How to tell a moth from a butterfly

Some people just don't like bugs. Even those people, however, often have a favorable view of butterflies. The colorful patterns of their wings make them like little abstract paintings on the wing, a mobile art gallery in the garden, as it were. In my yard, fortunately, there are a number of different butterflies and colorful moths.

The first step to identifying one of these insects, is to determine whether it is a butterfly or a moth. A good close-up view of the antenna can usually put this question to rest. Although there are some exceptions to the rule, the antennae of butterflies are structurally different than those of most moths.

The antenna of a moth, upon close inspection, is feathery in structure with a main "stem" and smaller little "branches" all along its length. The two photos below show the difference. Note that the butterfly antenna as shown in the top photo almost always has a thickening at the end, almost like a club.

Antenna of the Eyed Brown Butterfly with club-like end
Photo by Brad Sylvester, copyright 2011, all rights reserved

Antenna of the Polyphemus moth showing feathery branches
Photo by Brad Sylvester, copyright 2011, all rights reserved

Another way to tell the difference between a moth and a butterfly is the cocoon or chrysalis. Most all moths spin a thread cocoon. This cocoon will usually be on the ground or attached firmly to a solid object such as the underside of the eaves of your house. Butterflies, on the other hand, tend to form a chrysalis which has smooth walls and is usually hanging in the air from a branch. Of course, by the time you see it flying around, it's too late for this test.

Generally. moths are active at night, but are occasionally seen moving about during daylight hours (at least they are in my yard). Butterflies are active during the daytime. This helps, but is really not reliable enough of a clue to make the determination between moth and butterfly on its own.

Another clue is the way the insect holds its wings when at rest. Butterflies wings stay extended and may be upright and together, sticking straight out to the sides or slowly moving back and forth between the two positions. Some of the larger moths do this too, but many moths at rest have swept-back wings that fold back along their body.

Moths and butterflies have two sets of wings, the forewings and the hindwings. To fly effectively, the forewing and the hindwing need to be held together. In many moths, there's a kind of tiny row of hook-like structures on the hindwing that catch and hold bristles from the forewing. In butterflies, however, usually just have a lobe of the hindwing that overlaps the forewing. However, it can be very hard to see this unless you are handling the butterfly or moth which risks damaging them.

Another clue, but not a sure thing, is the color of the wings. Moths tend to be nocturnal and hide during the daytime, so most often, their wings are a drab brown, gray, white, or other color that allows them to more easily blend into their surroundings. Butterflies, on the other hand, tend to be bright and visually distinctive, using their coloration and intricate patterns to self-identify their species to prospective mates. Their wing patterns may also serve to deter potential predators, sometimes exhibiting large false eye spots that make their wings look a bit like a large predatory face.

Butterflies often, but not always, have long slender bodies, while many moths have short, more bulbous bodies. Again, though, this is not always a reliable indicator.

Although each individual clue may be less than 100% reliable, when you start stacking them together and all of them point to either moth or butterfly, then their value increases. The antenna difference is the easiest and most reliable differentiator between a moth and a butterfly, although, as I mentioned there are some exceptions to the rule.

Once you have it narrowed down to moth or butterfly, you at least know where to start looking for a specific species identification