They are totally harmless, and I find them quite fascinating myself. I was trapping some cats several years ago along a stretch of land behind a milk processing plant. Someone had dumped a few cats back there, and when I spotted one of the emaciated cats she was in the middle of the road stalking a mantis that was doing all it could to avoid being dinner for the cat! I picked the mantis up, put it in a safer place, and vcaught the cat. She got adopted out, the mantis was not squished..happy ending for all!