The species has two subspecies, Mellisuga minima minima (Linnaeus), which is found in Jamaica, and Mellisuga minima vieilloti (Shaw), which is found on Hispaniola. The subspecies differ subtly in coloration and potentially in song. Throughout this account both subspecies are collectively referred to informally as the Vervain, except where there are noteworthy differences between the subspecies, in which case the full scientific name is used.
The two subspecies appear to be morphologically distinct. Compared to M. m. minima, M. m. vieilloti males are darker (both dorsally and ventrally), and the female is darker ventrally (Ridgway 1911). Ridgway (1911) also claims M. m. vieilloti has longer wings and tail than M. m. minima, although this is not supported by his own data (table 1); if anything, M. m. vieilloti may be slightly smaller than M. m. minima, but, additional measurements of M. m. vieilloti, especially its body mass, would help clarify this (table 1).
The nominate race in Jamaica was originally named Trochilus minimus by Linnaeus (1758). The generic name Mellisuga was introduced by Brisson (1760). The Hispanolan subspecies Mellisuga minima vieilloti was described by Shaw (1812). Mellisuga minima has since had over a dozen names, mostly given by 19th century systemetists who overlooked (or ignored) the previous names assigned to the bird. These older names are summarized in Wetmore and Swales (1931) and Ridgway (1911).
The common name of the Vervain Hummingbird was given by Gosse (1847), who said: "I have ventured to give the [Vervain Hummingbird] a new appellation, derived from its habit of buzzing over the low herbaceous plants of pastures, which our other species [of hummingbird] do not. The West Indian vervain (Stachytarpha) is one of the most common weeds in neglected pastures, shooting up everywhere its slender columns, set around with blue flowers, to the height of a foot. About these our little Humming-bird is abundant during the summer months, probing the azure blossoms a few inches from the ground."
Phylogenetically, the Vervain lies within the ‘bee hummingbird’ clade (Bleiweiss 1998; McGuire et al. 2007), which is not to be confused with the Bee Hummingbird (Mellisuga helenae Lembeye). Bond (1950) hypothesized that the genus Mellisuga was related to the genus Archilochus, and molecular phylogenetic analyses appear to place Mellisuga minima minima sister to the genus Archilochus (McGuire pers. comm.). Notably, the phylogenetic analyses lack samples from the Bee Hummingbird, Mellisuga helenae. The Vervain Hummingbird was historically considered the sole member of the genus Mellisuga, with the Bee Hummingbird (Mellisuga helenae) instead placed in the genus Calypte (Peters 1945). Bond (1950) placed the Bee Hummingbird in the genus Mellisuga with little explanation; it appears the main characters supporting this move are tiny size and similarities in their song (Bond 1936; 1950). The relationship between Mellisuga minima, Mellisuga helenae, and the remaining 'bee' hummingbirds would therefore be worth analysis with modern phylogenetic methods.