Spilomena (Pemphredonini) — Life in a Southern Forest

Workbook

The first record of Spilomena for our home list here in the forest, and my first opportunity to have a close look at the key diagnostic characters for the genus.

These wasps are small to minute, so it is no surprise that field sightings are uncommon. However they do turn up in Malaise traps – and that is how we collected this specimen.

specimen #016

At this stage I’m not making any attempt at a species level identification. There has been no recent revision of this genus in Australia, and there are currently only nine named species … but I have it on very good authority that there are many more species, and that a major revision is in progress. I look forward to the publication!

Specimen collected in Malaise trap (GMP66330) during the week of 17th-24th January, 2026. Specimen ID 016.

See summary table on crabronid hub page (for identification to tribe)

see Pemphredoninae page (for identification to genus)

Genus level identification of Spilomena

The following set of characters place specimen #016 in tribe Pemphredoninae, and within subtribe Spilomenina (Menke 1988) in particular.

Featured

#016, female

eyes entire, widely separated, slightly convergent above

#016, female

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

#016, female

2 submarginal cells; enlarged stigma

#016, female

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).

#016, female

2 discoidal cells; 1 recurrent vein

#016, female

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).

#016, female

gaster with very short petiole

#016, female

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.

#016, female

gaster with very short petiole

#016, female

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

#016, female

10 flagellar segments

#016, female

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.

#016, female

antennal set low; clypeus short & not densely silvered

#016, female

This is characteristic of Pemphredonini, as distinct from Psenini.

#016, female

vertex declivous behind ocelli

#016, female

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.

#016, female

The features listed in the above gallery could equally apply to Spilomena or Arpactophilus. Indeed, the two genera are so similar that they might be effectively considered a single taxon.

“Arpactophilus and Spilomena are similar in general habitus and really differ only in the complete absence of an occipital carina and a very short frontal carina in the latter. Whether the two genera can be kept separate on th ebasis of these two characters remains to be seen.” (p. 740, Menke 1988).

By my reckoning, specimen #016 is convincingly Spilomena.

Spilomena cf Arpactophilus

The features that distinguish Spilomena from Arpactophilus involve fine details of the head shape. Specifically:

short frontal carina, not reaching halfway to the mid ocellus (much longer in Arpactophilus, absent in both Australomena and Ceratostigmus)

Featured

#016, female

absence of an occipital carina (present in Arpactophilus, at least ventrally)

Featured

#016, female

The absence of well-defined scapal basins was previously considered a third defining character, contrasting with their presence in Arpactophilus (Bohart & Menke 1976). However, Menke (1988) did not include this in his revised diagnosis. Indeed, several Arpactophilus lack scapal basins (or ‘antennal scrobes’, Matthews & Naumann 2002).

Other common – but not strictly diagnostic - characters of Spilomena include:

vein cu-a of hindwing straight, not appendiculate (whereas in most Arpactophilus it is angled and appendiculate)
finely textured integument (the head and thorax of many Arpactophilus is coarsely ridged)
small size (some Arpactophilus species reach 8mm in length, although some are as small as 3mm).

References

Bohart, R.M. & Menke, A.S. 1976. Sphecid Wasps of the World: A generic revision. University of California Press.

Matthews, R.W. & Naumann, I.D. 2002. Descriptions and biology of nine new species of Arpactophilus (Hymenoptera: Crabronidae), with a key to described Australian species. Journal of Hymenopteran Research 11(1): 101-133 freely available from BHL

Menke, A.S. 1988. Arpactophilus reassessed, with three bizarre new species from New Guinea (Hymenoptera: Sphecidae: Pemphredoninae). Invertebrate Taxonomy 2: 737-47

This is a workbook page … a part of our website where we record the observations and references used in making species identifications. The notes will not necessarily be complete. They are a record for our own use, but we are happy to share this information with others.