The mesothoracic intercoxal perforation in Chrysomelinae and its evolutionary significance (Insecta: Coleoptera: Chrysomelidae)

In the thorax of many beetles the posterior parts of the mesoventrite and anterior parts of the metaventrite are flexed inward to form mesocoxal cavities laterally and a transverse double-walled infolding medially. This study focuses on the structure of the median part of the infolding in Chrysomelinae (=Timarchini + Chrysomelini; 42 species studied). Whereas the infolding is intact in Timarchini and very few Chrysomelini, it is almost always perforated in Chrysomelini. The perforation connects the body cavity anterior and posterior to it. It is usually small or absent in wingless Chrysomelinae, but there are exceptions. The phylogenetic interpretation of the intercoxal perforation in the entire Cucujiformia is unclear: among Chrysomelidae, the perforation was found in a representative of Hispinae, but was absent in the cryptocephaline and criocerine species examined; among non-chrysomelid Cucujiformia the perforation was found in an anthribid, but was absent in the examined tenebrionid and erotylid species. From a functional perspective, the perforation likely reduces the movability of the intercoxal area. The origin of this perforation appears as an unusual type of evolutionary structural transformation where ± widely separated (but opposed) parts of the body wall belonging either to two external projections or to a single infolding (depending on the point of view) became attached to each other along their external surfaces, and then dissolved. As a result, the parts of the body cavity on both sides of the perforated walls came to communicate in a new place.