Horsehair Worms 

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Photo by Alastair Rae

By Dan Frasier

You’re actually fishing worms.

Tall waiving grass alive with thousands of hoppers and a stiff breeze. No setting can more quickly raise a flyfisherman’s heart rate. The promise of errant leaps by the insects causing aggressive and splashy rises by the fish can make the tying fingers of any sport trembly. So we look for conditions that are right. An abundance of hoppers or crickets is a good start, but often times we look for a little wind. Something to coax the crunchy fish snacks within striking distance of the fish.

But what if the wind wasn’t what was causing the hoppers to hit the water? Enter the horsehair worm; Spinochordodes tellinii to be more exact. ( The life cycle of this aquatic worm is fascinating. The larvae are found in the moist edges of bodies of water. There they are ingested by grasshoppers and crickets. The larvae then mature inside the insect; leaching nutrients out of the host and into its own skin. As it matures, the worm will molt many times, eventually growing to be up to 4 times the length of its host.

Now the worm has a problem. In order to mate it must find other horsehair worms and it must be in water.

Here’s where things get weird. The worm, still inside the hopper, begins to release a protein called WnT, among others that are believed to mimic neurotransmitters in the brain. In other words, they start to take over the thoughts of the hoppers. Apparently one of the thoughts that the worm plants in the insect goes something like this.

“Hey… you know what would be really fun right now? A cannonball into that stream!!”

The hopper then leaps into the water where the worm begins to wriggle out of its host’s body within 2 seconds. Most worms have completely left the zombie hopper in 90 seconds leaving the poor host insect to drown. Numerous studies across the US have identified horsehair worms along with their host’s bodies in the stomach contents of fish, including brown trout. It’s believed that these fish accidentally took in the worms while eating the fallen hoppers. One study of a small stream suggested that 60 percent of the fish’s diet consisted of hoppers and crickets and that horsehair worms were the primary cause of the hoppers ending up in the water. And that is just one strain of this type of parasites. Different strains are known to take over the bodies of all kinds of terrestrial insects like ants and beetles. In fact, “The adult worms are free-living, but the larvae are parasitic on arthropods, such as beetlescockroachesmantidsorthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide.” (

As creepy as this process is, there are some serious flyfishing implications that anglers should think through considering this new information.

First, there are real conditions that are more conducive to terrestrial fishing than others, but wind or grass conditions or even the lack of a hatch don’t seem to be the most important ones. The waterbody’s population of horsehair worms may be an important driving factor in the quality of terrestrial fishing, along with the particular strain of worms that inhabit that water. Utilize the internet to see if any studies have been done on that particular stream. Secondly, fish are triggered to eat by familiar movements of their food source. We call it action, or presentation, or drift or all kinds of things. But really it’s simply the answer to the question, “Is it moving like the food the fish are looking for?” Understanding that, and the speed with which a hopper is often consumed by a fish, and perhaps the action created by a slender 4-inch worm wriggling out of the body of a hopper that has just hit the water is a trigger to the fish. We’d be remiss if we didn’t at least attempt a few hoppers with long rubber legs extending from the back of the abdomen.

So the next time you hear someone say, “I think I’ll start with a hopper. Terrestrials seem to bumble their way into the water all the time.” Just know that there is probably something far more sinister going on then just an errant jump and a breeze.

Dan Frasier
Gink & Gasoline
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9 thoughts on “Horsehair Worms 

  1. Now that’s just freakie.
    That juicy information will make for some great Drift Boat conversation
    next time down the river and I’ll probably be cracking a few hoppers open to see if horsehair worms exist in the Delaware system.
    …Anyone interested in my shares of

  2. Great, I already have nightmares about neurotoxin producing parasites that burrow into bodies. Now I have a new set of nightmares thanks to this.

    Also I look forward to tying new hoppers with a rubber leg butt worm!

  3. First time i’ve ever seen these creatures was ironically at ice off in the BC Selkirk east slopes. COULD NOT believe my lying eyes. long stringy naematodes moving life pencil thin black leeches undulating level in the water . I can’t explain it other than to say the fish were not biting on anything on that spring outing. Tried tying a piece of larva lace on a hook but no luck. Apparantly the ball up mostly when at rest so i’ll try that..

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