Diagnostic description.

Aulacus bicolor is a distinctive species and is easily distinguished from other Australian species based on the combination of orange and black colouration and the infuscation surrounding fore wing vein 1-Rs (Fig. 11A).

Female. Body size 10.0 (9.6–10.9) mm, ovipositor length 11.9 (9.4–11.9) mm, fore wing length 7.8 (6.6–8.0) mm (Fig 11A); colouration head black, antenna orange except last two flagellomeres black, mesosoma orange in the anterior half and largely black in the posterior half, scutellum and mesopleuron orange, fore and mid tarsomeres 4–5 and claw black, hind legs dark brown except for variable amounts of orange on coxae, metasoma dark brown except T1 and T2 orange, wings hyaline except for infuscation of apex 1st marginal, 4th submarginal cell and vein 1-Rs (Fig. 12G), hind wing veins spectral; head quadrate, width 1.3 (1.3–1.4) × length, postocular space 0.5 (0.4–0.6) × eye length in dorsal view (Fig. 12C), raised transverse carina above toruli absent, malar space 0.2 × height of eye (Fig 12A); length of scape 1.5 × pedicel length, 1st flagellomere 1.6 (1.6–1.8) × length of scape, 0.5 × 2nd flagellomere; mesosoma length 1.9 (1.7–1.9) × height, mesoscutum in lateral view truncated antero-dorsally (Fig. 12D), length 1.1 (1.0–1.0) × width in dorsal view; ovipositor guide oblique, positioned medially (Fig.12F); ovipositor sheath length 1.5 (1.5–1.6) × fore wing length (Fig. 11A); fore wing vein 2-Rs+M 1.5 (1.41.5) × 1-Rs+M, three hamuli equidistant (Fig. 12G); metasoma clavate,T1+T2 length 3.4 (3.1–3.4) × width in dorsal view.

Male. Body size (9.6–11.1) mm, colouration similar to female except propodeum, hind coxa and apex of flagellomere 2 to tip dark brown (Fig. 11B).