Ventral margin of clypeus even, without teeth or tubercles, truncate to weakly concave; labrum usually flat, always broadly exposed. Occipital carina broadly absent dorsally, present laterally, extending to base of mandible, widely separated from hypostomal carina. Notauli varying from nearly absent to complete and ending in mesoscutal midpit, often sculptured anteriorly; mesoscutal midpit present, varying from punctiform to narrowly elongate. Precoxal sulcus varying from unsculptured and nearly absent to crenulate and extending nearly full length of mesopleuron. Fore wing stigma broad, discrete, with r1 arising from or distad midpoint; second submarginal cell relatively short, with 2RS and 3RSa approximately equal in length; m-cu antefurcal to postfurcal; RS complete to wing margin; 1st subdiscal cell closed (2cu-a present). Hind wing RS poorly developed, often barely indicated distally; m-cu nearly always present, though often weakly developed. First metasomal tergite with or without dorsope. Metasomal tergum 2+3 either smooth or sculptured. Ovipositor sheath varying from barely exserted to extending 0.5 × length of metasoma.

Fischer (1971) placed Atoreuteus and Bathystomus as synonyms of Diachasma, but both have been treated separately in subsequent studies (Wharton 1988, Quicke et al. 1997). Belokobylskij et al. (2003) listed them under Diachasma, with a note that not all authors agreed with this placement. Yu et al. (2012) also left them in Diachasma. Wharton (1988) noted major differences in external morphology between Atoreuteus and Bathystomus, and these were corroborated by venom gland differences observed by Quicke et al. (1997). Similarly, Wharton (1988) described differences in labral and mandibular morphology that differentiate these two from more typical Diachasma. The type species of both Atoreuteus and Bathystomus also have an exceptionally large pronope. Their placement remains enigmatic, though the shape of the labrum suggests a basal position within Opiinae. They likely merit recognition as separate genera, but we leave them in Diachasma, with reluctance, pending a more rigorous analysis of opiine relationships.

The new species described below belongs to an Australian/New Guinea species group that is also morphologically distinct from typical members of Diachasma (as represented by species such as Diachasma fulgidum and Diachasma hispanicum (Fischer)). These austral species include Diachasma australe (Fischer), Diachasma extasis Fischer, Diachasma kaltenbachi Fischer, and Diachasma tasmaniae Fischer. This group can be characterized as monophyletic by the distal origin of r1 from the stigma (distinctly beyond the midpoint). The included species can be further recognized by the combination of a postfurcal fore wing m-cu, propodeal sculpture with well-developed median longitudinal carina on anterior half, bordered by relatively unsculptured areas, and T1 with well-developed dorsope. Most species also have fore wing 2CUa shorter than 2cu-a.

The type species, Diachasma fulgidum, is unusual in lacking a well-defined malar sulcus and having a relatively flattened scutellum with polished posterior margin.

Specimens of the species newly described below were compared to primary types and other specimens of nominal species of Diachasma previously described from Australia and New Guinea. During this examination, we discovered that a specimen in HNHM labeled as the lectotype male of Diachasma rufipes Szépligeti, 1905, is a member of the genus Notiopambolus van Achterberg & Quicke. The new combination is Notiopambolus rufipes (Szépligeti). Several species of Notiopambolus have been described from eastern Australia (van Achterberg and Quicke1990, Belokobylskij 1992). It is likely that rufipes is a senior synonym of one of these previously described species, but we are unable to place it at this time.

Australia, New South Wales, Lord Howe Island

Holotype. Female (ANIC), first and only data label, first line: Aust:NSW second line: Lord Howe Is. third line: 17.31.v.1980 fourth line: S.&J.Peck

3 males, same data as holotype (CNC, TAMU).

Face faintly punctate, punctures separated by 5–7 × their diameter, surface weakly shagreened to smooth between punctures. Frons with small, median pit between median ocellus and toruli. Clypeus 2.25–2.65 × wider than high, faintly punctate; ventral margin weakly concave, nearly truncate. Eye moderately large, 2.5–2.75 × longer than temple in lateral view. Malar sulcus distinct, deep throughout; malar space 0.5–0.6 × basal width of mandible. Mandibles deflected ventrally, broadly exposing labrum, apical teeth bent sharply inward relative to base, basal portion as broad as long, with prominent flanges forming dorsal and ventral margins (Figs 5, 6). Antenna with 26 (female) and 30–32 (male) flagellomeres; first flagellomere 3.5–4.0 × longer than wide, 1–1.05 × longer than second. Length of maxillary palps distinctly greater than height of head. Mesosoma 1.5–1.55 × longer than high, 1.85–2.05 × longer than wide. Propleuron smooth, polished, without diagonal sulcus. Pronotum dorsally with small median pit; crenulate groove along posterior margin continuing laterally, extending to ventral margin of pronotum laterally; posterior margin of pronotum laterally crenulate. Notauli deeply impressed, extending from anterior margin to narrowly elongate midpit, crenulate anteriorly, unsculptured on most of disc; supramarginal carina distinct throughout, mesoscutal humeral sulcus crenulate; midpit occupying nearly half length of disc (Fig. 11); median mesoscutal lobe elevated along edge of anterior declivity in holotype. Scuto-scutellar sulcus broad, twice wider than long. Precoxal sulcus (Fig. 10) deep, broad, nearly extending to anterior margin, widely separated from posterior margin, varying from crenulate to almost completely smooth. Propodeum with smooth anterior-lateral areas separated by median longitudinal carina on basal 0.3, carina merging posteriorly with barely discernible median areola largely obscured by fine rugosities covering posterior 0.5 of propodeum. Fore wing stigma discrete, wedge-shaped, gradually widening distally, 4.2–4.5 × longer than wide, r1 arising 0.7 × distance from base, less than half width of stigma at this point; 2RS 1.2 × longer than 3RSa; 3RSb extending to apex of wing; m-cu widely postfurcal; 2CUb arising at or near anterior side of 1st subdiscal cell, 2CUa varying from absent to nearly so. Hind wing RS weak but distinct at base, absent distally; m-cu weakly pigmented and very weakly impressed, long, extending nearly to wing margin, curved basally. Metasoma with T1 1.35–1.45 × longer than apical width, about 1.7 × wider at apex than at base; densely striate, dorsal carinae distinct from base to apex, more nearly parallel-sided and widely separated in male paratypes than female holotype; dorsope large, distinct. T2+3 smooth, very sparsely setose, nearly bare. T2 spiracle on dorsal edge of lateral crease separating median tergite from lateral tergite. Ovipositor sheath about half length of mesosoma, sparsely setose. Color (Figs 9, 10) pale yellow (males) to dark yellow (female) except as follows: pronotum laterally, mesopleuron dorsally, metapleuron, propodeum posteriorly, and T1 dark brown, female more extensively darker than males; ocellar field, most of face, and temple behind eye brown; flagellum and tergal margins light brown; propleuron dorsally and propodeum anteriorly yellow-brown; apical teeth of mandible red; labrum, palps, lower gena, trochanters, and all coxae white, remainder of legs, scape, and pedicel faintly yellow.

This new species is almost identical morphologically to Diachasma tasmaniae, with very similar wing venation, body sculpture, and coloration (including female darker than male). The mandible, however, is distinctly different. The mandible of Diachasma tasmaniae is typical of other members of this species group, lacking the flange-like dorsal and ventral margins of the Diachasma dentatum mandible. Both Diachasma tasmaniae and Diachasma dentatum belong to a group of species readily recognized by the combination of a distinct dorsope, r1 arising from the distal portion of the discrete fore wing stigma, and a relatively short second submarginal cell, with 2RS longer than or less commonly equal in length to 3RSa. All other described species in this group have normal mandibles similar to those of Diachasma tasmaniae and are darker in color than Diachasma tasmaniae and Diachasma dentatum.

Unknown.

The species name is Latin for toothed, calling attention to the unusual form of the mandibles.

Intraspecific variation in sculpture and quantitative measurements poses challenges for species delineation in this group. The face is more visibly shagreened in two of the males of Diachasma dentatum, for example, but the sculpture is weak in all four specimens compared to most but not all specimens of Diachasma tasmaniae available for examination (n = 11). The eye varies in size between sexes and among species of this species group, but in known members of this group, the eye is large compared to that of the relatively small eye of Diachasma fulgidum.

The placement of dentatum in Diachasma, along with tasmaniae and the other morphologically similar species noted above, will ultimately need to be re-evaluated. There are compelling morphological differences that strongly suggest that these species are not congeneric with the type species, Diachasma fulgidum, nor closely resemble the type species of Atoreuteus and Bathystomus. There are a large number of available genus group names in the Opiinae, including several for species with a well-developed dorsope (as discussed in detail by Wharton 2006) and we are thus reluctant to create a new generic name since one or more of these may prove applicable with a broader analysis of opiine relationships. The described species from Australia and New Guinea are also morphologically diverse, as exemplified by the key to species presented above, and many new species await description, based on material examined in TAMU and ANIC. As noted above, resolution of relationships will necessitate consideration of many opiines currently placed in other genera and is beyond the scope of this work.

Though Lord Howe is an oceanic island well off the coast of New South Wales, this newly described species is remarkably similar in many respects to Diachasma tasmaniae. Diachasma tasmaniae was originally described from Tasmania and New South Wales (Fischer 1995) and is recorded here for the first time from Victoria (specimens in TAMU and ANIC).