Easy peasy, I hear you say. But your assumptions might be wrong - one might be brightly coloured, the other a master of disguise, a dull beige or brown.
Although butterflies and moths share the same classification Lepidoptera, take the time to look at the Chocolate Pansy Butterfly Junonia iphita, for instance, dull and leaf-coloured, or the Harnessed Moth Apantesis phalerata, with extravagantly coloured pinky yellow wings, and you might be surprised.
There are several differences that set them apart, and once you know them, you will find identifying them easy.
Time of day
This is obvious - butterflies are active during the day and moths have evolved to be nocturnal, travelling by the light of the moon. This is a terrible tale of fatal attraction - moths are attracted to light, and it’s a familiar sight – they gather around your outside lights, bewitched by the light but confused by the proliferation of artificial ‘moons’ and in the process of their confusion, get eaten by predators or simply overheat. Though scientists say there is still research to be done to fully understand moth behaviour, they do know that lamps have thrown a spanner into the moth’s evolutionary programming. It seems they’re not the brightest bulbs in the box, so to speak, but what moth evolution couldn’t anticipate was the spread of 24/7 electric light in our modern world.
Wings – open or closed?
Firstly, wings when resting – moths rest with their wings open, whereas normally butterflies fold theirs up, but just to confuse you, butterflies sometimes do something called ‘sun-baking’. They are ‘ectotherms’, which means they rely on external sources for body heat, so open up their wings to warm their body temperatures up to enable them to fly - so open or closed wings isn’t a sure way to tell which is which.
Moths’ antennae tend to be leaf or feather shaped, whereas butterflies’ ones are long and bare with a bulbous end. Even though this is normally the case, there are some moths that have slender antennae rather than feathered, such as the Zodiac Moth, however the antennae on these moths tend to be straight rather than curved.
Getting a bit more technical here - moths have a frenulum, which is a wing coupling device that ensures the front and back wings travel together during flight. Butterflies do not have this.
Pupae – cocoon v pupa
In the metamorphism from caterpillar to butterfly or moth, caterpillars spin a pupa made of hardened protein, but moth caterpillars form a silky pupa, which is known as a cocoon. Again, despite this being a generalised rule, there are some exceptions - examples include Gypsy Moths that form a pupae that resembles a butterfly pupa and the Parnassius Butterfly that make a cocoon.
Butterflies have two eyes just like we do. But butterfly eyes are compound eyes because they have multiple lenses. That means butterflies can see many different things in many directions all at the same time. The two species have adapted to have different eyes to suit their light conditions. In general, moths have two different types of eyes. While compound eyes provide a good area of view but only a low acuity, simple eyes can detect motion and colours with more details. Also, moths’ eyes have advanced contrast sensitivity, which is essential for moving around at night.
Widely speaking, moths tend to be stockier and have furry bodies, whereas butterflies are smooth and lean. This difference in appearance is due to scale size - butterflies possess far smaller scales. And the scales? Have you ever touched the wings of a moth or butterfly and got some ‘powder’ on your fingers? That powder is actually made up of tiny scales, like on a fish. These scales give butterflies and moths their scientific name Lepidoptera (Lepido = scale, and ptera = wing).
Camille Pissarro, a famous artist, once said: ‘Don't be afraid in nature: one must be bold, at the risk of having been deceived and making mistakes’. I guess there are a lot of bold moths out there that haven’t learned from their mistakes!