Taxonomy

The nightcrawler is among the largest worms found in the United States. It is 12.5 to 20 cm (5 to 8 inches) long when mature.

Nightcrawlers are reddish brown in color. The worm's body is divided into 100 or more body segments.

Mature worms have a swollen segment, called the clitellum, about a third the way from the front end of the body.

Nightcrawlers are separated from other species by their dark color and the distance from the front of the worm to the clitellum.

Immature worms do not have a clitellum and are difficult to identify.