The short version of this argument already ran in Why Moths Fly to the Light: antennae, wing position, and a shrug about exceptions. It earned its own follow up because the full comparison has more useful marks than that piece had room for, and thousands of people a month are apparently searching for exactly this.
Five field marks
- Antennae. A butterfly's antennae end in a small club, like a golf tee. A moth's are feathery, comb like, or simply threadlike, built to catch pheromones drifting on the air rather than to look tidy.
- Wings at rest. Butterflies fold their wings upright behind their backs. Moths rest with wings flat, tented like a little roof, or spread open against whatever they've landed on.
- Body build. Moths run stockier and fuzzier, with shorter legs built for a night of flying into things. Butterflies are the sleeker, more streamlined cousin.
- The coupling hook. Most moths have a frenulum, a small hook and bristle structure that clips the front and back wings together in flight. Butterflies mostly lack it, a fussy detail but a genuinely reliable one under a hand lens.
- Pupation. A moth caterpillar usually spins a silk cocoon. A butterfly caterpillar forms a chrysalis, a hardened shell with no silk wrapping at all.
The exception that ruins the easy version
The tidiest rule, that moths fly at night and butterflies fly by day, is also the least reliable one. Plenty of moth species are strictly daytime fliers and get mistaken for butterflies constantly, which is the whole reason the other four marks above are worth learning. If night versus day were dependable, this article would not need to exist.
From the archive
The nocturnal file is kept in Moth and Bat Shirts, including Luna Moth Field Notes and Death's Head Hawkmoth Field Notes. For the folklore and the reason moths circle a porch light in the first place, see Why Moths Fly to the Light.