The branch of natural science concerned with the correct placement and hence naming of plants and animals is taxonomy – "the sullen art of taxonomy", as Jonathan Weiner called it. The theoretical underpinning is systematics. The systematic units of taxonomy are called taxa or taxons.
Lepidoptery, or lepidopterology, is the study of Lepidoptera (the singular is Lepidopteron), that is, 'those with scaled wings' because their wings are covered with little delicate shingles or scales. According to a count by John B. Heppner of the Florida State Collection of Arthropods, 146,565 species of Lepidoptera have been described. He estimates that their total number will be around 255,000, so 43 percent are still awaiting discovery. They form the second largest order within the class of insects – the study of which is entomology. Lepidopterology thus is a subdivision of entomology, entomology in its turn of course being a subdivision of zoology which is a division of biology.
It is rather a field apart, though. As biology has become bioscience, the focus shifted from inventorying nature to explaining it at the cellular and molecular levels. The image of the entomologist sneaking across alpine meadows to hunt down his objects of study with a tartan net and a glass jar (an image Nabokov relished) has become even more anachronistic than it was in Nabokov's day. When funding can be procured at all, it is for systematic survey collecting. With mounting concerns about diminishing biodiversity, however, science seems slowly to realize that it is a pity so few experts are left who know what species fly out there and what kind of life they lead. At an alarming rate, many are going extinct without ever having been recorded.
Lepidoptera usually are considered to consist of two large subgroups. At first the distinction looks slight and arbitrary:
(1) Rhopalocera, that is, 'those with antennae in the form of clubs,' the great majority of them them flying by daylight. When resting, most of them fold their wings on their back.
(2) Heterocera, that is, 'those with differently shaped antennae,' a much larger group. In many languages, the Heterocera are equated with night-flying Lepidoptera, as opposed to the day-flying Rhopalocera. That, however, is not quite correct. Many of them fly by day or at dusk. For those that do fly by night, colors are of little use, so they often are dully colored. But the most reliable way to tell moths apart from butterflies is not by their nocturnal activity nor by their ensuing less colorful overall appearance (there are gaudy moths and inconspicuous butterflies) but by the shape of their antennae. All those that don't have little clubs at the ends of the antennae are likely to be moths. Moths usually rest with their wings outstretched, not folded.
Another distinction is that between large and small Lepidoptera, Macrolepidoptera and Microlepidoptera. The vast realm of Microlepidoptera usually includes only the very tiny ones, with wingspans down to four millimeters. The upper size limit is arbitrarily drawn at families like the Tineidae (15 mm) or Psychidae (20 mm). However, in many of them there are a number of species that are just as large as small or average Macrolepidoptera. For instance, there are many Microlepidoptera which are larger than the Pygmy Blue (Brephidium exile) whose wingspan is just 10 to 19 millimeters. (In spite of its dwarfish size and its weak flight, this little butterfly migrates from the warmer regions of the Americas, where it is based, as far north as Oregon.)
Even though these subdivisions seem to be of a rather precarious character, most catalogues and manuals adhere to them strictly. The immediate consequence is that much of the literature focuses on the showier species and excludes their unloved relatives, the predominantly night-flying, therefore inconspicuous and mostly small moths. It is difficult to come by handy information on moths, and extremely so when they are from some exotic fauna. The moth fauna of some regions like New Guinea still is more or less unknown.
The fundamental distinction between butterflies and moths seems to be based on a very minor and unimportant feature – that little club at the end of the antenna. However, it serves just as an easily recognizable marker for a whole array of differences that are grounded in evolution. Phylogenetically, moths are older than butterflies. They evolved much farther back in time. According to the fossil record, the first moths branched off the insect lineage some 200 million years ago, 235 million years after the appearance of insects, and all the important families of moths were around when the first butterfly appeared some 80 million years ago. The oldest butterfly fossils date from only 48 million years ago. Right at their inception, the modern group probably split into two lineages. One was that of the skippers (today considered a family of butterflies, Hesperiidae) which retained several moth-like features. The other line was that of the true butterflies, sometimes called scudders to distinguish them from the skippers.
Thus, in an evolutionary context, the distinction between moths and butterflies that looks so haphazard at first sight does make sense. So does the distinction between Microlepidoptera and Macrolepidoptera. Again, the Microlepidoptera are the older group. All families of the Microlepidoptera had evolved when the first Macrolepidoptera appeared some 115 million of years ago: the ancestors of the geometer moth, and the noctuid moths soon after. It is not some accidental descriptive feature that makes the difference, like time of flight, overall size or some minor anatomical characteristic. Unwittingly the traditional subdivisions were reflecting phylogeny, cutting the evolutionary tree at different points.
 John B. Heppner, "Classification of Lepidoptera, Part 1. Introduction," Holarctic Lepidoptera, 5 (Suppl. 1), 1998, p. 1–148