Did You Know?
Butterflies and moths have much more in common than in contrast. Evolutionarily, they are very closely related, largely feed on the same foods, and both have wings made of modified hair in the form of scales.
Butterflies and moths are members of the order Lepidoptera. The classification of the order Lepidoptera is not subject to scrutiny, but the classification within the order is still sketchy. The main problem for Lepidoptera classification is that butterflies are an evolutionary subgroup of moths.
This means that though butterflies can be classified as a monophyletic suborder―Rhopalocera is the accepted convention―moths can't be grouped as a lineage without including butterflies.
So, moths and butterflies are still classified only vaguely. To add to it, the physical markers often used to distinguish between the two, have prominent exceptions. The question of differentiating between moths and butterflies stands on shaky grounds, at best.
However, physical characteristics can indeed be used to distinguish moths from butterflies, albeit with exceptions, since the majority of one class usually share a characteristic unseen in the other.
Differences Between Moths and Butterflies
The butterfly family, Rhopalocera, is actually named from the ancient Greek words rhopalon and ceras/caeras, meaning 'club' and 'horn' respectively. The antennae of butterflies are slender and virtually always end in a curled ball, called a club. Moth antennae, on the other hand, always lack the club at the end, and are often feathery or hairy in appearance.
This is a very subjective marker, but effective nonetheless. Moths have stout bodies, and their undersides are often hairy.
However, there are notable exceptions to the latter, since many moths, saturniid moths in particular, can be vividly colored and extremely appealing. In this case though, the public perception about butterflies and moths is usually correct.
Method of Metamorphosis
The caterpillars then feed for several days/weeks, before transforming into an adult butterfly or moth. This transformation is called metamorphosis. Butterfly larvae achieve metamorphosis through the stage of a chrysalis, whereas moth larvae need a cocoon.
The difference between the two is that a chrysalis is a new layer of skin, whereas a cocoon is an external structure. When a butterfly larva is ready to pupate, it attaches itself to a tree/leaf, and sheds its skin, revealing the hard chrysalis underneath.
As a rule of thumb, though, if you see it on a warm spring afternoon, it's almost certainly a butterfly. If it charges at your lantern at night, it's almost certainly a moth.
Most moths have a unique anatomical feature called a frenulum. They have an arrangement that allows them to lock their front and rear wings. It consists of a hook on the rear wing that attaches to the front wing in a manner similar to Velcro. Not all moths have this locking system, but butterflies don't have it at all.
Distinguishing between butterflies and moths is best done on a trial-and-error basis. The antennae are, without doubt, the most prominent distinguishing factor, but easily observable superficial indicators such as the resting position and coloration are, more often than not, an accurate guide.
More importantly though, moths outnumber butterflies by such a huge margin (almost 90% of the order Lepidoptera is made up of moths) that it is statistically much more likely that the creature you encounter would be a moth rather than a butterfly.