Matthew Cobb, who has a penchant for parasites, sent me this video that was recently posted on YouTube. WARNING: if you’re squeamish you may find it distressing. But that’s nature, folks!
The caption says this:
ok so i was just editing my latest montage and this huge spider came out, so i sprayed it and killed it, then this fricken alien worm came out of it!!!
One commenter noted (and this looks accurate) that it’s a “Gordian worm,” also known as a “horsehair worm.” They constitute their own phylum, the Nematomorpha. Curiously, although they infest some terrestrial arthropods, like this spider, nematomorphs must reproduce in water. The article on the phylum in the Encyclopedia of Life notes that they might be quite devious about this:
Many types of parasites are known to modify the behavior of their host in ways that benefit the parasite. Based on anecdotal observations, it has long been suspected that at least some mature nematomorphs, which must reach water to mate and reproduce, manipulate the behavior of their terrestrial insect hosts, causing them to seek water and jump into it. Investigations by Thomas et al. (2002) found clear evidence of this phenomenon in 8 tettigoniid orthopterans infected by the nematomorph Spinochordodes tellinii, as well as in the gryllid cricket Nemobius sylvestris infected by the nematomorph Paragordius tricuspidatus. In experiments, however, they found no evidence that hosts actively seek out water; rather, they suggested, infected hosts seem to display erratic behavior that eventually brings them close to water, which they then enter. Consistent with the findings of Thomas et al. (2002), Sanchez et al. (2008) found that this behavioral manipulation has two phases, first causing the cricket to wander into atypical habitats and next causing it to commit suicide by entering water.
The evolution of the ability of parasites to manipulate their host’s behavior in an adaptive way is one of the most fascinating areas of organismal biology. In almost no case do we know how they do it. It’s mind control, pure and simple.