To kick off National Moth Week, I thought I’d start by trying to address one of the more common questions I received when I worked at an insect zoo: what’s the difference between a butterfly and a moth?
Just because it’s a common question doesn’t mean it’s easy to answer!
For one thing, what most people mean when they ask that question is “How can I tell if the insect I’m looking at right now is a butterfly or a moth?” So they don’t want to hear that butterflies are monophyletic and moths are paraphyletic , and they don’t want to hear about features they can’t see, like whether the fore- and hindwings hook together .
There are a lot of ways to generally tell butterflies and moths apart, but most have exceptions and it’s easy to make mistakes if you always rely upon one method.
Moths have feathery antennae, butterflies have straight antennae with clubs on the ends
This one is a bit of an oversimplification: while some moths do have feathery antennae, others have straight antennae. Butterflies generally have threadlike straight antennae with clubbed ends.
In general, butterflies with large feathery antennae tend to need them to find mates. A silk moth (Family Saturniidae) like this Atlas Moth (Attacus atlas) that lives only a few days as an adult has to be able to scent pheromones produced by potential mates from miles away.
Thick, straight antennae like those of this Corn Earworm Moth (Helicoverpa zea) are also pretty common among moths:
This Painted Lady (Vanessa cardui) exhibits typical clubbed butterfly antennae:
But (you knew there’d be one), moths in a few families have clubbed antennae. A few butterflies lack clubs. Skippers (Family Hesperiidae) have hooked club tips to their antennae.
Moths are drab, butterflies are colorful
More accurately, day-flying species tend to be more colorful than night-flying species, and moths tend to be noctural, so there tend to be a lot of drab moths and colorful butterflies. But there are butterflies which are highly camouflaged, as well as butterflies which prefer to fly at dawn and dusk. There are also brightly colored day-flying moths.
For example, the Madagascan Sunset Moth (Chrysiridia madagascarensis and related Uraniid moths may be one of the most colorful lepidopterans in the world (photo by Cody Hough, Creative Commons Attribution-Share Alike 3.0 Unported license).
Whereas many people I met initially thought the dusk-flying owl butterflies (Caligo sp. and others) of South America were actually moths.
Generally speaking, if it’s flying around at night you can probably assume it’s a moth. But if it’s flying around during the day or at dusk, it could be either.
Moths have fatter, fuzzier bodies than butterflies
This one is mostly true, at least among larger species–but it’s pretty hard to tell when looking at a tiny butterfly or moth how relatively fuzzy and fat it is. And some butterflies, like skippers, have proportionally fatter, fuzzier bodies than most butterflies.
Moths rest with their wings spread open or folded across their backs, butterflies rest with their wings folded vertically
In general, most moths rest with their wings spread open, like this Atlas Moth:
Or with their wings folded across their backs, like this African Peach Moth (Egybolis vaillantina) (photo by Muhammad Mahdi Karim, GNU Free Documentation License, Version 1.2):
While butterflies typically rest with their wings closed vertically above them like this tropical lacewing butterfly (Cethosia sp.):
But it’s not uncommon to see butterflies basking with their wings open like this male Greater Eggfly (Hypolimnas bolina):
Moths in the family Geometridae sometimes rest with their wings closed vertically, and some skipper butterflies tend to hold their wings at a 45 degree angle from their bodies.
So, what does this all mean?
Telling the difference between a butterfly and a moth in the field isn’t that difficult in practice, but because there’s no simple rule and every guideline has exceptions, it’s difficult to explain how to do. The best advice I can give is to look at all these features and try to evaluate overall whether the insect looks more like a butterfly or a moth.
And I won’t go into weird groups like the moth-butterflies (Family Hedylidae)! Most people are unlikely to encounter them.
Pop quiz time!
What are these, moths or butterflies?
A. From Wilson44691 on Wikipedia, licensed under a Creative Commons Attribution-Share Alike 3.0 Unported license (I will edit in a link after people have a chance to guess!):
B. Taken at Great Sand Dunes National Park, September 18, 2010:
C. Also taken at Great Sand Dunes National Park:
(All tropical butterfly photos not credited to someone else were taken by me at The Butterfly Pavilion in Westminster, Colorado.
This post is for National Moth Week, July 23-29, 2012.
 This means, in essence, that butterflies are a small group within the moths, all related to each other. There’s no taxonomically valid way to group all moths together with a common ancestor without including butterflies. Butterflies are, basically, specialized moths.
 Moths have a special frenulum on their hindwings which hooks to barbs on their forewings, keeping their wings moving together. Butterflies have no similar structure, so their front and back wings move independently, giving them their distinctive fluttering flight. Unfortunately, while this feature will reliably distinguish butterflies and moths, you must capture the animal and know what you’re looking for, so it’s not what the average person is after.