How beetles explode : new insights into the operation, structure, and materials of bombardier beetle (Brachinini) defensive glands

Abstract

Bombardier beetles possess one of the most remarkable defense mechanisms in nature, using explosions inside their bodies to synthesize and eject a hot, noxious spray at attackers. The chemical reactions that enable this process are well understood, but many aspects of the beetles' two-chambered defensive glands, which house the explosions and produce the defensive spray, remain unexplored. In this Thesis, I describe our recent progress in understanding the operation, structure, and materials composition of the defensive glands-topics which have to date received little treatment in the literature-of the best-known bombardier beetles, the brachinines (Carabidae: Brachininae: Brachinini). Chapter 2 deals with the pulsed-jet character of brachinines' sprays, which is in contrast to all other types of bombardier beetles that emit their sprays as continuous streams. Brachinine sprays comprise a number of spray pulses emitted in a rapid sequence, each pulse formed in a discrete explosion event inside the reaction chamber of the defensive gland, with the frequency of pulsation ranging from 300 to 1000 Hz. Using a combination of high-speed synchrotron x-ray phase-contrast imaging of live beetles, anatomical studies of the excised defensive glands, and mathematical analyses, we determined that spray pulsation arises due to explosion-induced displacement of the inlet structures to the reaction chamber periodically cutting o the ow of reactant solution into the reaction chamber. In Chapter 3, the interior cuticular microsculpture of the reaction chamber is studied using scanning electron microscopy and synchrotron x-ray phase-contrast microtomography. The microscupture is found to be highly complex, with a number of distinct spiny microtextures localized to specific regions of the reaction chamber. Quantitative details of the spine lengths and spacings are reported, and on the basis of the similarity of some of the features to the beetles' external abdominal microscupture and the micro-textural transitions observed inside the reaction chamber, we hypothesize that the reaction chamber microsculpture is homologous with the exterior microsculpture, consistent with the fact that the defensive glands are invaginations of the abdomen. Chapter 4 reports our preliminary investigations of the materials composition of the defensive glands. We use scanning electron microscopy to examine the fibrous composite structure of the gland cuticle and employ various light microscopy techniques to understand spatial variations in the cuticle sclerotization and chemical composition. The reaction chamber is found to exhibit dramatic spatial variation in sclerotization, including several lightly sclerotized regions, and possible functions of these regions are proposed. Additionally, the inter-chamber valve is found to contain the rubber-like protein resilin, likely as an adaptation to allow the valve to consistently make and hold a tight seal during each explosion, in analogy to rubber gaskets used in technological valve applications.