Diagnostic description.

Aulacus douglasi is one of three species with general orange and black colouration, resembling general aposematic colouration typical of other aculeate hymenoptera (e.g Vespidae).

Female. Body size 17.4 (10.8) mm, ovipositor length 21.8 (14.1) mm, fore wing length 11.8 (9.4) mm (Fig. 25); colouration orange with variable black patches on lateral lobes of mesoscutum, dorsal mesopleuron and metasomal T3 and T4, ovipositor sheath brown; wings tinted, fore wing 1st marginal, 1st submarginal, basal cell infuscated, costal cell and pterostigma yellow, hind wing veins spectral (Fig. 26G); head shape quadrate, width 1.3 (1.3) × length, postocular space 1.1 (1.0) × eye length in dorsal view (Fig. 26C), raised transverse carina above toruli, malar space 0.2 (0.2) × height of eye (Fig. 26A); length of scape 1.6 (1.8) × length of pedicel, 1st flagellomere 1.8 (1.8) × length of scape, 1.0 (1.0) × 2nd flagellomere; mesosoma length 1.4 (1.4) × height, mesoscutum in lateral view rounded antero-dorsally (Fig. 26D); mesoscutum length 0.8 (0.8) × width in dorsal view (Fig. 26E); medial lobe reticulate–rugulose; ovipositor guide transverse positioned medially (Fig. 26F); ovipositor sheath length 1.8 (1.5) × fore wing length (Fig. 25); 2-Rs+M 0.4 (0.4) × 1-Rs+M, four hamuli, equidistant (Fig. 26G); metasoma pyriform, T1+T2 length 2.3 (2.8)× width in dorsal view.

This species is superficially similar to A. festivus and A. vespiformis in general appearance and distinctive infuscation of the fore wing, but can be separated by the distinctive raised transverse carina above toruli (A. vespiformis with indistinct projections), ovipositor length long, 1.5–1.8 × fore wing length (shorter in A. festivus, 0.9–1.0 × fore wing length; A. vespiformis 1.5 × fore wing length) and mesosoma colouration mainly orange with small variable patches of black (mesosoma colouration mainly black with variable patches of orange in A. festivus and A. vespiformis).