Discussion on answer by Green: What kind of natural armor would stop bullets?

@ScottMilner The variable $d$ is the distance the bullet travels through the armor, not the distance to the shooter. Stopping the bullet in less distance requires greater pressure. So, the armor has to be at least 2cm thick to stop the bullet before breaking pressure is reached.

@MarkH not even that, unfortunately. You're looking at it moving 2 cm while stopping. To be undamaged, you'd need to look at stress/strain curves for mantis material and know the specific shape and specific deformation, or have enough margin that it didn't matter. For ablative armor it's even harder.