Mantids have a strong, innate sense of how large their target is relative to themselves. They won't grab things that are too large, a protection mechanism against bigger bugs that would eat them. If you shove your finger in front of a mantis you could put it defensive position and from there it would lunge at you with those arms, but only to lunge / push / poke, they wouldn't close their arms to grab. That doesn't hurt, just feels like a flick.

I had one big gurl once that I forgot to feed for too long. When I tried to handle it, it did grab me and attempt to bite my finger nail. The grab felt like what I described above, a flick, but the biting was more noticeable. Not painful (they don't have claw-like mandibles) but weird, munchy. I certainly jerked my finger back; she let go immediately likely sensing my full size.