If you fed your mantises brightly colored moths and they didn't die, it's likely they were poisonous but the mantises are immune to their specific kind of toxin.
It's fairly easy to avoid poisonous insects. If it's clearly making an effort not to be found or caught, it's fast and hard to catch or well camouflaged, it's probably harmless.
Even if you do accidentally feed your mantis something nasty, there's a good chance they will instinctively recognize and reject it upon tasting it. Most poisonous insects have closely related species or at least species that employ the same or similar poisons, so even a mantis from Africa will know better than to eat an American moth that tastes just like those other slightly different ones from home that instinct tells it are not good to eat at all...
If a bug is brightly colored, easily seen during the day, and not particularly quick or wary it is probably toxic.
