HOLOTYPE female. “Brasil, MA[ranhão] [Centro Novo do Maranhão] REBIO – Res[erva] Biol[ógica do] Gurupi 03°14'05"S, 46°41'83"W “Arm[adilha] Luminosa móvel 07–15.I[Jan.].2011, F. Limeira-de-Oliveira & M. M. Abreu, cols.” (CZMA). Paratypes: same data as holotype (5 females, 22 males, CZMA; 2 females, 20 males, INPA; 5 males, MPEG; 5 males, MZSP).

Mostly light yellow, slender, and soft-bodied specimens. Thorax and abdomen with yellow bristles. Antenna with three flagellomeres after postpedicel. Wing unusually long; usually with cup cell open, without petiole if cell is closed. Abdomen unicolorous. Female tergite 9 distinctly narrower medially; tergite 10 sub-rectangular.

Holotype female. Body length: 8.9 mm. Specimen mostly light yellow. Head (Fig. 1) with eyes black (green in life, ) more or less suboval in profile, rounded laterally in frontal view, with very short yellowish bristles which are barely visible under higher magnification. Frons (Fig. 2) narrow, somewhat parallel sided, slightly divergent dorsally and ventrally, frontal index about 2.7, smoothly tomentose, with a median inconspicuous groove, and short, inconspicuous brown bristles. Ocellar tubercle (Fig. 2) somewhat prominent, as high as ocellus. Subcallus (Fig. 2) very small, tomentose, separation from frons indistinct. Parafacial narrow, tomentose, with long black bristles. Face convex laterally, deeply sunken medially, tomentose, without bristles, separate from parafacial by deep groove. Antenna (Fig. 3) with scape and pedicel short, plump, yellow to brown, and with robust black bristles; flagellum light yellow with robust black bristles, apparently with six flagellomeres; postpedicel swollen when observed in lateral view, with three distal flagellomeres, the first flagellomere almost totally fused to postpedicel based on a distinct incomplete suture on medial side (see Fig. 3 from a clarified antenna of a different paratype specimen); second flagellomere as long as first and with an indistinct suture; third flagellomere, the distalmost, longer than two preceding flagellomeres. Palpus (Fig. 4) with first segment somewhat swollen, second slightly narrower and slightly curved, distinctly bristled. Proboscis short, as long as palpi, membranous, with long, narrow, soft and bristled labellum.

Thorax with scutum and scutellum light brown to dark yellow, sparsely yellow bristled, with yellow pruinescence. Pleuron slightly clearer than scutum, yellow with light grey to yellow pruinescence.

Legs (Fig. 1) entirely yellow except distal half of tarsomeres 5 brown; most legs with yellow bristles, except fore tibia black bristled. All tarsomeres 1 of equal length. Hind tibial spurs slightly shorter than mid ones.

Wing (Fig. 5) 9.1 mm long, 2.9 mm wide, narrower than usual for tabanids, diffusely brownish, with costal margin slightly darker; pterostigma ill defined. Vein Sc bare dorsally and ventrally; vein R₄ with short appendix; vein CuA₁ with even row of small setulae; cell cup open. Halteres with stem yellow and capitulum brown and white.

Abdomen (Fig. 1) long, narrow, entirely yellow, with short golden bristles dorsally and ventrally. Terminalia: Tergite 9 (Fig. 6) narrow medially, expanded laterally; tergite 10 subrectangular in dorsal view, divided medially; cercus subtriangular. Sternite 8 (Fig. 7) wider than long, with somewhat distinct gonapophysis. Genital fork as in figure 8.

Male. Body length: 9.0 mm; wing length: 9.1 mm. Habitus similar to female specimens except head holoptic, antenna (Fig. 9) slightly weaker, cell cup narrowly open (sometimes narrowly closed, without petiole), abdomen slender and of a lighter tone, first 3–4 abdominal segments light yellow, somewhat translucent, remaining brown. Terminalia (Fig. 10): epandrium with concavity basally; cercus subquadrate in lateral view; gonocoxite slightly arched; gonostylus bifid (Fig. 11); ejaculatory apodeme and gonocoxal apodeme similar in length.

The specific epithet is a noun in apposition and refers to Reserva Biológica do Gurupi, where the specimens were collected.

Brazil, Maranhão.

Pinned, not dissected, in good condition except for a damaged left wing. We chose the best preserved specimen, among the few females collected, as holotype because in most tabanids species the primary types are females.

One female specimen without short appendix on vein R₄. Female size varying from 8.6–9.6, mean 9.0 mm (n = 3). Male size varying from 8.0–10 mm, mean 9.1 cm (n = 10).

Protosilvius gurupi sp. n. is smaller than other Protosilvius species, as the biggest specimens (9.8 mm) are slightly shorter than the smallest species, Protosilvius priscus (10 mm); these differ by three flagellomeres after the postpedicel in the former and four flagellomeres in the latter. Female specimens would key out to Protosilvius termitiformis in couplet 3 of Fairchild´s (1962) key by the following characters: short and sparse bristled specimens and abdomen unicolorous. Protosilvius gurupi has an open cup cell and narrow female tergite 9 (Fig. 6), whereas Protosilvius termitiformis has a closed cup cell and wide female tergite 9 (Fig. 20). According to Chainey and Hall (1996), female specimens of Protosilvius differ from Boliviamyia Chainey & Hall by a frons without callus, a slender palpus without a dorsal groove and apparently absent mandibles and both sexes have the antennal flagellum with a very short and/or irregular postpedicel and very long and slender apical flagellomeres.

Bionomics. Light traps are a common method for collecting many male and some female tabanids. All specimens of both sexes of Protosilvius gurupi sp. n. were collected in light traps, not one in the Malaise traps mounted nearby. The specimens were constantly collected in the light trap, either while the car was slowly moving or not. We believe the specimens are not nocturnal but they were attracted to trap when the light reached the specimens bedding in the vegetation. The collection was made in the Amazonian Region, in the state of Maranhão, in the rainy season, far from any drier area for at least 300 kilometers.