The opimacus is found in some medieval encyclopedias. It is a worm that fights with serpents. It is much smaller and weaker than a serpent, but it wins the fight through skill and boldness. It attacks by attaching itself just below the serpent's head, from where it cannot be removed.

The name opimacus is a corruption of ophiomachus, "snake killer", a name for a kind of locust mentioned in the Septuagint and Vulgate versions of Leviticus 11:22 (A Book of Creatures, Opimachus).