The largest and arguably prettiest moths in North America are called Giant Silk Moths. They include such beauties as the Cecropia, Polyphemus, and Promethea Moths. Since they are so big (with wingspans of 4 to 6 inches) and often so boldly colored, you might think you would readily see them. But that generally is not the case. Since they fly only at night, and since the adults live only a few days (they do not eat, but simply reproduce and die), finding Giant Silk Moths tends to be a very rare and special thing.
This is a Luna Moth which we found resting outside Mary's dorm. It's a male, as you can tell by its large, feathery antennae. Males use their antennae to detect a chemical released by the females to attract them, and they can detect it from miles way. You can also tell it is a male by its elongated body below; females, full of eggs, have short, plump abdomens.
If you find a Giant Silk Moth, you are fortunate indeed. It is worth stopping and marveling at what a beautiful creature it is, and how fortunate you are to behold it.