Did you know horsehair worms turn their insect hosts into suicidal zombies? That's a dramatic way to say it, but it's fairly accurate.
Horsehair worms are in their own phylum (Nematomorpha), with about 2,000 species worldwide. This is an estimate because we don't really know how many species there are. They all live in freshwater for part of their life cycle.
What about the other part of their life cycle? This is where things get bizarre. Adult horsehair worms mate in water, and the females lay eggs in water. When the eggs hatch, the young worms quickly form a protective covering called a cyst. These cysts are often eaten by insects, particularly grasshoppers and crickets. Once inside an insect, the protective covering dissolves, and the juvenile worm starts living inside the insect, absorbing nutrients from the food the insect eats. The worm grows bigger and bigger, sometimes weighing as much as the insect itself.
Here's the really weird part. When the worm becomes mature inside the insect, it has a chemical effect on the insect's brain—the insect is irresistibly drawn to horizontally polarized light. The result? The insect is drawn to water, and it inevitably jumps in and drowns.
Then the horsehair worm emerges from the drowned insect and swims away to find a mate, thus starting the cycle again.
I know it seems gross, but these kinds of interactions are amazing, don't you think?
The photo shows a horsehair worm that has emerged from a drowned grasshopper.
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