Entomologists have a lot of explaining to do, and not just about their life choices! With a million plus (and counting), species of insects, arachnids and their relatives known to science, all going about their relatively inconspicuous lives, it’s not surprising we often get a lot of questions about them.
Some of the common questions thankfully have easy answers. Like, are tarantulas poisonous? Do mosquito-eaters really eat mosquitos? Do daddy-long leg spiders really have the most toxic venom in the world? And the all-important, what the heck is this bug?
Entomologists love to answer these questions because it makes us feel super smart and full of nerdy glee. However, there is a deceptively simple question we get all the time that might surprise you has no easy answer…
What’s the difference between a butterfly and a moth?
Most of the time entomologists will give a very simplified answer to this question. We often say that butterflies are diurnal, while moths are nocturnal, butterflies have clubbed antennae, while moths have feathery or threadlike antennae, butterflies are brightly colored, while moths are dull and cryptic. This answer works for a lot of species, but in reality, there are a LOT of exceptions to these generalities!
Take a look at these two Lepidopterans. Can you tell which is the butterfly and which is the moth?
the Madagascar sunset moth, Chrysiridia rhipheus (left), is considered by many to be the most beautiful Lepidopteran in the world, and it is a moth! It flies during the day, which explains those bright colors warning predators that it is toxic if consumed. Although its wings have an uncanny resemblance to swallowtail butterflies, the antennae are not clubbed, indicating that it is a moth. In THIS case.
Remember I said there were a ton of loopholes?? Take a look at the giant butterfly-moths (family Castniidae). These moths are large, colorful, fly during the day, AND they have clubbed antennae.
Honestly it’s enough to make you want to pull your setae out!
Speaking of setae, hairiness also isn’t a reliable character. A lot of moths are very fuzzy, it’s true. But check out this ridiculously fuzzy butterfly:
So what is going on here?! Well, if you are ready for the long-winded, but hopefully interesting answer, get ready. The truth is that evolutionarily speaking, butterflies ARE moths! Say what now?
All moths and butterflies belong to the order Lepidoptera, which is one of the largest and most diverse insect orders on the planet. To understand their evolution, we must travel back to the late carboniferous, about 299 million years ago. Here, among the luxuriant ferns, primitive amphibians, giant dragonflies, and bright green mosses, we would find the most recent common ancestor of all the Lepidoptera: a small, nocturnal moth bumbling about feeding on moss with chewing mouthparts. The quintessentially lepidopteran nectar-slurping proboscis evolved later, first as an innovation to feed on sap and water, and eventually nectar.
Plants initially started producing nectar in the cretaceous (about 150 million years ago), not for moths, but for bees! But because moths had already developed straw-like mouthparts, many Lepidoptera lineages proliferated with the rise of the flowering plants, trading in their nocturnal hours for day shifts, to capitalize on day-blooming flowers. Butterflies, the Superfamily Papilionoidea, are one of the lineages whose members overwhelmingly cast off their evening wear for fancy daytime apparel.
Butterflies like swallowtails and monarchs are so large and charismatic, that they really do steal the show! But they are a standout minority. Out of roughly 160,000 species of Lepidoptera known to science, only 19,000 of those belong to the butterfly superfamily.
Take a look at the Lepidoptera family tree and you will see how extensive it is. Butterflies are not at the apex of the tree, but rather sit in the middle with lots of other Lepidoptera superfamilies evolving in parallel with them.
There are a lot of other Lepidoptera lineages and species that also evolved to be diurnal, like the stunning day-flying tiger moths.
And butterflies are also not the only Lepidopteran pollinators! There are tons of flowers that bloom at night, like jasmine, honeysuckle, and yucca, which are overwhelmingly pollinated by moths. In fact, scientists are only just uncovering the extensive pollinating work that moths do.
Nocturnal butterflies, like the American night butterflies, are quite rare, but they do exist. And some butterflies, like skippers, those stout little cuties with the big clubbed antennae, are frequently found pollinating at dawn and dusk.
So, we know that butterflies evolved from moths, but butterflies are in their own superfamily after all. What separates them from all other moth groups?
Well, until the 1990’s, entomologists thought they had an answer to this. They believed that there were three morphological features of butterflies that distinguished them from all other members of the Lepidoptera.
One is the easy-to-spot clubbed antennae. Second is the tendency for butterfly forewings and hindwings to overlap and beat together in flight. Most moths, in contrast, have a velcro-like structure on their wings that keeps them stuck together. And thirdly, most butterflies have a set of sensory scales on the heads that most moths lack.
While none of these characteristics define a butterfly on its own, it was believed that if a Leipidopteran had ALL THREE in combination, then it was most certainly a butterfly!
And then……the American night butterflies (family Hedylidae), a group previously thought to be moths, were added to the butterfly superfamily, and all this went out the window. These plain Lepidopterans have a combination of a velcro hindwing coupling, threadlike antennae, and most species are nocturnal, all moth-like characteristics!
So, where does that leave us? Certainly, generalizations about butterfly characteristics hold true in enough cases that we can identify many of them in the field or laboratory. But, as of now, the only way we can definitively distinguish between butterflies and all other Lepidoptera are a few unique DNA sequences. Sigh, time to pull out those lab goggles.
But Lepidoptera specialists like David Bettman at the California Academy of Sciences are confident that one day unique physical characteristics will be found that distinguish the butterflies from all other Lepidoptera without the use of DNA. But these characteristics are likely to be subtle and only present in the larval or pupal stage, and will still likely require a microscope and some expertise to see!
And so a seemingly simple question takes us down quite the rabbit hole of taxonomic quandaries! Boundaries between groups of species, or even between individual species, aren’t always clear cut. But reconstructing how these amazing insects came to be is still ongoing and we are getting closer to understanding their evolutionary story, how butterflies got so dang fabulous, and maybe even more importantly, that moths have been fabulous all along.
In-depth discussion of the latest research on Lepidoptera evolution can be found here.