Diagnosis Body brown, with area between ocellar triangle and compound eyes paler; fore legs light brown, mid and hind legs brown, tarsi and apex of tibiae yellow. Antennal segments I and VII–VIII light brown, II and VI brown, III brown with apical neck pale, IV brown medially with base and apex lighter, V pale; fore wings brown with pale band sub-basally. Head strongly sculptured, constricted behind eyes and basally. Antennae 8-segmented, III & IV with long apical neck and forked sense cones, on IV the cone does not extend beyond antennomere V. Pronotum reticulate, without internal markings. Mesothoracic endofurca without spinula, metathoracic endofurca without spinula and with stout arms, reaching mesothorax. Mesonotum without a median longitudinal split. Metanotum reticulate, without a strong triangle; one pair of setae; campaniforme sensilla present medially. Tarsi 1-segmented. Fore wings with first row of setae interrupted medially, three setae distally; second row with 8 setae equally spaced between them; setae stout, wing cilia wavy. Abdominal tergites III–VIII with oblique striae on lateral thirds, a few reticles sublaterally; median setae short and distanced from each other; posterior margins with craspeda; tergite VIII with teeth arising from craspedum on lateral thirds; X without a longitudinal split. Abdominal sternites with three pairs of setae, pairs 1 and 2 near posterior margin, 3 ahead of posterior margin. Both sexes macropterous.
Male similar to female, but smaller and with thinner forewing setae; abdominal sternite III with an oval pore plate, sternites V–VII with concave and transverse pore plates.
Intraspecific variation Not recorded.
Genus and similar species information The genus Oneilliella has five Neotropical species, all with a dark and strongly reticulated body; metanotum sculpture not forming a strong triangle; metafurca stout and long, without spinula, reaching the mesosternum; and abdominal tergite X without a complete longitudinal split.Oneilliella williamsi is similar to O. chicoi, both with antennomere III bearing a long apical neck and forewings with stout setae; but O. williamsi has the base of antennomere III brown, forked sense cone on antennomere IV reaching at most segment V, and three distal setae on forewing first vein. A key to the Neotropical species of Oneilliella is available in Lima et al. (2023).
Distribution around the world Described from Trinidad and Tobago, recorded from Brazil.
