Occasionally people who looked into drinking troughs or other
stock watering places were startled by the appearance of long, rather stiff
worms, about the size and range of colors as the hairs from
horses' tails. These worms seemed to appear suddenly, at full size.
Well, everyone made the natural assumption that these worms were indeed horsehairs which had fallen into the water and somehow or other become alive on their own.
A perfectly sensible theory, based on observation.
However, when early naturalists attempted to produce horsehair worms on their own, by soaking horsehairs in water for prolonged periods of time, they invariably failed.
We know more now, but not everything.
The current story goes like this: juveniles of these species (they have their own phylum, Nematomorpha) live in the body cavity of a host, usually an insect, often a grasshopper. The larva's digestive tract is rudimentary; usually it has no mouth. Nutrients are absorbed through the skin.
When the young horsehair worm is near maturity, it "influences" its host to seek water. (How? No one but the worms knows.) Somehow or other the juvenile worm senses when the insect has reached a suitable body of water. At this time, it emerges, and the host dies.
The long, rather stiff worm enters the water. At this stage in its life it cannot feed, and depends entirely on nutrients absorbed from its now-dead former host. These worms hardly move, and can live quite a long time. Circulatory and even respiratory systems are absent. They are only (barely) mobile reproductive machines.
More assumptions. That this feeding trough or whatever contains not just our one worm, but at least one other, of opposite gender. This being the case, the two mate, and the female lays eggs in long gelatinous strings which are wound around water plants.
When the eggs hatch, they are larvas with a spiny eversible proboscis armed with three stylets. More good luck (this is getting monotonous): a grasshopper happens by at just the right moment, and eats the larva. And so the story starts over.
You'd be thin too.
As the improbable chain of events in this tale gets longer and longer, it becomes obvious that there is still a lot we don't know about these animals.
PHYLUM Nematomorph, 230+ known species