Taxonomy is a science of its own, sometimes closer to law or history than to the biological sciences. Its main objective is to safeguard that every taxon has one and just one name, that it has its name all to itself, and that this name is formed in accordance with certain rules. If one and the same animal has been given several different names, they are called synonyms. If several different animals have been given the same name, they are homonyms. Synonyms and homonyms render any nomenclature ambiguous. Taxonomy thus must endeavor to rigorously avoid synonymity and homonymity. This sounds simple enough, but with hundred of thousands of animals being named for centuries all over the world, it is not simple at all.
Since 1901 there have been international regulations on nomenclature. There is an International Commission on Zoological Nomenclature (ICZN) that rules which names are admissible ("available") and which are not. In 1961, it has issued the International Code of Zoological Nomenclature, a set of rules which explicitly and in great detail govern what the scientific name of an animal must be like, down to the spelling of its Latin and Greek words. The purpose is "to promote stability and universality in the scientific names of animals". To achieve its aims, it had to do away with a lot of nomenclatorial rubbish that had accumulated in the two centuries following »Linnaeus. Counting almost 200 pages and 88 articles, the Code is guided by four main rules.
The most important one is the Law of Priority (Article 23): "The valid name of a taxon is the oldest available name applied to it". If the same taxon is inadvertently named more than once, the newer names are mere "junior synonyms" and have to be abandoned. So sometimes taxonomists have to dig deep into the literature to find out who published the very first Original Description (OD) of an animal, whether it was the same or different from one described by later authors and whether the description and the type it was based on really correspond.
The other important rule is the Law of Homonymy (Article 53): "Any name that is a junior homonym of an available name must be rejected and replaced". A homonym is one and the same name used more than once. The Code stipulates that no specific name may be used more than once within the same genus, and no generic name within the same kingdom. This means that no two genera of animals are allowed to have the same name but that a specific or subspecific name may be used over and over again as long as it does not happen within the same genus.
For this reason many different species and subspecies in different genera have the same name. For instance, in one sole volume (3) of Seitz's encyclopaedic work on the butterflies of the world there are as many as forty-nine species and subspecies that go by the name of pallida ('the pale one').
This sorry fact must persuade everyone to proceed with caution when trying to identify an insect mentioned in the literature under less than its full name. It is never sufficient to find a matching specific name. One has to make sure that it belongs to the right genus, and when the generic name has changed a few times or is not quite certain for other reasons, this often means a lot of searching. For instance, if the only information provided is the specific name snowi, it must be determined whether the insect in question is the Rusty Gully Skipper Ochlodes snowi or Snow's Lustrous Copper Lycaena cupreus snowi. In this case, the only further hint to go on is that snowi occurs in Wyoming and at a very high altitude. The locality seems to point toward the copper, the skipper usually occurring farther to the south. The altitude makes it a near certainty and quite definitely excludes the skipper.
Another rule of the Code (Article 19) says that a given name may not be altered even if it contains a spelling error. Any alteration is considered just a misspelling. So names, once published, may not be improved upon. If Dr. Holland derived a name from Russian Vice Admiral V.M. Golovnin and spelled it golovinus though the correct spelling would have been golovninus, it will have to stay that way.
Finally, there is the Law of Name-Bearing Types (Article 72). It rules that the name is assigned not to the description but to the specimen the description was based on. This is called the holotype. The holotype should be carefully preserved in some collection and must be available for inspection if the need arises. The specimen of the opposite sex deposited along with the holotype is called the allotype. If the original holotype for some reason is lost or destroyed, a new specimen has to be selected to serve in its place; it is called a neotype. The specimens deposited along with the holotype are called paratypes. Higher taxa also have their types. For each genus a type-species is selected that seems best to represent its characteristic features. If in the course of a revision the type-species is removed from a genus and placed in another one, the whole genus collapses and is no more.
Chosing the holotype or the type-species is an arbitrary act, for nobody is perfect, and the ideally typical organisms early systematicists believed in just do not exist. As in the course of time more gets known about a certain species or genus, the types selected may even turn out to be grossly atypical. This is just too bad, for in order not to add to the confusion, types are changed only most reluctantly. Being a bad example of what it is to typify is not enough – a change is accorded only when it has been shown that the type specimen has been misidentified by the author and really belongs to an entirely different taxon.
The problem, of course, is that no plant or animal ever comes with a tag saying "I am a such and such". In fact, nature does not present itself to its human members as neatly compartmentalized into taxa in any obvious and self-evident way. It is a jumble of subtly interrelated and intergrading forms, and it is forever changing; every system is only a snapshot made at a certain point in time.
Any two specimens of no matter what species will hardly ever by completely alike. Several kinds of differences have to be expected and subtracted.
First there are the individual differences acquired during a lifetime. One specimen may be young and fresh, the other old and faded or battered and torn. As a matter of fact, the nearly perfect specimens one sees in butterfly exhibits usually are fresh ones, mounted by skilled hands, and not typical of the average member of their species one would encounter on the wing. After a few weeks of adult life, most butterflies show the marks of narrow escapes and the general wear and tear of life.
Secondly, there are the individual differences based on genetic variation which tend to increase the higher up in the tree of evolution an organism is placed. Even if butterflies and moths are no virtuosos of genetic variation, there are some like the Great Tiger Moth (Arctia caja) where virtually every specimen has different wing markings.
Thirdly, there may be gender differences. In fact, sexual dimorphism is pronounced in many butterflies and moths. In several species males and females look so different that at first they were believed to belong to different species. This is sometimes acknowledged by their specific name, dispar, meaning 'the unequal one'. Examples of extreme sexual dimorphism may for example be found in the genus Lycia, a group of geometer moths whose females lack wings and cannot fly, giving them a most un-mothlike appearance.
Fourthly, in butterflies and moths that produce several broods during a season there may be seasonal variants. Thus the spring generation of the Map Butterfly Araschnia levana (called f. = form levana) has brown wings with black markings while the summer generation (called f. prorsa) has black wings with yellow spots and reddish lines.
Finally, there is a great deal of geographic variability due to differences in the biotope. Whole colonies or populations of a certain species may differ from another slightly, one being a little larger or more brightly colored or having its bands or eyespots displaced. Sometimes populations change in a certain geographical direction. This continuous variation of a species from colony to colony is called a cline. Clines occur when populations are not isolated completely and a certain gene flow is maintained between neighboring ones. Along a cline, a species or subspecies may intergrade into what is classified as another species or subspecies.
What the taxonomist needs is some feature, or bundle of features, that is very stable across the whole species but distinguishes it clearly from neighboring ones. In butterflies and moths, this usually is not the size and overall color pattern which the layman might suggest. Usually it displays considerable individual variation, and there are different species that on the surface look quite alike; for instance, the very point of the mimic is to look like its model. A more reliable feature often is the venation of the wings. The most telling feature in all insects, however, is the structure of the genitalia. Insects have no bones. Among their hardest body parts are the extremely well-defined little sclerotinal structures of their genitalia which serve to hook onto the opposite sex. They have to clasp so tight they can mate even in flight. Moreover, this coupling apparatus has to fit across the whole species and thus is likely to be less variable than other body parts, while there is no penalty for not fitting beyond the limits of the species, in terms of reproductive success. So Nabokov's occupation with butterfly genitalia certainly was not a personal whim but general practice in entomology. He introduced new names for the genitalic structures he examined under the microscope, practically all of them in his paper on South American blues (Lep9). "Humerulus, alula, bullula, mentum, rostellum, sagum, surculus, Bayard's angulation or Point, Chapman's Process – are terms I invented thinking them up as I went", he wrote in 1953.
The art of the taxonomist thus presupposes a twofold way of looking at an object of study. For one thing, he has to note the minutest differences until no specimen seems like the other any more. At the same time, he has to be on the lookout for similarities, to deduct all the observed or potential variability, weighing differences and similarities until he arrives in his mind at a sort of scheme on which to found a taxon. That's why it does not suffice to take just one specimen and then describe it. Descriptions of one sole specimen tend to be shaky. Wherever he can, the taxonomist has to study series of them.
If a lepidopterist has some very unusual specimen at hand he will of course be tempted to publish its description and name it, lest somebody else beat him to it. However, it may turn out that he had only been dealing with a freakish individual of some well-known species or with a sterile hybrid. This has happened to Nabokov once, in the case of 'Lysandra cormion.'
Sometimes it has happened that out of carelessness or lack of knowledge the types that were assigned do not coincide with the published description. In such cases disastrous consequences are bound to occur. This is what has befallen the genus Lycaeides Hübner. The early German naturalist Jacob »Hübner, in his Verzeichnis bekannter Schmetterlinge of 1819, misidentified the type-species he assigned to it, and the nomenclatorial confusion that ensued has not yet been quite overcome. The eminent British systematicist Francis »Hemming called it "the most complicated case of a nominal genus based upon a misidentified type-species to be found anywhere in the butterflies". Nabokov was working right in the middle of this muddle.
At least for the well explored butterfly faunas of Europe and North America, it was thought that the constant renaming due to expansion and reordering of the system would come more or less to a rest by the end of the twentieth century and that recent names would be ever more likely to be the definite ones. Instead, the end of the twentieth century has seen an acceleration of the renaming process. Between the English and the French edition of the Collins Field Guide to European butterflies (1997 and 1999, respectively), there have been scores of generic name changes. Many names have not even lasted two years. In the European butterfly fauna, more than 70 percent of the generic names have undergone some change within the last fifty years.
The efforts to get the genera systematically and phylogenetically right sometimes seem to obscure the original purpose of naming – to provide each species with a unique and stable appellation under which it can be easily and unequivocally recognized.
 Letter to Alexander B. Klots, in Nabokov's Butterflies, p. 498. Five of these nine terms have found their way into the Dictionary of Insect Morphology (eds. Lajos Zombori & Henrik Steinmann, Handbuch der Zoologie / Handbuch der Zoologie, iv.34, Berlin [de Gruyter] 1999. These are ...
● humerulus "the distal part or articulated piece of the angulately bent falx of some male Lepidoptera" (Lep8 109; Lep9 9, 11, 16, 17, 18, 19, 23, 26, 29, 37, 41, 42, 48, 49; Lep13 273)
● alula "the bilateral auricular lobes of the manica in some groups of male Lepidoptera" or "the out-turned flaps of subzonal sheaths" of the aedagus = penis, according to Nabokov, Lep9 48 (Lep9 8, 11, 12, 15, 21, 28, 39, 48)
● bullula "the small vesicular swelling with membranous wall lying between the rostellum and the mentum of the valve in some groups of Lepidoptera, especially those of Lycaenidae" (Lep9 13, 21, 28, 34, 40, 52)
● rostellum "the ventrally curved chitinized keel on the lower edge of the penis in some Lepidoptera" (Lep9 9, 11, 13, 16, 21, 22, 23, 25, 28, 29, 31, 33, 34, 37, 49; Lep14 482)
● sagum "the paired, mantle-like membranes covering the phallus of Lycaenidae" (Lep9 11, 15, 17, 18, 19, 21, 25, 28, 29, 30, 34, 35, 36, 39, 45, 50, 51; Lep13 273)
The term mentum for the ‘chin' of an insect was in use long before Nabokov. He used the word to denote a structure in the male genitalic armature close to the rostellum (Lep9 9, 13, 21, 23, 26, 34, 52). In this sense, it has not made it into the dictionary. Chapman's process is in Lep9 8, 12, 15, 48; surculus in Lep9 21, 23, 25, 27; Bayard's point in Lep9 33; Bayard's angulation in Lep9 9 52.