Elachistelmis tetramera sp. n.

(Figs. 1–6, 9–11, 15–21)

Type material. Holotype male: “ SURINAME: Sipaliwini District/ 2°10.973’N, 56°47.235’W, 210m / Camp 2, on Sipaliwini River/ leg. A.E.Z. Short; UV-light/ 27.viii–1.ix.2010; SR10–0827–LT2” Holotype deposited in the NZCS.

Paratypes: 45 specimens, same data as holotype. Paratypes will be deposited in: 5 in NZCS, 5 in NMPC, 5 in BMNH, 10 in NMW, 10 in SEMC, 5 in NHNM, and 5 in USNM.

Diagnosis. This species can be distinguished from all other described species of Elminae by the combination of its small size (ca. 1.0 mm) and tetramerous tarsi (Figs. 15 & 17). This is possibly the smallest described riffle beetle in Elminae and the only species of elmid that does not have a 5–5–5 tarsal formula. Other characters that are useful for separating this species from its congener are hind wings with setal fringe, the pointed apex of the prosternal process (Fig. 19), and the rufus coloration with testaceous elytral humeri and elytral apices.

Description. Holotype Male. Body elongate, subparallel, dorsum moderately convex (Fig. 9). Length, 0.90 mm; greatest width, 0.45 mm. Dorsal surface sparsely setose. All appendages testaceous. Pronotum reddish-brown; elytra dark drown with testaceous patches at humeri and apices (Fig. 9). Ventral surface testaceous and densely covered with recumbent setae.

Head microreticulate. Eye hemispherical, separated by a distance three times the radius of eye. Antenna with sparse, short setae, apical antennomere spindle-shaped (Fig. 11). Clypeus explanate laterally. Labrum rounded apically; surface with fine, sparse punctation and covered with sparse, golden setae (Fig. 4). Lacinia and galea long; apical palpomere of maxillary palpus spindle-shaped (Fig. 1). Labium apically with a brush short, flattened setae and short hair-like setae (Fig. 2). Labial palpus set into a furrow at base of labium (Fig 2). Apical palpomere of labial palpus broad and cylindrical, bearing a patch of sensilla at apex (Fig. 2).

Pronotum 0.35 mm long, 0.30 mm wide, roughly as wide at base as at apex, widest point at posterior third, broadly convex, with two complete, strong, sublateral carinae; lateral edges convex; anterolateral angles acute, slightly explanate and slightly depressed behind each angle; posterolateral angles acute, slightly explanate; discal area free from impressions, covered with very sparse setae and fine, dense, shallow punctures, punctures more dense anteriorly (Fig. 10).

Prosternum very long in front of procoxae, approximately two times as long as prosternal process; with sparse setae, coarsely microreticulate without sculpturing medially, with strong, sinuate longitudinal carinae extending anterior of procoxae. Prosternal process expanded posterior to procoxae, forming a diamond-shaped projection; lateral margins concave, with curve for reception of procoxae; apex broadly pointed (Fig. 19). Mesoventrite with sparse setae, coarsely microreticulate (Fig. 11). Metaventrite coarsely microreticulate posterior to mesocoxae, densely setose laterally, with long flattened setae, glabrous at center line; disc with distinct, deep, longitudinal groove medially. Procoxae and metacoxae moderately widely separated; mesocoxae slightly more widely separated (Fig. 11). Legs long and slender. Femora glabrous. Protibiae, mesotibiae, and metatibiae (Figs. 15 & 17) with cleaning fringe at apex. Tarsi tetramerous on all legs, claws long and stout (Figs. 15 & 17).

Elytron with seven rows of weak, shallow punctures; punctures separated by a distance roughly six times the diameter of the puncture; intervals with fine, very sparse setae, smooth and without sculpturing (Fig. 10); elytron with two carinae (sublateral carina in basal 0.77, lateral carina reaching elytral apex), with dense setae of plastron lateral of lateral carina; elytral plastron setae long, (ca. 0.2 mm), with short lateral projections on the entire length of the seta (Fig. 21). Hind wing with long setal fringe. Lateral margin of elytron with fringe of hooked setae (Fig. 20). Elytral interior with patch of cuticular processes distally (Fig. 16)

Abdomen with five ventrites; cuticle densely covered with setae, with glabrous patch medially. First ventrite with intercoxal process broadly, shallowly depressed and distinctly carinate adjacent to metacoxae; carinae extending longitudinally behind metacoxae for half the length of ventrite; area posterior to metacoxae coarsely microreticulate. Second ventrite of male with two ventrally produced sensilla medially. Last visible ventrite broadly rounded, with lateral hook-like projections (Fig. 11).

Aedeagus as illustrated, elateriform, median lobe of aedeagus 1.5 times as long as wide (Fig. 5).

Female. Externally similar to male (tend to be larger in size), lacking two ventrally produced tubercles (sensilla) medially on second abdominal ventrite. Ovipositor as illustrated; Gonostylus with row of stout teeth laterally, with single seta between each tooth (Fig. 6).

Intraspecific Variation. This species varies widely in coloration, with most individuals displaying strong testaceous markings on elytral humeri and apices, while others display a more uniform color pattern. The species also varies in size, from 0.9–1.2 mm long.

Etymology. The specific epithet, “ tetramera ” is a reference to the tarsal formula of members of this species, which is 4–4–4, in contrast with the all other known members of the family Elmidae, which have a 5–5–5 tarsal formula.

Habitat. The specific microhabitat for this species is unknown, as they were collected at a UV light approximately 30 m from the Sipaliwini River (Short and Kadosoe 2011). The specimens were collected in a blacklight trap along with several hundred other elmid specimens comprising seven different genera.