Nomenclature and classification of insects
Why classify and what is the use of classifying ?
Insect classification, and thereby entomological nomenclature and more particularly insect scientific names have undergone many reorganisations and modifications over the last decades. The general public is not familiar with scientific nomenclature, whether zoological or botanical. Moreover, their notion of what a species is or represents is quite vague. o name an animal or a plant species, people generally use the words « a kind of », a sort of », « a variety », « a race ».
Such approximate and therefore imprecise language highlights how difficult it is for the public to name or apprehend some “thing”, whether animal or plant, that more or less looks like some “thing” else. For scientists, the word “species” has a well-defined meaning: it is the basic unit (also called taxon) of systematic classification.Although the concept of “species” is currently interpreted in different ways by the scientific community, its main feature is inter-fecundity, i.e. the capacity for individuals belonging to a same population to interbreed and give birth to viable, fecund offspring in natural conditions.
Accuracy and rigour are needed more than ever. They allow traceability, language uniformity and universal accessibility when family, genus, species and sub-species names are expressed in scientific terms. Besides, scientific terms give access to targeted, accurate bibliographical research data that would not be accessible with only vague species indications. Naming organisms accurately (and only scientific names are accurate) even in the agricultural sector, is an indispensable and unavoidable prerequisite of any research, any experimentation and any study, in laboratory or on site.
For a better reading of this text, we thought it would be useful to give a reminder of some essential rules that apply to insect nomenclature and classification. The rules also apply to other organisms than insects.
What is classification?
The classification of organisms relies on a hierarchical, pyramid-structured system that consists in creating groups and sub-groups that constitute taxonomical categories: classes, orders, families, genera, species, sub-species... (Table 1).
Each group or sub-group gathers together animals or plants (insects in our case) that possess common, usually morphological, features. If we move toward the top of the pyramid, the degree of resemblance between groups decreases (for example, in the Arthropod branch, animals that do not display much resemblance with one another are found, such as crabs, spiders, insects and centipedes). Conversely, if we move toward the base of the pyramid, resemblance increases (for example, within the Hexapod (insect) class, the Diptera (flies) and Hymenoptera (bees) orders display higher resemblance with each other). Table 1 :
|POSITION SYSTÉMATIQUE DE LA MOUCHE DOMESTIQUE : Musca (Musca) domestica Linnaeus, 1758|
|Genus :||Musca Linnaeus, 1758|
|Sub-genus :||Musca Zanthiev, 1967|
|Species :||domestica Linnaeus, 1758|
Musca domestica domestica Linnaeus, 1758
Musca domestica calleva Walker, 1849
Musca domestica curviforceps Sacca & Rivosecchi, 1957
Within a same genus, it is sometimes possible to differentiate (morphologically, and sometimes geographically or biologically) between one or several groups of species. The status of sub-genus is attributed to each of those groups when scientifically relevant.
The species that belong to sub-genera have therefore a certain number of proper or similar features in common. For example, within the large Otiorhynchus (Coleoptera Curculionidae) genus, which comprises more than 1,000 species and sub-species in Europe, 78 sub-genera have been defined.
By convention, the sub-genus name is placed in-between the genus and species names, between brackets, which results in the following writing: Otiorhynchus (Dorymerus) sulcatus (Fabricius, 1775) or Otiorhynchus (Cryphiphorus) ligustici (Linnaeus, 1758). The species O. meridionalis was not classified into any particular sub-genus, so by definition, it belongs to the Otiorhynchus sub-genus (sensu stricto) and is named as follows: Otiorhynchus (Otiorhynchus) meridionalis Gyllenhal, 1834.
A group of individuals belonging to a given species can find itself isolated geographically, biologically or ecologically. After a certain time, that group has acquired proper features (from a genetic, morphological, biological, chromatic… point of view) that differentiate it from the nominal species it belongs to. Then all the individuals in that group are considered as belonging to a sub-species that differs from the nominal species. There is no reproductive barrier between a sub-species and the nominal species within a same taxon; as a result, they can cross-breed. For example, the Western corn rootworm Diabrotica virgifera LeConte, 1858, has a sub-species called Diabrotica virgifera zeae Krysan & Smith, 1980. The nominal species Diabrotica virgifera virgifera and the sub-spieces Diabrotica virgifera zeae are not listed in the same annexes on quarantine lists, and only Diabrotica virgifera virgifera was anthropogenically introduced in Europe, whereas the sub-species zeae was not. This shows how important accuracy is regarding that matter. When a taxon contains one or several sub-species and one does not refer to a sub-species in particular, the reference sub-species (or “type sub-species”) is designated using the last term of the species name twice, which yields the following writing : Diabrotica virgifera virgifera LeConte, 1858.
Considering insect numbers (presently more than one million known valid species) and their morphological diversity, one can easily expect insect classification to be complex, and it is complex indeed! We do not aim to get into the labyrinth of entomological systematic, knowing that it is constantly being revised at all levels according to the new insights provided by molecular biology, amongst others. Certain classification categories have been given conventional name endings (Table 2). Thus all super-families end in –oidea (e.g. Bostrichoidea is the super-family Anobiidae belong to).amily names end in –idae (e.g. Anobiidae, Dermestidae, Cerambycidae, Curculionidae, …). There also exists conventional endings for other taxonomical levels (sub-family, tribe), but they are rarely used in practise, so they are not used in this site. Table 2 :
|Conventional nominal endings (eg. the housefly)|
|Genus :||Musca Linnaeus, 1758|
|Species :||domestica (Linnaeus, 1758)|
Let us speak and write the right language : the advantages of a universal classification
Several hundred more pages are added to the “Good Book” of insect knowledge each day, and more than 3,200,000 works have been published about these arthropods since the beginning of the 18th century. In order to name species, it is therefore suitable to have a universal language as accurate as possible and also taking into account advances in systematic. The few lines below provide the reader with information about the basics of zoological nomenclature.
Species name writing :
Insect names have to follow the strict requirements of the International Code of Zoological Nomenclature. Lets us recall that the basic classification unit is the species.The International Code, or Linnaean system, was established by the famous Swedish entomologist Carl Linnaeus, who established that animal or plant species were to have two Latin- or Greek-derived names : a genus name, followed by a species name.The two names are completed by the name (or names) of the person who described the species (author, discoverer or inventor). Thus, the housefly is named scientifically by the following full name : Musca (genus group) domestica (species group) Linnaeus, 1758 (author, descriptor, discoverer, inventor, followed by the date of the original description).To name the housefly scientifically, one therefore writes Musca domestica Linnaeus, 1758.
The name of each taxon is permanently tagged to a “nomenclature type” (holotype, paratype,…), which is the equivalent of a reference or of a standard metre in a way, and is made of one or several specimens deposited in a public (museum) or private collection. By convention, genus, species and sub-species names are written in italics, and authors’ names in Roman font. Only the names Linnaeus (or Linné in French) and Fabricius can be abbreviated using the letters L. and F., respectively, but it is preferable to write the names in full as we do in this text. When one refers to an undetermined (unidentified) species whose genus is known, sp. (singular) or spp. (plural) abbreviations are commonly used right after the genus name. In the same way, “sub-species” is abbreviated ssp. in the singular and sspp. in the plural. Unlike genus and species names, these abbreviations have to be written in Roman font.
The use of brackets in modified combinations. NB : unchanged combinations do not contain brackets
If the name of a species group is combined to the name of a genus group other than the original generic name, the name of the author of the species group has to be written between brackets. Let us consider the example of the American cockroach Periplaneta americana : the species was described in 1758 by Linnaeus under the name of Blatta americana. Later, the species was rightly classified in the Periplaneta genus. The name of the person who first described the species is written between brackets, and the proper writing of that cockroach’s name reads Blatta americana (Linnaeus, 1758). Genus and species names can change over time, once or several times, especially at the genus level.
Such was the case for tens of thousands of insect species, like the drugstore beetle (Stegobium paniceum) for example. The species was originally described by Linnaeus under the name of Dermestes paniceum in 1758. At that time, many Coleoptera Dermestidae, and even those now belonging to other families, were described within the Dermestes genus. The research studies in systematics carried out later showed that the Dermestes genus grouped together species that deserved to be transferred into other pre-existing or new families or genera. That is how Dermestes paniceum Linnaeus, 1758 first became Anobium (Artobium) paniceum (Linnaeus, 1758), then Sitodrepa panicea (Linnaeus, 1758) and finally Stegobium paniceum (Linnaeus, 1758) (current genus name and valid current name of the species).
As mentioned above and in order to avoid possible confusion, in all published documents it is recommended or compulsory, depending on the case, that the two genus/species names be followed by the name of the person who first described the species (also called the author, the discoverer, or the inventor); besides, depending on the contents of the document in which the name is written, it is followed (or not) by the year when the species was first described. Let us recall that as a rule scientific genus and species names are written in italics. The first letter of the genus name is always in block-case, whereas the first letter of the species name is always in lower case.Moreover, scientific names, ranging from the sub-species level to the order level and beyond, are never written with accents or dieresis or other spelling attributes.
In zoology, the word synonym is used when more than one scientific name applies to the same taxon (species) .
Explication : All the names in that taxon, valid name included, are synonyms. The reason is as follows :
The case used to be quite frequent, and for dissemination and publication accessibility reasons, it concerns hundreds of thousands of species ; thus, some species have more than 140 synonyms. We can better understand why nearly 3 million insect species have been described, while hardly one million of them are actually valid species, also called “good species”.
The scientific community, i.e. specialists in this case, is in charge of classifying species as synonyms when justified. In that case, priority rule is used. As a rule, only the first available name, i.e. the oldest name (the senior synonym), is acknowledged as the official scientific name (val. name), while the other, i.e. the more recent synonym (the junior synonym) is considered as an invalid name, and the species that bears that name is invalid or non-valid. For example, Anobium punctatum (De Geer 1774) = the senior synonym, so the name is valid ; Anobium striatum Olivier 1790 = a junior synonym, so the name is invalid : Nom invalide The Code of Zoological Nomenclature mentions many other cases and various conservation measures that apply to nomenclature synonyms, but we do not find it necessary to give a reminder of them here.
Following the same principle and for the same reasons as for species, those synonyms can affect families and most often genera. For example, the genus name Ptinus is a junior synonym of the name Anobium. Thus, the common furniture beetle (Coleoptera, Anobiidae) used to be known as Ptinus punctatum, but it is now named Anobium punctatum. Although a lot of species have taxonomic synonyms, some do not have any. Moreover, some species synonyms have been used in applied entomology writings, whereas others have never been.
For each valid species, we only indicated the synonyms used at least once in agricultural or applied entomology publications. During bibliographical searches, we recommend searching the references of each species using not only the valid name but also the junior (genus or species) synonyms. For example, in the case of a bibliographical search about the common furniture beetle (Anobium punctatum), in order to carry out an exhaustive search, the suitable procedure will consist in looking for references using the following name combinations : Anobium striatum, Birrhus domesticus, Ptinus punctatum, Anobium ruficolle, Anobium ruficorne.
NB : families can sometimes shift in taxonomical position. For example, Ptinidae are now part of the Anobiidae family, as a sub-family.
Vernacular or common names :
The vernacular name is the common name generally used to designate an animal or a plant. While there exists only one valid scientific name for each taxon (“genus species”), a same taxon can have several vernacular names (also termed common name, vulgar name or usage name). The vernacular name is sometimes the word-for-word translation of the scientific name (1*), but most often it is quite different (2*).
Vernacular names usually apply to the most common and most easily recognisable species (daytime butterflies, large coleoptera…) and to many species of agronomic or economic interest. However, it should be known that in France, most insects do not have a vernacular name, and vernacular names are often quite vague and do not allow their users to tell accurately between species. Sometimes a same vernacular name designates all the species in a family or in a genus (3*).
Common names are more or less precise depending on countries. In Canada, for example, vernacular names are widely used by agronomist and foresters, and are generally precise enough. In many places (particularly in tropical regions) vernacular names are the only names that local populations can use to designate plants but also the most common insects
The vernacular names mentioned here are taken from agronomical and applied literature. The names preceded by an asterisk are those we found most commonly used. Of course, the scientific name has to be chosen as the key-word for all precise bibliographical research.
(1*) Lyctus brunneus and Lyctus linearis designate the powder post beetle and the European Lyctus beetle (Coleoptera Lyctidae), Blatta orientalis designates the oriental cockroach (Dictyoptera Blattidae), Necrobia rufipes designates the copra beetle (Coleoptera Cleridae).
(2*) The lesser grain borer (Coleoptera Bostrychidae) is named Rhyzopertha dominica (Fabricius, 1792).Le Bostryche ou capucin des grains (Coleoptera Bostrychidae) se nomme Rhyzopertha dominica (Fabricius, 1792).The furniture beetle or woodworm (Coleoptera Anobiidae) is named Oligomerus ptilinoides (Wollaston, 1854)
(3*) The name "woodworm" designates all the species of the Anobiidae family and the common name Dermeste designates a lot of Coleoptera of the Dermestitidae family, notably those belonging to the genera Dermestes, Attagenus and Anthrenus.