Diagnosis.--Key characters: Ommatidial size coarse (Fig. 39b); prosternal process planar, very thin, forming a vertical plane between narrowly separated procoxae; apex not or barely expanded (Fig. 14); acute lateral pronotal tubercles absent; antennae with mesal spines (Figs. 8b, 9b); antennal carinae present (Fig. 45b); tibial carinae present (Fig. 22); pronotum round, wider than long (Fig. 41e); elytra and pronotum with uniform covering of short, dense velvety vestiture; each elytron with two longitudinal depressions for nearly their entire length; very long flying hairs covering appendages; femoral apices rounded (Fig. 26); profemora clavate (Fig. 37b); meso- and metafemora clavate and pedunculate (Fig. 37a); elytral apices weakly bispinose. Length: 9-11 mm.
Distribution and Diversity.--Bolivia, Brazil, México to Central America. Monotypic.
Discussion.--Terpnissa fell as a basal member of the Mallocera and allies clade (Fig. 59) based on the implied weighting phylogenetic analysis. The pubescence of Terpnissa differs from the Mallocera and allies clade primarily due to its uniformity in distribution and color. In Appula, Mallocera, Protomallocera, and Pseudomallocera, the pubescence varies in density and coloration. In Terpnissa the meso- and metafemora are clavate and pedunculate while in the other genera, they are linear to gradually enlarged. Terpnissa lacks lateral pronotal tubercles and femoral spines which are present in most species within the Mallocera group of genera. Terpnissa resembles Nyssicostylus with regard to the pedunculate meso- and metafemora and long flying hairs on the appendages. It differs in having a rounded pronotum laterally (slightly laterally tuberculate in Nyssicostylus) and a dense vestiture of pubescence on the pronotum and elytra (shiny and nearly glabrous in Nyssicostylus).