If your really really interested here is the section from the International Code of Zoological Nomenclature:
31.2. Agreement in gender. A species-group name, if it is or ends in a Latin or latinized adjective or participle in the nominative singular, must agree in gender with the generic name with which it is at any time combined.
31.2.1. A species-group name that is a simple or compound noun (or noun phrase) in apposition need not agree in gender with the generic name with which it is combined (the original spelling is to be retained, with gender ending unchanged; see Article 34.2.1).
Examples. The specific name in Simia diana (Simia and diana both feminine) remains unchanged in Cercopithecus diana (Cercopithecus masculine); and the noun phrases in Melanoplus femurrubrum (Melanoplus masculine; but rubrum agreeing with femur, neuter) and Desmometopa m-nigrum (Desmometopa feminine; nigrum neuter, agreeing with m, because letters of the alphabet are neuter).
31.2.2. Where the author of a species-group name did not indicate whether he or she regarded it as a noun or as an adjective, and where it may be regarded as either and the evidence of usage is not decisive, it is to be treated as a noun in apposition to the name of its genus (the original spelling is to be retained, with gender ending unchanged; see Article 34.2.1).
Example. Species-group names ending in -fer and -ger may be either nouns in apposition, or adjectives in the masculine gender. Cephenemyia phobifer (Clark) has often been used as C. phobifera, but the original binomen was Oestrus phobifer; since Oestrus is masculine, phobifer in that binomen may be either a masculine adjective or a noun in apposition; hence it is to be treated as a noun in apposition and not changed when combined with the feminine generic name Cephenemyia.
31.2.3. If a species-group name (or, in the case of a compound species-group name, its final component word) is not a Latin or latinized word [Arts. 11.2, 26], it is to be treated as indeclinable for the purposes of this Article, and need not agree in gender with the generic name with which it is combined (the original spelling is to be retained, with ending unchanged; see Article 34.2.1).
Example. Species-group names such as melas, melaina, melan; polychloros, polychloron; celebrachys; nakpo (from the Tibetan word meaning black) remain unchanged when transferred from combination with a generic name of one gender to combination with one of another gender. But melaena is a latinized adjective (derived from the Greek melaina) and must be changed when so transferred, with an appropriate Latin gender ending (-us masculine, -um neuter).
from
http://www.iczn.org/iczn/index.jsp?article=69&nfv=
Best,
Chuck