Holotype (♂): “VENEZUELA: Amazonas/ 0°50'N, 65°59'W; 2100 m/ Cerro de la Neblina, camp II; beetles in flight over sunlit stream/ 16:00hrs. 31.i.1985/ leg. W.E. Steiner et al.” (USNM). Paratypes (8): VENEZUELA: Amazonas: Same data as holotype (SEMC, USNM, 7, including DNA voucher SLE 1374); same except 0°52'N, 65°58'W, 1450 m, camp XI, 25–28.ii.1985, seine of rapids in small mountain stream, leg. P.J. & P.M. Spangler, R. Faitoute (USNM, 1).

Primocerusgigas is among the largest species of the genus. It can be distinguished from similarly sized species by the moderately sized eyes being separated by a distance of 7.5 × the width of an eye (Fig. 10H).

Body length 4.9 mm, width 2.8 mm. Body elongate oval, moderately convex (Fig. 10F). General coloration dark brown. Elytra with ground punctures shallowly marked, seta-bearing systematic punctures slightly enlarged, and serial punctures absent. Posterior elevation of mesoventrite with simple transverse ridge. Metafemora with hydrofuge pubescence covering slightly more than basal half of anterior surface. Apex of fifth abdominal ventrite truncate. Aedeagus (Fig. 14I) with basal piece nearly 1.1 × longer than parameres; parameres slightly longer than median lobe, truncate and obliquely directed at apex; apex of median lobe narrowly pointed.

Named with the Latin word gigas meaning giant, in reference to the large size of this species compared to most members of the genus.

Primocerusgigas is only known from Cerro de la Neblina in the Venezuelan Amazon, at elevations between 1450 and 2100 m (Fig. 15A).

Label data indicates the beetles were collected “in flight”, with one specimen collected by seining rapids in a mountain stream.