Rhopalum coriolum (Crabronini)
Workbook
In spring of 2022 we discovered these small wasps nesting in a patch of well-packed coarse sand. Since then we have studied their nesting habits over consecutive breeding seasons (manuscript in preparation). On this notes page I detail their morphology, make a species-level identification, and describe the previously unknown male of this species.
There are currently nearly 100 described Rhopalum species in Australia, with additional undescribed specimens in collection (Pulawski 2010). Leclercq is responsible for the majority of that work, including the most recent review and key to species (Leclercq 1997).
It is worth noting the close similarity between Rhopalum and Podagritus. For a more detailed discussion of this, see my notes on Podagritus:
Rhopalum are small wasps, typically black with relatively few colour markings. Species distinctions largely rely upon subtle differences in shape (e.g. pronotal angles; clypeus margin; interantennal space), although there are more obvious clues too (e.g. head width; metasoma shape).
During 2022 and 2023 I collected five specimens, all from a nesting site in our local forest.
Specimens: 2209A (female); 2308A (female); 2309C (female); 2305A (male); 2305B (male).
One female (2308A) and one male (2305A) will soon be lodged with Museums Victoria.
Features of Rhopalum coriolum
Our local species is an excellent match for Rhopalum coriolum, based on the original species description (Leclercq 1957) and the features presented in the most recent revision (Leclercq 1997). Importantly, the key diagnostic characters that distinguish this species from R. variitarse are evident. Comparison with images of the holotype provide further justification for referring these specimens to R. coriolum.
At the time of his major revision of Australian Rhopalum, Leclercq (1997) had not discovered any additional specimens beyond the female holotype. The male remained unknown. In 2022 I observed mated pairs in the field, and subsequently collected two male specimens from the same nest site. These specimens (2305A & 2305B) closely resemble the females, including in several diagnostic characters for the species.
Key steps to R. coriolum
The following annotated set of images step through the key to females (Leclercq 1997; translated extracts also below, for reference).
Note re mandibular teeth: the distinction between bidentate and unidentate mandibles is unclear in the key, and I have deviated at Step 51. That is, although there appear to be two teeth at the mandible apex, I have followed the ‘unidentate’ option which then leads to Step 52 (not the bidentate option, which leads to Step 59). Importantly, the image of the holotype female clearly shows evidence of a second, lower tooth … most evident when viewed from slightly below (see images of the holotype).
The holotype
The holotype was collected in October 1930 at Mt. Victoria, NSW (Blue Mountains region) by A.N. Burns, identified by Jean Leclercq in 1956, and officially designated in the published description the following year (Leclercq 1957). It is held in the Museums Victoria collection, database number T-11665. https://collections.museumsvictoria.com.au/specimens/1013418
Dr. Ken Walker from Museums Victoria kindly provided additional, close-up images of the head for comparison with my collected specimens. They match!
This is the only R. coriolum specimen I am aware of in museum collection. Presumably Leclercq examined all Rhopalum specimens held by Museums Victoria and the Australian National Insect Collection (ANIC) at the time of his revision (Leclercq 1997). However, as Leclercq makes no mention of reviewing the collections of other Australian institutions, it is possible that there are additional specimens of R. coriolum held elsewhere and/or in drawers of undetermined Rhopalum. These caveats aside, identified (‘determined’) species are no doubt in short supply.
For this reason our small contribution it likely to be quite beneficial to researchers in future.
The missing males
We first sighted males at the nesting ground in September 2022. These two field shots highlight the overall similarity in shape and colour pattern between the sexes. And the striking difference in size!
A closer look at the collected males (2305A & 2305B) shows just how very alike the sexes are. In terms of the diagnostic features of the species, they are virtually identical to the female. Minor differences include: the head (less broad), the metasoma (more slender), and the scapes (more extensively yellow beneath).
Bits & pieces
Earlier work, including the original descriptions of R. coriolum (Leclercq 1957) and R. variitarse (Turner 1915).
References
Leclercq, J. 1957. Le genre Rhopalum (Kirby, 1829) en Australie. Bulletin et Annales de la Société Entomologique de Belgique, 93 (VII/VIII): 177-232 (available here)
Leclercq, J. 1997. Hyménoptères Sphécides Crabroniens d'Australie, du genre Rhopalum Stephens, 1829. Notes Fauniques de Gembloux, 32: 3-101 (available here)
Pulawski, W.J. 2010. Rhopalum nasale, a new species from Australia (Hymenoptera: Crabronidae). Journal of Hymenoptera Research 19: 139-143
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.