Worms up to Twelve Times the Length of Their Host, and That’s Just The Beginning

Generally speaking, crickets are not an aquatic species. When infected by the parasitic hairworm, however, they can be found jumping to their death into the nearest body of water. The lifecycle of these and all known species in the phylum Nematomorpha is relatively simple – as adults they are free-living aquatic animals, but in their youth they spend their time loitering, disrespecting their elders, and infecting terrestrial arthropods [1]. One species, Paragordius tricuspidatus, primarily targets the wood cricket, Nemobius sylvestris, a small species of cricket native to western Europe and northern Africa. In its larval stage it is microscopic, and enters the body of a cricket via ingestion. Once it has entered a suitable adult insect, it develops into a worm about 4-6 inches in length – dwarfing its tiny, half-inch host. In order to reproduce, however, it has to get back into open water. This is when it starts to mess with the cricket’s head. Crickets infected with sexually mature hairworms begin to jump into any body of water they can, drowning themselves and allowing the worms to escape into the water, where they hopefully get to mate. This strategy of host manipulation has been honed to the hairworm’s benefit: infected crickets have the highest rate of diving attempts at the same time as the worms reach their highest fecundity – the worms make it to the water exactly when they are able to produce the most offspring, meaning they spend as long as they can in the safety of the cricket’s guts [2].

So, what makes these crickets so eager to jump to a watery grave? A study by Thomas et al found a few different physiological changes occured in the brains of infected crickets compared to uninfected ones. The concentrations of a few amino-acids (the basic building blocks of proteins) differed. Three amino-acids in particular – taurine, valine, and tyrosine – were highly correlated with manipulation status: their levels were different between the brains of insects that were infected and tried to jump into the water and insects that were infected but didn’t try to jump [3]. The amino-acid concentrations in the brain are indicative of what proteins are being produced, which in turn have a major effect on brain function. Another major change that goes on in the brain of these bugs is the increased cell growth in the mushroom body cortex, an area of the brain found mainly in insects and other arthropods that plays a part in sensory learning and memory [4]. The cells here appeared to be dividing at nearly twice the normal rate in parasitized crickets, implying that neurogenesis – the development of new nerve cells in the brain – might be the cause of the strange behavior seen in infected crickets. The cell growth seen isn’t all that surprising, taurine – one of the amino-acids that is seen to increase in the brains of infected crickets – plays a role in both neurogenesis and in neuromodulation. Higher levels of taurine are correlated with lower levels of excitation in the nervous system. This translates to some pretty confused sensory intake, and can change the way the insects interpret both internal and external signals. This means that there is possibly a very simple explanation for the crickets’ desire to take a swim: hairworms might just make their hosts feel thirsty enough to hop right into the first stream they find.

[1] Thomas, F., Schmidt‐Rhaesa, A., Martin, G., Manu, C., Durand, P., & Renaud, F. (2002). Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts? Journal of Evolutionary Biology, 15(3), 356–361. https://doi.org/10.1046/j.1420-9101.2002.00410.x
[2] Sanchez, M. I., Ponton, F., Schmidt-Rhaesa, A., Hughes, D. P., Misse, D., & Thomas, F. (2008). Two steps to suicide in crickets harbouring hairworms. Animal Behaviour, 76(5), 1621–1624. https://doi.org/10.1016/j.anbehav.2008.07.018
[3] Thomas, F., Ulitsky, P., Augier, R., Dusticier, N., Samuel, D., Strambi, C., … Cayre, M. (2003). Biochemical and histological changes in the brain of the cricket Nemobius sylvestris infected by the manipulative parasite Paragordius tricuspidatus (Nematomorpha). International Journal for Parasitology, 33(4), 435–443. https://doi.org/10.1016/S0020-7519(03)00014-6
[4] Gronenberg, W., & López-Riquelme, G. O. (2004). Multisensory convergence in the mushroom bodies of ants and bees. Acta Biologica Hungarica, 55(1–4), 31–37. https://doi.org/10.1556/ABiol.55.2004.1-4.5

Updated 4/11/2019 to include links