Diagnostic description.

Aulacus grossi is easily distinguished from other Australian species based on the combination of the small body size, 4.5 mm, occipital carina present laterally and distinctive colouration, body orange except head; metasoma and antenna tip black.

Female. Body size 4.5 (4.4) mm, ovipositor length 3.8 (3.5) mm, fore wing length 3.6 (3.2) mm (Fig. 45); colouration body orange except head, metasoma and antenna tip black, ovipositor sheath dark brown; wings hyaline (Fig. 46G), hind wing veins spectral; head shape quadrate, width 1.1 (1.3) × length, postocular space 0.5 (0.7) × eye length in dorsal view (Fig. 46C), occipital carina present laterally, raised transverse carina above toruli absent, malar space 0.2 (0.4) × height of eye (Fig. 46A); length of scape 1.3 (1.2) × length of pedicel, 1st flagellomere 1.0 (1.1) × length of scape, 0.6 (0.5) × 2nd flagellomere; mesosoma length 1.6 (1.7) × height, mesoscutum in lateral view rounded antero-dorsally (Fig. 46D); mesoscutum length 1.0 (1.1) × width in dorsal view (Fig. 46E); medial lobe with transverse carinae; ovipositor guide oblique, positioned medially (Fig. 46F); ovipositor sheath length 1.1 (1.1) × fore wing length (Fig. 45); 2-Rs+M 1.4 (1.1) × 1-Rs+M, two hamuli, equidistant (Fig. 46G); metasoma pyriform,T1+T2 length 1.9 (1.7) × width in dorsal view.