Look at it this way: Earthworms eat dirt. Eating an earthworm is like eating dirt with some slime wrapped around it. Chickens don't eat dirt (not counting grit for their gizzard). A chicken would prefer a nice grasshopper or cricket with greens inside their bodies, because then you have greens wrapped in protein. Giving thought to what the live food eats, and comparing it to what a chicken might eat, is helpful for giving chickens live foods they'll probably like.