Out of the cricket, into the river

Horsehair worms increase insect suicide rate

Nematomorpha:  Gordiida

What happens when you accidentally crush a cricket under foot while standing in a mixture of forested and logged land in the foothills of the Peruvian Andes?  First, it doesn’t end well for the cricket.  And second, you may wish to wipe off your boot.  But an additional possibility is that a strange creature emerges from the deceased cricket. A very long, threadlike animal, 165mm — almost six and a half inches — long.  This is what happened in May, 2018.  And in this case, the parasitic worm turned out to be a species new to science:  Acutogordius olivetti.

If the “gordius” part of the genus name sounds familiar, it should be.  Nematomorphs, with the common names horsehair worms or Gordian worms, the group of worms to which the new species belongs, are prone to rolling up into something akin to a ball of twine.  This intertwined mass of worm has been compared to the proverbial Gordian knot.  You remember the Gordian knot encountered byAlexander the Great when he arrived in Gordium, the capital of Phrygia, in 333 BC.  The chariot of the ancient founder of the city, Gordius, had long remained tied to a pole with an impossibly complex knot.  Legend had it that the knot could only be undone by the one destined to be conquerer of Asia.  One account has Alexander drawing his sword and slicing through the knot with a strong blow.  This story, of course, is the source of the common phrase “cutting the Gordian knot,” referring to a necessarily bold approach to a complicated problem.  Let’s unravel a bit more about the story of this weird worm and its relatives, but first a note about the geographic significance of the discovery.

Until recently, not a single horsehair worm was documented to occur in Peru.  Given their wide distribution, this was clearly for lack of trying on the part of worm hunters.  In the span of a few years, scientists have documented at least different eight species, but no one supposes this is even close to the total Peruvian horsehair worm fauna.  The incredible diversity of potential host arthropods alone suggests a very bright future for horsehair worm exploration in Peru, and throughout South America.

So, what, exactly, is a horsehair worm?  They are members of the class Nematomorpha, with mature worms found in both sea and freshwater.  The threadlike adults are free-living, while larvae are parasitic on various species of arthropods, notably crickets, grasshoppers, cockroaches, mantids, beetles, and crustaceans.  Typical examples are 50 to 100 mm (or 2 to 4 inches) long.  The longest adults encountered are usually no more than 39 inches, but there is an almost unbelievable record of a worm about 2 meters in length.  In contrast to their great lengths, the worms are only 1 to 3 mm in diameter.  About 350 freshwater species have been discovered and named, but it is estimated there may be as many as 2000 species, or more.  Knowledge lags for the simple reason that the world has few parasitic worm experts, but no shortage of parasitic worms.

Nematomorphs have only longitudinal muscle fibers and a non-functioning gut.  They have no excretory, respiratory, or circulatory systems.  Their version of a nervous system consists of a ventral nerve cord along the length of the body and a nerve ring circling the anterior end of the worm.  There are two sexes and internal fertilization, followed by gelatinous strings of eggs being laid.  Rings of circular hooks and terminal stylets are thought to help the worms enter their host arthropods, where they live for weeks or months, molting as they increase in size.  What passes for a sex life involves males and females aggregating into tightly packed, slithering balls while mating, another, perhaps even better, source of the Gordian knot allusion than the coiling up behavior of individual worms.

Nematomorphs are related to nematode round worms that look similar to the uninitiated, especially viewed from a distance.  The earliest fossils of a worm resembling extant horsehair worms is from Burmese amber from the early Cretaceous, about 100 million years ago.  So, it’s been a dangerous world for crickets out there for a very long time.  Only five known salt water species are known.  They are planktonic, with larvae infecting decapod crustaceans, especially crabs.  Living in the ocean with such an ever-present threat would be enough to make anyone crabby.

The large cavity inside the body of an insect is filled with haemolymph — the insect version of blood which is not red, has no haemoglobin, and does not transport oxygen.  This body cavity is called the haemocoel.  This is where baby horsehair worms call home after some unsuspecting insect ingests them.  It is the place where they hang out for the whole of their juvenile life, absorbing nutrients through their skin, and growing longer and longer.  But that’s not even the weirdest part, not by a long shot.

Acutogordius olivetti horsehair worm that emerged from a cricket (Eubliastes sp.) attracted to a mercury vapor lamp in the Andean foothills of Peru. The specific epithet refers to the Olivetti river that runs nearby the collection site in Madre de Dios. Photo by Peter Billingsley. Reproduced from Figure 1A in the publication describing the species (cited below), with permission of Dr. Ross Piper, School of Biology, University of Leeds, UK.

Living inside a cricket, being carried around the forest floor for the duration of its childhood, how does a horsehair worm get from its host to a river or stream so that it can enjoy its adulthood, free at last?  Here’s the really strange part.  These parasites have an effect on the host insect’s brain, causing a disproportionate number of them to seek out water, jump in, and drown themselves, thus transporting the worm to its desired habitat destination as efficiently as an Uber driver.  At least in the case of one mantid host, and possibly more generally, it has been shown that the host becomes attracted to horizontally oriented light rays and is thus drawn to the reflective surface of water.

Should you worry about being infected by a Gordian worm?  Not.   Not, that is, unless you are an insect, which is why I slept like a baby last night after researching this story.  In full disclosure, there are a few documented incidents of accidental parasitism in both humans and dogs, but this is not the preferred situation by any of the involved parties, not even the worms.

Acknowledgment

I thank Dr. Ross Piper, University of Leeds, for permission to reproduce Figure 1 from his publication with Dr. Andreas Schmidt-Rhaesa describing the new species.

Further Reading

Schmidt-Rhaesa, Andreas and Ross Piper (2021) A new species of Acutogordius Heinze, 1952 from Peru.  Spixiana 44: 21-24.