It wouldn't hurt the mantis once a while. although it is not as nutritious as gutloaded flies.but how can you do that when you're on vacation?Found out from Chuck (spiderpharm) that all kinds of flies emerging from pupa excrete waste accumulated during the puparium stage, the excretion does content high concentration of amonia. Although it might not affect the mantis feeding on it, it is better to wait and gutload your flies with nectar, honey, sugar, etc before letting them go into the mantis cage.