The inner margins are entire (without emarginations/notches), and broadly separated. They are essentially straight, slightly convergent above.

This is in contrast with the majority of crabronids, which have either one or three submarginal cells.

The stigma is particularly large, covering about “half as much area as marginal cell” (p. 192 Bohart & Menke, 1976).

Bohart & Menke refer to both the discoidal cell (DC) and subdiscoidal cells (sud DC) when they describe this arrangement as two discoidal cells and a single recurrent vein (p.186 & Fig. 41F in Bohart & Menke 1976).

In lateral view, the base of the first gastral segment is obviously narrowed. This raises the question about whether it should be considered petiolate or sub-sessile. Note that many/most Australian species in the subtribe Spilomena have a sessile gaster.

A dorsal view is needed, to determine the length to width ratio of the petiole.

From a more dorsal perspective, the shape of the petiole is more apparent. It is clearly shorter than wide.

Among Pemphredoninae genera, males have 11 segments in the flagellum. This specimen is therefore a female. Note that it presumably has just 6 visible tergites, but given the small size and orientation of the gaster it is hard to tell.

This is characteristic of Pemphredonini, as distinct from Psenini.

In a comparison of key features, Ceratostigmus reads much like Spilomena. Notably, in Ceratostigmus the petiole is very short. However the shape of the head is distinctive. In Ceratostigmus the vertex is produced behind the ocelli. That is clearly not the case in specimen #016.