Mind control
Once grown inside its host, the worm releases proteins that disrupt the insect’s nervous system and remove its fear of water. The land insect seeks out a stream or pool and jumps in, and the worm bursts out through its rear end within minutes, sometimes leaving the host alive and still moving.
The Gordian knot
Adults emerging into the same water twist together into a dense writhing knot, which is where the name Gordian worm comes from.
Status
Not Evaluated and common worldwide in fresh water and damp soil. Harmless to people and pets, and a natural check on cricket, grasshopper, and cockroach numbers.
Horsehair Worm: common questions
Can a horsehair worm infect a human or a pet?
No. Horsehair worms are parasites of insects, and they cannot develop in a mammal; their larvae need to pass through an aquatic insect and then a land insect such as a cricket to reach adulthood. People occasionally find a live worm in a toilet, a water bowl, or a swimming pool and worry, and there are rare reports of a worm being accidentally swallowed with water and later passed, but it does no harm and does not establish itself. A worm in a dog's or cat's water dish came from a cricket the pet or a bird brought in, not from the animal itself.
How does the worm make a cricket drown itself?
The worm spends weeks or months growing inside the insect, coiled up and filling most of the body cavity. As it matures it begins producing and releasing a cocktail of proteins that act on the host's central nervous system, including molecules that mimic or interfere with the insect's own signalling. Infected crickets lose their normal strong avoidance of open, reflective surfaces and their fear of water, become more active, and are drawn toward light reflecting off water. The insect walks or jumps in, and the worm exits into the pool it needs to reproduce. The host is essentially reduced to a delivery vehicle.
Why is it called a Gordian worm?
Because of how the adults look when they gather. When it is time to breed, worms emerging from hosts into the same body of water writhe together into a tight, tangled, slowly moving ball that can contain dozens of individuals and is genuinely difficult to pull apart. To classically educated naturalists this recalled the Gordian knot, an impossibly complicated knot in Greek legend that Alexander the Great solved by slicing it with his sword, and the name Gordian worm stuck to the whole group alongside horsehair worm and hairworm.
Are horsehair worms good or bad for the garden?
On balance they are a natural check on some garden and crop pests, since crickets, grasshoppers, and cockroaches are among their main hosts, and a worm-infected insect stops feeding and then dies. They do not damage plants, do not bite, and are not a sign of dirty water; in fact finding the free-living adults usually just means the local pond or puddle has a healthy population of the insects they parasitise. The only downside is the momentary alarm of finding what looks like a self-propelled hair in the water bowl.