Sorry, Hartearem,; I didn't mean to offend you, I did read your info right through,and I asked if you were in HS (again!) because I never know, at that age if you have had a HS level intro to science.
If your balloon enters the stratosphere, it will pass through a zone of -20C temp. and will pass through it again on the way down. I'm sure that you and your uncle will cover all the angles, but a video cam will not tell you the mantids' cause of death. Also, if you maintain a "normal" temp and pressure, what is the purpose of the experiment? (I think that someone mentioned the temp issue already) Do you plan on recording telemetry readouts, oxygen concentration, barometric pressure, temperature, etc? Also, I am sure that you know that a "commercial" camera will not operate below 0C so the whole chamber will need to be temp controlled unless you go to an industrial grade camera.Perhaps i am missing the fundamental reason for your experiment. Are you testing the ability of the mantis to stay alive --we already have a pretty good idea about that -- or the ability of your capsule to sustain invertebrate life, perhaps before trying it on a mouse, a monkey, and who knows, a Hartearem?
In any event, I look forward to hearing what conclusions you draw from your experiment.