The Collaborative International Dictionary of English v.0.48:

Gordius \Gor"di*us\, n. [NL. See Gordian, 1.] (Zool.)
   A genus of long, slender, nematoid worms, parasitic in
   insects until near maturity, when they leave the insect, and
   live in water, in which they deposit their eggs; -- called
   also hair eel, hairworm, and hair snake, from the
   absurd, but common and widely diffused, notion that they are
   metamorphosed horsehairs.
   [1913 Webster]