Brachystegus dubitatus (R. Turner 1914)

Brachystegus dubitatus (Turner)

Nysson dubitatus Turner, 1914:255 [; Coimbatore, South India; holotype in British Museum (Natural History)].—Turner, 1917:184 [, Pusa, India; description].—Maidl and Klima, 1939:135 [listed].

Brachystegus dubitatus (Turner).—Pate, 1938:160 [tentatively assigned to Brachystegus].—Bohart and Menke, 1976:473 [listed as synonym of B. decoratus (Turner)].

Brachystegus dubitatus and B. decoratus (Turner) are discrete species, not synonyms. Both species were described from Coimbatore, South India, the former from a unique female, the latter from a pair of which Turner designated the male as type. Turner's key distinguished the females by the red first abdominal segment of B. dubitatus. The legs of B. dubitatus were described as red (black in B. decoratus), and the size smaller, 6 mm as contrasted to 9 mm for the females.

I have seen Turner's types of the two species as well as two additional females of B. decoratus from South India, and a pair of B. dubitatus from South India and a single female from Sri Lanka. The color differences of the legs are confirmed in the two species, as well as a size difference, B. dubitatus (, 5.8–6.3 mm; , 6.7) being smaller than B. decoratus (, 8.0–9.5; , 8.0). The base of the dorsal surface of the first abdominal segment is more thinly silvery tomentose in B. dubitatus. The extent of yellow abdominal markings also is different. In B. decoratus the dorsum is yellow with a narrow black median line, the lateral yellow spots covering almost the whole exposed area of segments II–V and the dorsal surface of I. In B. dubitatus the yellow spots are more widely separated and narrower, the normally exposed bases of the segments being dark. Finally, the apex of tergum VII is bidentate in the male of B. dubitatus, the apical margin being concave, whereas in B. decoratus the apical margin appears tridentate because the margin between the lateral teeth is noticeably convex.

Brachystegus dubitatus must be more widely distributed in Sri Lanka than is indicated by my single specimen from Labugama Reservoir Jungle at an altitude of 100–150 m in the Wet Zone with an average annual rainfall of some 2400 mm. This species and its congener, B. basalis (Smith), are very uncommon both in Sri Lanka and India. Their rarity is exceptional considering that the only other Ceylonese nyssonine, Nysson rugosus Cameron, is both ubiquitous and extremely abundant in Sri Lanka.

The following description is based on the available three females and one male from Sri Lanka and India.

FEMALE.—Length 5.8–6.3 mm. Black, the following yellow: basal third or half of mandible, small spot laterad of and behind posterior ocelli, streak on basal half or more of outer surface of anterior tibia, tip of posterolateral propodeal spine, and large posterolateral spots on terga I–V, decreasing in size after II, moderately separated on midline; the following light red—rest of mandible except tip, occasionally scape, flagellum beneath on basal half or more, legs except yellow area on fore tibia, first segment except yellow areas and occasionally in middle posteriorly, and sterna II and VI except only base of II in Sri Lanka specimen. Vestiture dense, appressed, silvery on clypeus, front, temple, pronotal dorsum, mesopleuron, propodeal dorsum except enclosure, and sternum II, thinner on upper part of declivous surface of tergum I. Wings slightly infumated in marginal cell and apically, stigma and veins brown.

Eyes diverging above, interocular distance at anterior ocellus 1.6 times that at base of clypeus; clypeal apex thickened, bare, bidentate in middle; frontal carina moderately developed and extending about one-third toward anterior ocellus; upper front with large pits, those near ocellar area narrowly separated, both the pits and interspaces with dense tiny punctures from which the tomentum arises (Figure 9); posterior ocelli with low oblique ridge along inner margin.

Pronotal dorsum with a transverse ridge; scutum, scutellum, and mesopleuron contiguously pitted, the pits larger than on front; propodeal enclosure dorsally with a few longitudinal rugae, posteriorly irregularly rugose; posterolateral propodeal tooth moderately developed; posterior propodeal surface with a few longitudinal rugulae in middle, laterad of which are some rugulae radiating outward and upward from abdominal insertion.

Disk of tergum I with moderately large, subcontiguous punctures, some at apex larger; punctation of terga II–V becoming successively smaller, subcontiguous except for a few larger ones apically; pygidium strongly convex, closely and moderately punctate, apex rounded, distance between lateral carinae at base 0.8 times length of pygidium; sternum II gently convex toward base.

MALE.—Length 6.7 mm. Coloration and vestiture as in female except anterior tibia without yellow streak.

Sculpture as in female except as follows: last flagellar segment enlarged, concave beneath; tergum VI posteriorly with a lateral carina that terminates as a backwardly directed tooth; apical margin of tergum VII emarginate so that apex appears bidentate with rounded teeth.

SPECIMENS EXAMINED.—WESTERN PROVINCE, Colombo District: 1, Labugama Reservoir Jungle, 2–4 Feb, Krombein et al. (USNM).

INDIA, South India: 1, Coimbatore, 23 Jan, T.V.R. coll. (London; type); 1, Tranquebar, Apr, Nathan (Corvallis); 1, Kurumbagaram, Karikal Terr., Apr, Nathan (Corvallis).