This common name refers to only one family.
These are often called Earthworms.
Earthworms frequently end up in the stream after they make a wrong turn crawling around on a rainy day, and trout relish them. It is interesting that fly fishers frown upon the San Juan Worm, a popular artificial imitaiton, considering that this form of terrestrial (Terrestrial: Insects which live on land and are fed on by trout only when they incidentally fall into the water are known as "terrestrials" to fly anglers, and they're very important in late summer.) hatch-matching is just as rooted in biology as any ant or beetle fisherman's sport.
Since worms fit easily onto a hook, many fishermen skip all the feathers & thread tying and go straight for their favorite all-natural fly pattern: the real, live, wiggling "Garden Hackle."