Nearly all Pemphredoninae have 2 submarginal cells and an enlarged stigma.

The shape of the submarginal cells varies considerably between species.

A distinct occipital carina is a feature of most crabronids (solid, straight arrows). The extent may vary, sometimes useful in distinguishing species within a genus. In Arpactophilus it is present, at least ventrally.
[Note that in this particular species, there is an additional, strong genal carina – dotted, curved arrows]

Spilomenina is unusual in completely lacking an occipital carina. The rear of the head looks remarkably smooth and rounded (see examples below).

The median ridge (arrows) running to the mid ocellus is typical of Arpactophilus. In Spilomena, it is very short, barely reaching half way to the ocellus.

Deep depressions above the antennal sockets (arrows) are characteristic of several Pemphredoninae, including Arpactophilus. In contrast, there is only the barest indication of such ‘scapal basins’ in Spilomena.

Many Pemphredoninae have a groove alongside the eye (arrows). Often it is along the inner and/or upper margin of the eye, while In others (such as here) the groove completely encircles the eye. It may also be pitted (foveate’), as here.