In the Glossary of Characters I have described the sternellum as follows:
"Sternellum (= furcasternum) (thorax) - the second sclerite of the ventral part of each thoracic segment. In weevils it is generally distinct on the prothorax, forming for example the posterior part of the canal in Cryptorhynchinae and a small clear sclerite just behind and between the fore coxae in most others. On the mesothorax it is rarely if ever distinct, but would lie between the mid coxae. On the metathorax it is small and lies between the hind coxae, if discernible. Wood (2007-14), after Hopkins, 1909) figures the sternellum for each thoracic segment. "
Lawrence et al (2010) in their glossary of morphological terms for the Handbook of Zoology, provide a definition of the 'prosternal process':
"Prosternal Process. Posterior projection of the mesal portion of the prosternum which extends between the procoxae and may overlap the mesoventrite or fit into the mesoventral cavity. The term “spinasternum” has been used by those studying Curculionoidea for the posterior end of the prosternal process, when the central portion of that process has been obliterated to accommodate the enlarged and contiguous procoxae."
These two definitions clearly contradict one another, although Lawrence et al note of the prosternum "it is likely that this sclerite has a complex origin, like comparable structures in the mesothorax and metathorax".
I have dissected a number of weevil thoraxes, and the 'sternellum' is separate from the prosternum in everything I've looked at. In the Cryptorhynchinae the two are fused but generally are marked by a suture between them; I think this unlikely if the posterior plate is simply an extension of the prosternum. Images in Davis (2009) (Figs 494-498) show the suture in some Baridinae (auctt). In some Brentidae the sternellum and prosternum are separated internally by a deep vertical sclerotised structure, while being demarcated by a suture on the surface. Another characteristic in some Cryptorhynchinae is for there to be setae or scales within the rostral canal; if these are present the form of them is often different on the sternellum than the prosternum, again not something I'd expect if the two sclerites were the same.
The use of the term 'spinasternum' would be incorrect; this is an intersegmental sclerite, an there is no evidence that the sclerite in question is intersegmental.
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