This over-active, threadlike worm, appropriately called a Horsehair worm, just wouldn't keep still to get its picture taken! It moved in amazingly sinuous and beautiful curves, often seeming to loop back and knot itself making it very obvious why they are also known as "Gordian knot" worms.
The larval stages of these nematodes are parasitic, colonizing the bodies of grasshoppers, beetles, and other such invertebrates. They pose no threat to humans, pets or livestock. Adults are not parasitic and feed on their own, without depending on other life forms. Up close, they're featureless, showing no definitive distinction of a head or tapering tail.
We found this one wriggling about amongst our aquatic plant samples - they live near and in water, the parasitic form often 'driving the host' to drown itself, to get to water. They're found worldwide.