As caudatadude suggests, earthworms are excelent. They are in fact the ultimate caudate fodder!!!
Thyey are highly nutritious, rich in calcium and they just love them. Just make sure you get them from a chemical free area or culture them yourself.
In addition to a diet based on earthworms, you can add variety by offering frozen bloodworms (very handy, quite good for a frozen food), brine shrimp, isopods, slugs, crickets, waxworms, a wide variety of fresh water crustaceans (Daphnia, gammarids, shrimp...), tubifex, fly maggots....