If the light in the room is on for you to see, that's enough to make their pupils constrict. Sometimes even in high light the pupils will dilate if they get overly stressed or excited (such as when they are about to pounce on prey). Generally, they're nocturnal, so it doesn't take a lot of light to make their pupils small.

For the mantis, a jar doesn't have enough ventilation for most things. Get a small critter-keeper or something, use eco-earth as substrate. Find some twigs and bake them at 350 degrees for 15 minutes in your oven (to kill parasites and bugs) and put those in there. They need something to climb. Also, most inverts don't actually drink, so you need to keep the environment hydrated. Mist well at least once a day. Desiccation is one of the biggest causes of death in inverts.
I've never kept mantises, but I have kept millipedes for several years.