There can be various reasons because of which mantises can die. They require proper care to live. They need height for molting and require tall aquarium. If you seal the tank and there is no entrance of oxygen then mantises can die. Temperature and humidity should be very correct for mantises. Praying mantises can also die because of high humidity or very low humidity. You have to keep a balance between high and low humidity. Even just after a male or female mantis mate with each other, male mantis can die because female try to eat it. Therefore, it is very important to properly take care of mantises.