Range: Cosmopolitan Habitat: Inside hosts such as praying mantises, crickets, beetles, millipedes, grasshoppers, cockroaches, crustaceans, and more as larvae; Ponds, streams, lakes, rivers, and more as adults Size: 1 inch - 6.5 feet (2.54cm-2m) long Diet: Host nutrients Threats: Fish and crustaceans Life span: Several months or more

Transcript:

Today on Animal Fact Files we’re talking about horsehair worms. These animals were named horsehair worms because they were originally believed to be pieces of horse hair that had come in contact with water and miraculously became animate. There’s even a scientist who sat and watched strands of horse hair in jars to test this hypothesis. That’s science in action for you.

There are somewhere around 350 described species of horsehair worms, but it’s believed there may be upwards of 2,000 species alive today - they just aren’t all classified - or have even necessarily been discovered. These worms are found on every continent besides Antarctica, though that’s not really a surprise both because Antarctica has almost no arthropods and just because it’s freaking Antarctica.

Adult horsehair worms live in water, and are almost exclusive to freshwater inhabitants, though there are a couple exceptions that live in marine water. These worms may be seen in ponds, streams, lakes, rivers, and basically anything that retains water - like a cat’s water bowl. Trust me, if that sounds unpleasant, it gets worse - at least if you’re an arthropod. Horsehair worm adults also occur in moist soil and leaf litter. They need water, but don’t necessarily have to be submerged. Horsehair worm larvae on the other hand may be found… elsewhere.

Horsehair worms are an entirely parasitic animal phylum. Basically what that means is that every known species uses another animal (or animals), called the host, in order to develop and obtain nutrients. There are ectoparasites like ticks and there are endoparasites like today’s subjects. The larvae of horsehair worms spend their lives inside a host - or multiple hosts depending on how things work out.

Horsehair worms begin life as eggs. Female horsehair worms can produce millions of eggs at a time. These eggs are typically fertilized by a male horsehair worm’s sperm, though there are some horsehair worm species that can reproduce via parthenogenesis - in which eggs develop without sperm. Horsehair worm eggs, which have been laid in long strands in sediment or under debris, hatch about two weeks to three months depending on the water temperature. At this point, the larvae need to be consumed by a host. They can’t move around much, so some of this is left up to chance. Sometimes they get lucky and a definitive, or primary, host, or the animal in which they will develop into an adult, will come drink up some water and ingest a larva or two; but often the larvae are consumed by a “traveling” host.

The larvae won’t develop in these hosts, but they’ll wait inside this animal’s body until it is consumed by the horsehair worm larva’s definitive host - at which point it can begin developing into an adult. Some traveling hosts for horsehair worm larvae include mollusks and aquatic arthropods. Definitive hosts for horsehair worm larvae include: the praying mantis, crickets, beetles, millipedes, grasshoppers, cockroaches, and crustaceans. Now, perhaps you noticed that most of these definitive hosts are terrestrial animals. So then how, you might ask, would an adult horsehair worm end up back in the water? And that’s where the wonders and total creepiness of parasitism comes into play.

It takes about two to eight months for the larvae to develop into adult horsehair worms. When they’re ready to emerge horsehair worms cause their hosts to jump into water - effectively committing suicide so the adult worm can escape. By this point, the worm has grown to fill just about all of the host’s body cavity, and even if it did survive the ordeal of having an adult horsehair eject from its abdomen, it wouldn’t be feeling 100% afterwards. Horsehair worms may be about an inch (2.54cm) long to more than six and a half feet (2m) in length! And while it’s typically only one adult horsehair worm per host, multiples do happen! Like we said, though, they like arthropods, so you don’t really have anything to worry about. In fact, if you did accidentally swallow an adult horsehair worm, it would likely make its way out of your body through your mouth or nose - at least, that’s what they do when they’re eaten by other animals in the wild!

For more facts on horsehair worms check out the links in the description. Thank you to Enrico for today’s request! Give a thumbs up if you learned something new today, and thank you for watching Animal Fact Files.