Most likely the food, if mantids die mysteriousely with out traces its because its internal, and is always by the food or a block in the mantid's sytem, one tiny bit of poisoning can kill a mantids, like rotten fruits, chemicals, mold, eaten by roches and/or crickets, or slightly dying insects you feed to mantids can kill your mantis. just look for any signs in the cricket or feeder of any malfunctioning parts like legs that dont work( in a dying, starving cricket they show a sign of just their back legs working or crawling) in roaches, if their container is smelling like dead animals and they are eating egg carton, and its not digesten it can plug the the mantis up and block it from absorbing nutrition. just be aware of any strange things happening to feeders and avoid them. always feed your mantids the heathiest bugs and if your garden or yard has pestisides or chemicals, avoid bugs you suspect could have some on them or in them. flies also can be toxic at times.
Joe
