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Kingdoms are subdivided into units or taxa (singular taxon) such as class, family, genus (Table 2.1). There are no rules about how many species constitute a genus, how many orders constitute a class, or whether families are divided into subfamilies. However, it is essential that a ‘taxon’ forms a natural grouping. Consequently, research, especially molecular phylogeny, causes taxonomists to re‐arrange the hierarchy of individual species and groups of organisms on a regular basis. A class, family or any other category within one group of organisms is therefore not evolutionarily comparable with those in another group.

Table 2.1 The taxonomic hierarchy with specific reference to the sheep nasal bot fly Oestrus ovis.

Taxonomic division	Taxon name	Common name
Kingdom Subkingdom Branch Infrakingdom Phylum Subphylum Class Infraclass Division Subdivision Superorder Order Suborder Superfamily Family Subfamily Genus Subgenus Species Subspecies	Animalia Bilateria Protostomia Ecdysozoa Arthropoda Hexapoda Insecta Pterygota Neoptera Endopterygota Panorpita Diptera Cyclorrhapha Oestroidea Oestridae Oestrinae Oestrus ovis, Linnaeus, 1758	Animals Moulting invertebrates Insects and related species Insects Winged insects True flies Higher flies

Not all taxonomists agree on the appropriate division for a grouping (taxon). For example, some workers consider there to be two suborders of Diptera: the Nematocera and the Brachycera and that the term Cyclorrhapha should be considered a division of the Brachycera rather than a suborder. Note that only the genus name and lower taxonomic descriptors are placed in italics.

The International Commission on Zoological Nomenclature (ICZN) provides rules on legal aspects of nomenclature (e.g., precedence). However, it is not unusual for workers to continue using old names that have been superseded or to fail to agree on an accepted single name. For example, the blowflies known as Lucilia cuprina and Lucilia sericata within the United Kingdom and Europe are often called Phaenicia cuprina and Phaenicia sericata by workers in the United States.