Why didn't evolution help Luna Moth to have a mouth?

Evolution didn't prevent the moth having a mouth. It allowed the mutation, that involved not having a mouth, to persist, presumably (since it eventually prevailed) because it carried some other benefit, we cannot see without studying it. read more

The Luna moth is just one stage of life of an animal that spends most of its life as a caterpillar. Evolution selects for things that survive and reproduce. If something can do so without having a mouth during its"mating phase" of life, it's all good as far as evolution is concerned. read more

The Luna moth’s declining numbers in some areas can be attributed to a variety of factors, including parasitic predation (the Tachnid fly introduced in some areas to prey on gypsy moths and tent caterpillars also attacks the Luna moth caterpillar,) foliar spraying and habitat destruction. read more

Moths can range in size from smaller than a pencil tip to bigger than a songbird. The Atlas Moth, of Southeast Asia, considered the largest in the world, has a wingspan of nearly a foot (30 centimeters) — more than that of a Baltimore oriole. read more