Styx has been referred to as both a "missing link" and a "living fossil" due to its indeterminate taxonomic nature and unique morphological characteristics.

[2] Its wings are narrow, with short antennae, and it has a stout black-colored body which resembles to a Geometrid or Lymantriid moth.

[6] The females have centrally clustered trichoid sensillae on the foretarsus, a feature shared with the Old World butterfly genus Laxita.

Akin to other Nemeobiines, the larvae feed on members of the family Primulaceae, in this case the plant genus Myrsine.

[6] Styx infernalis has had a fraught taxonomic history, being reclassified within four separate butterfly families; Pieridae,[11] Erycinidae,[12] Lycaenidae,[6] and Riodinidae,[6] following its initial description by Staudinger in 1875.

[13] Structural differences with regards to the thorax, legs, wing venation, and eggs warranted reclassification out of the family Pieridae.

[7] It was formerly considered as part of the subfamily Styginae, within the Lycaenidae, with the genus Styx its sole member by Ehrlich (1958) and Scott (1985).

Robbins (1988) used a scanning electron microscope to highlight morphological differences with regards to the forelimbs to corroborate upon Harvey's proposed placement.

[4] Within the family, Styx was initially classified within the subfamily Euselasiinae, tribe Corrachiini due to biogeographical reasons rather than morphological means.

[3] Vane-Wright corroborates upon this idea, describing the butterfly as a "living fossil" akin to the Papilionid Baronia brevicornis of Mexico.

S. infernalis inhabits the tropical montane cloud forests or the Peruvian Yungas of central and southern Peru.

[3] Otto Staudinger's original rationale for naming the genus Styx was because that the butterfly "[seemed] to have come from the underworld" rather than resembling the colorful tropical vegetation that had surrounded it.