Article 57. Species-group names.
57.1. Application of the Principle of Homonymy to names of species and subspecies. The Principle of Homonymy applies to species-group names that are or are deemed to be spelled identically [Art. 58] and are published originally or subsequently in combination with the same generic name [Art. 53.3], including names of collective groups and of ichnotaxa at genus-group level [Arts. 10.3 and 42.2.1].
57.2. Primary homonyms. Identical species-group names established for different nominal taxa when originally combined with the same generic name (see also Articles 220.127.116.11 and 57.8.1) are primary homonyms [Art. 53.3] and the junior name is permanently invalid (but see Article 23.9.5) except when:
57.2.1. its use as a valid name (a nomen protectum) is maintained under the conditions specified in Article 23.9, or
57.2.2. it is conserved by the Commission under Article 81, or
57.2.3. it, but not its senior homonym, is included in a relevant adopted Part of the List of Available Names in Zoology (see Article 79.4.3).
Examples. The following are primary homonyms: Culex affinis Stephens, 1825 and Culex affinis Adams, 1903; Lycaena argus nevadensis Oberthür, 1910 and Lycaena nevadensis Zullich, 1928; Aporia hippia transiens Alpheraky, 1897 and Aporia crataegi transiens Lempke, 1953.
57.3. Secondary homonyms.
57.3.1. Identical species-group names established for different nominal taxa and subsequently brought together in combination with the same generic name are secondary homonyms [Art. 53.3] and the junior is invalid (but see Article 57.8.1), but a junior secondary homonym may be reinstated under certain conditions [Art. 59.2-4].
Examples. The specific names in the names Frontina acroglossoides Townsend, 1891 and Eophrissopolia acroglossoides Townsend, 1926 become secondary homonyms when both species are placed in Chaetogaedia.
57.3.2. Identical species-group names established for different nominal taxa are secondary homonyms when one was originally combined with a junior generic homonym and the other was originally combined with a new replacement name (nomen novum) [Art. 60.1] for that generic homonym.
Example. Xus albus Smith, 1900 (where Xus is a junior homonym) became Xoides albus (Smith, 1900) when Xoides Dupont, 1909 was established to replace Xus. If a new species Xoides albus Jones, 1910 were proposed, the two specific names would be secondary homonyms.
57.4. Subgeneric name irrelevant. The presence of different subgeneric names placed in parentheses between the same generic name and identical species-group names is irrelevant to the homonymy between the names concerned.
Example. The specific names of Aus (Bus) intermedius Pavlov and Aus (Cus) intermedius Dupont were both originally established in the genus Aus, and so are primary homonyms. The specific name of Aus (Dus) intermedius (Nomura) was originally established in the genus Xus, and so is a secondary homonym of the species names of both Aus (Bus) intermedius and Aus (Cus) intermedius.
57.5. Difference in spelling of generic names. Identical species-group names (or species-group names deemed to be identical [Art. 58]) established for different nominal taxa are homonyms when combined with the same generic name (but see Article 57.8.1) even if the spelling of the generic name with which one or more of the species-group names is combined is an incorrect spelling or an emendation [Art. 18.104.22.168].
57.6. One-letter difference. Except as specified in Article 58, a one-letter difference between species-group names combined with the same generic name is sufficient to prevent homonymy.
57.7. Precedence of names of species over those of subspecies. Of two homonymous species-group names of identical date, one established for a species takes precedence over one established for a subspecies [Art. 24.1] or over one deemed to be of subspecific rank [Art. 45.6].
57.8.1. Homonymy between identical species-group names in combination (originally or subsequently) with homonymous generic names having the same spelling but established for different nominal genera [Art. 53.2] is to be disregarded.
Example. Noctua Linnaeus, 1758 (Insecta) and Noctua Gmelin, 1771 (Aves) are homonyms, but homonymy between variegata Jung, 1792 in Noctua (Insecta) and variegata Quoy & Gaimard, 1830 in Noctua (Aves) is disregarded.
57.8.2. For the reinstatement of junior secondary homonyms in certain circumstances, see Article 59.2 - 59.4.