Episode 270 – Horsehair Worm: Parasite on the Prowl

“…and today we’re talking about a parasite on the prowl. But more on that later.”

By now you may have heard of Cordyceps, the fungus among use that can take over the brain of an ant. But is there such a threat in the kingdom Animalia. The horsehair worm is a parasite that bears no ill will, but it will make an unfortunate mantis very ill. Still, the journey from egg to host isn’t an easy one and the worm pays its dues to get a free ride through Life, Death, and Taxonomy.

Description of the Horsehair Worm

  • Not much to it. It’s a long, thin, wriggly black or brown worm. 
  • It looks like a piece of string or a shoelace that wants to live in your insides
  • A very close look at it’s head shows a head with a small light-sensing hole and two fleshy mandibles for latching onto things
  • Females tend to have a rounded and “slightly swollen” posterior while males don’t. So the ladies got more junk in they trunk

Measure Up

Welcome to the beloved Measure Up segment. The official listener’s favorite part of the show! The part of the show when we present the animal’s size and dimension in relatable terms through a quiz that’s fun for the whole family. It’s also the part of the show that’s introduced by you when you send in audio of yourself saying, singing, or chittering the words Measure Up into ldtaxonomy at gmail dot com. We don’t have a new Measure Up intro!


  • 90 cm
  • How many heights of Pumuckel the Shetland pony go into a horsehair worm?
  • Hint: Pumuckel is a contender for world’s smallest horse. He lives in Germany where he tours kindergartens and nursing homes.
  • 1.8 horses. Pumuckel is 20 inches (50 cm) tall.

Phylum Size

  • There are 351 known species in the phylum Nematomorpha.
  • How many species of freshwater Nematomorpha are estimated to be extent globally?
  • Hint: It’s about the same number of species as there are feet in the height of AFLAC Tower in Rowley, Iowa.
  • There are 2,000 species estimated to be in Nematomorpha.

Fast Facts about the Horsehair Worm

  • Range: Japan, Nepal, and Taiwan 
    • Very specific, why not China or Korea?

Major Fact: Parasite on the Prowl

The horsehair worm goes on a parasitic journey that spans several hosts and gets eaten twice. Eggs are laid in a river or pond and soon hatch into larva. 

The larvae do their best impression of a tasty morsel in order to be gobbled up by a small water insect. Water nymphs, as in the aquatic stage of development for many insect species (not the Greek mythological creature), eat the horsehair larvae before growing wings and leaving the pond. 

Once the horsehair larva is eaten it develops a hard outer shell to avoid digestion and waits.

With any luck, the host insect is on the menu for the parasite’s prime paradise: the praying mantis. 

Once the host is eaten by a mantis, the hair worm reaches adulthood. It grows up to many times the length of its host, which may only be around 10 cm. Host mantises, and sometimes crickets, will stop growing and may never reach full size. 

Now the worm has the nutrients it needs, but it needs water to reproduce. Mantises usually don’t like hanging out by the water. It’s a good place for predators like fish and birds to catch them, not to mention the threat of drowning. 

To get the mantis in the right place, the worm will have to hijack the controls. Researchers have found that infected mantids are attracted to horizontally polarized light. Light polarization happens in nature when light reflects off of the surface of water. 

The mantis wanders aimlessly until it sees light reflecting off of a pond and then jumps in. The horsehair worm leaves the mantis and lays its eggs. In most cases, the host washes away and drowns, but mantids that survive may live and recover from their parasite incursion. 

Ending: So head toward that horizontally polarized light, develop an anti-digestion pod, and hijack your way to a better life like the horsehair worm here in LDT.