The explosive defensive system of paussine and brachinine bombardier beetles (Coleoptera Carabidae): comparative morphology and ultrastructure

The ground beetle subfamilies Paussinae and Brachininae, have always been of interest to scientists because of their ability to explosively eject a hot (up to 100°C) and irritating quinonoid secretion from pygidial glands, in response to attack by predators. The peculiarity of this defensive system led many scientists to consider "bombardier beetle" as a monophyletic lineage, emphasizing the general similarities in morphology, function and chemical reactions, despite the different ways of ejecting the spray (directing the abdomen in brachinines; using the elytral flange of Coanda in paussines). However, studies concerning fine morphology and ultrastructure of the components of the pygidial defensive systems (reaction chambers, valves, ducts, reservoirs, and glands that secrete H2O2, hydroquinones, catalases and peroxidases) are still lacking (paussines) or incomplete (brachinines). In order to render these issues available to a comparative advanced discussion, we performed a morphological and ultrastructural study of representative species belonging to the genera: Metrius (Metriini), Pachyteles and Goniotropis (Ozaenini), and Paussus (Paussini) for Paussinae; and Brachinus and Pheropsophus (Brachinini) for Brachininae. Samples were examined through optical microscopy (dissections, classical histology), scanning (SEM) and transmission (TEM) electron microscopy, and focused ion beam (FIB) microscopy. The relative parts of the defensive systems for each paussine and brachinine species were described, illustrated and comparatively analyzed. We found significant differences between the two systems at both cellular and microstructural levels. Our preliminary results raise some challenges to the acknowledged hypothesis of evolution of this peculiar defensive system that parallel other evidences on the relationship between paussines and brachinines.