The male of Thereus oppia is distinguished from Thereus lomalarga and Thereus brocki by lacking a scent pad on the dorsal forewing (Fig. 3) and from Thereus orasus by having a darker gray/brown ventral ground color (Fig. 3). Both sexes differ from the other members of the species complex by lacking dark scaling along the basal edge of the postmedian line on the ventral wings (Fig. 3). Thereus oppia has small teeth on the ventral tip of the penis (Fig. 17), in contrast to Thereus brocki and Thereus orasus.

We examined a syntype of this species in the BMNH.

Thereus oppia occurs from Mexico to Costa Rica at a variety of elevations. Most localities where it occurs appear to be deciduous dry forest. It is allopatric with its sister species, Thereus brocki.

Territorial male behavior was observed at Ciudad Valles, SLP, Mexico in the early afternoon (vouchers in RCB), in contrast to the early morning territorial behavior of Thereus lomalarga.

Adults of Thereus oppia were found nectaring on Cordia (Boraginaceae) flowers at two localities in Veracruz, Mexico (vouchers in RCB).

From Janzen and Hallwachs (2015), a pupa was found 30 April 1993 on Struthanthus orbicularis (Kunth) Blume (Loranthaceae) at Sendero Carobonal, Santa Rosa, Area de Conservación Guanacaste, Guanacaste, Costa Rica, latitude 10.77594, longitude -85.65799. An adult male (voucher 93-SRNP-30, deposited USNM) emerged 16 May 1993. As an associated comment on the web site, “red-brown pupa with white markings laterally so that it looks just like a bird turd, sitting on the top of a mistletoe leaf in middle of large plant (this species is a sprawler, vine/shrub); host tree was leafless.” The leafless host tree is the reason that we consider the mistletoe plant on which the pupa was found to be the caterpillar food plant.