Night crawlers are large nocturnal earthworms that are sometimes kept as pets, but are mostly used as fishing lures and food for other animals. They need to be fed to be kept alive, no matter the purpose for keeping them. You don't have to do much, as far as nutrition goes, because of their natural purpose as one of the decomposers of the animal world. They'll eat almost anything, from ground meat to paper-based garbage. The one thing you have to worry about is the amount you give them to eat, because overfeeding can indirectly kill them.
What Night Crawlers Eat
Night crawlers eat decomposing matter, or anything finely ground and soggy enough to be consumed. For night crawlers used as pets or being reared as food for other animals, feed them things that will keep them healthy, so they'll live longer or so they won't poison the animals to which they'll eventually be fed. Chicken mash is a common food used by night crawler farmers. There are two types--chick starter and laying mash, with the latter being the more nutritious. If you don't care what happens to the night crawlers once they're fully grown (for people who raise night crawlers to catch fish), feed them fruit and vegetable skins, molded bread, and finely shredded paper-based trash. Make sure the food is damp, or they won't be able to consume it.
How to Feed Night Crawlers
Once you've decided what you'll feed the night crawlers, you have to give them the food. Night crawlers come to the surface to eat, and water draws them up as well, so place small amounts of food on top of the soil and wet it so that it's damp but not soaked--you don't want to drown the night crawlers. The night crawlers will come to the surface to feed.
When you're feeding the night crawlers, you have to take into account how slowly they eat food and what the food becomes when it is allowed to decompose before it's eaten. Some items, typically meats, may poison the soil when it decomposes and cause the night crawlers to die. Therefore, you have to give them just enough food to eat without much being left over. You can't pile food in the environment, unless you don't care about what happens to them because they're going to be fish lures. If you find that the night crawlers are dying, remove them from the environment and replace the old soil with fresh, damp soil before putting them back.