Corbels

In the context of commenting on and updating the glossary of weevil terms, as Chris Lyal encouraged us to do and as we need to do rather urgently for the Handbook of Zoology, I finally made the time to properly compile some older notes of mine and also of E.C. Zimmerman on the nature and definition of the corbel. In view of the query that was posted about this term a while ago, I intended to do this much earlier, but as usual there is never enough time for everything. Sorry about that.

Zimmie and I independently arrived at very much the same conclusion about this structure, he from his studies of the Australo-Pacific entimines and I from mine of largely African entimines and brachycerines, and we discussed the matter at some length shortly before his death. The following text is a compilation of our respective notes on this stucture. It is not a concise definition as yet but includes explanations, examples and comments on nomenclature.

Corbel (corbula, corbeille, Körbchen) (leg, tibial apex) – an oval to lenticular bevel at outer apex of (hind) tibia, originating in one of two ways.

The TRUE CORBEL arises from an inward tilting of the outer edge of the tibial apex, forming a bevel delimited along its INNER margin by the normal outer fringing setae of the tibial apex and along its outer margin by a row of enlarged tibial surface setae, to result in a basket-like structure whose inside may be bare or covered with finer setae or scales. This is the “closed” or “enclosed” corbel of previous authors. It occurs only in the curculionid subfamilies Entiminae and Brachycerinae sensu lato (Oberprieler et al. 2007) and may represent a synapomorphy for or within these two groups (but then with numerous secondary losses). In Brachycerinae it is well developed in, e.g., Arthrostenus, many Brachycerus, Desmidophorus, Tetracyphus, Trichocaulus and Turanocryptus, and incipient (without a definite secondary row of outer setae) in Aonychus and some Brachycerus. In Entiminae its structure is often diagnostic of tribes or similar generic groupings.

The FALSE CORBEL arises from a bare keel formed on the face (disc) of the tibial apex along the outer edge of the tarsal socket and is therefore delimited along its OUTER margin by the normal outer fringing setae of the tibial apex (the keel forming the inner margin), thus does not form a proper basket-like structure and is never covered with setae or scales. Termed "falsches Körbchen" (false corbel) by Faust (1883), this is the “semi-enclosed corbel” of later authors and “inner flange” of Thompson (1992), and while it is often indeed only a flange, it can also form a large, flat bevel (in the cyclomine genera Epicthonius and Occylotrachelus) or a distinct concavity (in most Molytinae: Psepholacini, e.g., Catocalephe, Dactylipalpus, Protohylastes, Psepholax, Therebus). It occurs sporadically in various curculionid subfamilies (but rarely in Entiminae and Brachycerinae; e.g., in Amystax, Liophloeus, some Ottistirini; and in some Echinocnemus, Icaris, Jekelia, Neonotaris, Ocladius, respectively) and has evidently evolved independently numerous times.

Because of their different origins, it is possible for a true and a false corbel to occur together (as in some Lepropus, according to Thompson (1992)). Both type of corbels evidently function as a platform for pushing off and are therefore most prominently developed on the hind tibiae (less frequently on the middle tibiae as well). The “open corbel” of previous authors is a misnomer for the normal condition of the tibial apex, i.e., without any kind of bevel at the outer edge. The tibia is then called “without corbel” (Faust 1879) or “simple” (Thompson 1992). Thompson (1992) provided excellent illustrations of the various types of tibial apex.

Thompson (1991) argued that the term “corbel” in entomology is a misnomer and should be abandoned, because the English word with this name is derived from the Latin corvus (a raven) and denotes a stone console used in architecture to carry superincumbent weight on walls. This English word is actually derived from the Latin corbellus, the diminutive of corvus, via the Old French corbeau (a crow). The French term originally used by Lacordaire (1863) for the little basket on the weevil tibia is corbeille, which derives from the Latin corbula, the diminutive of corbis (a basket). This terminological confusion thus appears particular to English only; the Latin corbula, the French corbeille and the German Körbchen as used by Lacordaire, Seidlitz, Pascoe, Faust and van Emden were always clear. There is therefore no need to abandon this term in weevil terminology, but the misleading descriptors “open”, “closed/enclosed” and “semi-enclosed” should not be continued as they confuse and obscure the different structure and origin of the two types of corbels.

R.G. Oberprieler & E.C. Zimmerman, Canberra, Australia.