Night crawler earthworms mating. Night Crawlers (Lumbricus terrestris) are a large worm that have a distinct, dark colored head end which does contain the primitive brain of the animal, and this tends to be the end of the worm that travels forward the most. They get their name because they crawl on top of the ground at night. The night crawler is not native to North America but is a European species that was introduced to the new world with the advent of European settlement. Both night crawlers, and most other worms, are hermaphrodites having both male and female reproductive organs. Most worms must still mate with another of their species in order to reproduce. When two worms or crawlers mate, they lie alongside one another, and both transfer sperm to each other. Each will lay one or more capsules (like a cocoon for the eggs), from which will emerge one or two fully formed tiny worms. The familiar thickened band near the front end of most worms is a structure called the clitellum which secretes the mucous and other substances that form the capsule containing the fertilized eggs.