The Trinomial System of Zoological Nomenclature

Formal categories

Genus
Used as a group for the most closely-related species.

Species
For the majority of distinguishable insect taxa a binomial system of formal names is applied. The species (abbreviated sp. or plural spp.) name is always accompanied by the genus in documents. Sibling species are morphologically similar species that have been shown to represent distinct, reproductively-isolated taxa. Their formal description is often avoided due to the difficulty of separating preserved specimens from other similar species.

Subspecies
The trinomial system may be applied to geographically distinguishable populations of a species, when subspecies (abbreviated ssp.) are described. A subspecific name is added after the species name (+ author's name for the new subspecies name when first used). The original species is referred to as the nominotypical subspecies. The species name is repeated to indicate a trinomial status followed by that author's name.

The differences between insect species can sometimes only be observed using a
microscope.

Informal categories

Clines
A cline is a gradient in a measurable character within a species. Sometimes populations within a cline are given subspecific, trinomial names. The term is most commonly applied to geographical variation when populations interbreed with neighbouring populations and show a gradual change, rather than an abrupt one.

Biotypes
Biotypes have no formal taxonomic status but they may be significant in biological control studies. The term refers to populations of a species which although morphologically indistinguishable, exhibit different biological characteristics. Quite often subsequent detailed studies reveal morphological differences in populations which enable their description as a separate species or subspecies.

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