World’s Most Disgusting Animal? Behold the Horsehair Worm

Horsehair worm and katydid. Photo by Dr. Andreas Schmidt-Rhaesa.

Horsehair worms, often perceived as among the most unsettling creatures in the natural world, have a way of challenging even the strongest of stomachs. Yet, there’s an undeniable intrigue to them.

Why do they evoke such a strong reaction? The answer lies in their larval stage, during which they inhabit the bodies of insects and other arthropods, growing to sizes that defy expectation.

Just have a look at this one exiting the posterior of a katydid:

Alastair Rae from London, United Kingdom via Wikimedia Commons

These worms, which are in the phylum Nematomorpha, are truly the stuff of nightmares, and not even horror movies could do them justice. Some species of horsehair worms, such as Spinochordodes tellinii and Paragordius tricuspidatus, even take their body-snatching life cycle a step further, infecting the insect’s brain and causing it to seek water to drown itself. This allows the adult horsehair worm to emerge safely from its host’s dead body and start the process over again by producing its own nasty, freeloading offspring.

In the videos below, watch what happens when a person kills a “zombie” praying mantis that is under control of a horsehair worm.

Could you imagine having something that large (in proportion to your body size) living and feeding inside you? Blech! Unfortunately, that’s just one of millions of instances this occurs every day. Most horsehair worms are aquatic, and when exposed to water, they can be prompted to emerge from their host’s bodies.

In the video below, watch what happens when someone places a live, infected praying mantis halfway into a jar of water.