These ferocious-looking creatures eat a wide variety of insects, including beetles, caterpillars, grubs, aphids, grass-hoppers, crickets and many more. And since they don't fly, they stay right in the area where they are released. Praying Mantids are completely harmless to people, pets, and the environment. In fact, they are so much fun that sometimes people buy them for pets. Each egg case, about the size of a ping pong ball, contains about 200 eggs, which hatch when the weather becomes warm.