A praying mantis will eat other insects..I`ve seen them stalk & catch grasshoppers and moths...praying mantis is good for the garden as they eat the insects eating your tomatoes, etc.  I`m not sure if they eat grass or leaves..I don't think so.

Dont kill grand daddy long legs..they keep black widows away..most spiders are harmless & they do trap mosquitos, flies, etc..