Issue 9, June 20, 2016

Horsehair Worm

Horsehair worms are being found in mud puddles and road ruts after rains. They are very slender, whitish to dark brown, nematode-like worms usually four to seven inches long. These are adult worms that have emerged from their insect hosts to reproduce. Male and female worms shed their gametes (sperm and eggs) into the water where fertilization takes place.

Horsehair worm.

Fertilized eggs hatch into infective larvae that are aquatic like their parents. These larvae find their way into crickets, grasshoppers, or cockroaches by means that are not understood. It is assumed that the larvae are ingested while these terrestrial insects are drinking, but these insects primarily obtain their moisture from their food and dew, where the infective larvae are unlikely to be found.

The larvae feed inside the insect, maturing into adult worms. Adult worms are large enough compared to the size of the host insect that they fill most of interior of the insect when fully grown. When ready to emerge, the adult worm somehow causes the dying host to plunge itself into water, an action that would likely cause the terrestrial insect to drown. The adult horsehair worm then emerges into the water to reproduce.

Horsehair worm adults are named due to their being common in horse water troughs and being similar in size and color to the hair of horses. These are one-host parasitic worms and are thus not a threat to humans or other mammals or birds. They are related to nematodes but are in their own phylum, Nematomorpha. Their only close relatives parasitize hermit crabs in marine environments. (Phil Nixon)

Phil Nixon

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