This praying mantis was hunting on the leaves of a pumpkin plant. He (she?) was one of the bigger ones that I have seen, about four inches long. Harmless to humans, they are voracious carnivores of almost anything else that they can catch – other insects, smaller mantises, and so on. Wikipedia says mantises will even eat small lizards and birds if they can catch them.
The female is also known to eat the male after mating.