Ynghylch

Horsehair worms’ adults are free-living in aquatic freshwater or marine environments and capable of moving across damp terrestrial areas. In the larval stage, these worms must find an arthropod host to complete their life cycle, often killing the host, making them obligate parasitoids.

Eggs are laid in sticky strings underwater or on nearby vegetation depending on the species. When the eggs hatch, the larvae can encyst to survive until they are ingested. They are then eaten alongside vegetation or drunk with water by a suitable host which varies between species. Some species such as Spinochordodes tellinii and Paragordius tricuspidatus infest hosts such as grasshoppers or crickets that feed on vegetation with larvae attached.

Outside the UK, other species specialise in parasitising terrestrial hosts such as mantids by first being consumed by mosquitos or mayflies. The intermediate hosts act as transportation from aquatic areas to their terrestrial hosts. Marine species in the genus Nectonema specialise in parasitising crustacean hosts underwater.

Some larvae have a sharp spike-like appendage on their anterior end that acts as a needle that penetrates host tissues.
Once inside the host the larvae live inside the primary body cavity. The larvae feed on nutrients inside the host absorbing them directly through their skin and moulting as they grow. It may take a couple of weeks to months to complete the parasitic stage. This process rarely kills the host as the parasite relies on the host’s nutrients. The horsehair worm needs to ensure the host is near water before emerging as an adult.

This leads to the most famous trait of some species of horsehair worms, the ability to highjack the terrestrial host’s nervous system, altering their behaviour to make them actively seek out water. Spinochordodes tellinii and paragordius tricuspidatus species achieve this by causing grasshopper and crickets to jump into waterbodies drowning themselves when the horsehair worm reaches its adult stage. Once it encounters water, the horsehair worm burrows from the body to return to water to breed, killing the host if not drowned already.