Horsehair Worms, Nematomorpha, are parasitiod Worms related to Nematodes, but with a rather more complex life-cycle. The adult worms are extremely long, sometimes exceeding two meters in length, but seldom more than a millimetre in width, and are found in marine and freshwater ecosystems. The larval stages of these Worms are parasites, growing inside the bodies of other animals, and typically have a two-host life-cycle, with the young Worms infecting a secondary host shortly after hatching, then modifying the behaviour of this host so that it is more easily consumed by the primary host, inside which the Worm reaches maturity, emerging as an adult, typically with fatal consequences for the host. There are two extant orders of Nematomorphs, the Gordiida, which are typically found in freshwater environments, and which mainly parasitise Insects, and the Nectonematoida, which are typically found in marine environments, and which typically parasitise Crustaceans. A single terrestrial species is known, Gordius terrestris, from Oklahoma, Texas, and Louisiana.
In a paper published in the journal Evolutionary Systematics on 2 October 2025, Qaisar Jamal, Muhammad Riaz, and Moeen Uddin of the Institute of Zoological Sciences at the University of Peshawar, and Andreas Schmidt-Rhaesa of the Leibniz Institute for the Analysis of Biodiversity Change, describe a new species of Gordiid Horsehair Worm, found living on snowfields in Khyber Pakhtunkhwa Province, Pakistan.
The new species is named Gordius nixus, where 'nixus' derives from 'nix', the Latin for snow. Specimens were found living on freshly fallen snow in two villages, Gat Koto and Shaltaloo, in Upper Dir District, and one village, Karang, in Upper Kohistan District. Sightings of similar Worms on snow are reported from several other villages. Examined specimens are 1 mm in diameter and between 110 and 185 mm in length, and are yellow-to-light brown in colour. No likely Insect host for the species was found near the sightings.
The biology of Horsehair worms is now well understood. Most reported sightings of these Worms from temperate regions are from the summer months, leading to the expectation that they overwintered either as eggs or inside a host species. Finding Horsehair Worms active on recent snowfalls is therefore surprising.
Jamal et al. note that reported sightings suggest that this is not the only so-far-undescribed species of Horsehair Worms in Pakistan. They also note that this is probably true for neighbouring countries as well, with only three records of Horsehair Worms from Iran, and no records from Afghanistan, In India, 22 species of Horsehair Worms have been recorded, but most from the east of the country, with only three species recorded in northwest India, one from Punjab and two from Uttarakhand. Seventeen species have been described from the whole of China.
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