Chemical Defense Mechanisms Constrain Elytron Shape and Body Size Macroevolution in Darkling Beetles (Coleoptera: Tenebrionidae)

Chemical defense mechanisms have been extensively studied across Coleoptera. Many beetle lineages have gained or lost chemical defense mechanisms independently, but the evolutionary consequences of such shifts have been largely unstudied. This study uses stochastic mapping to estimate the number of evolutionary gains and losses of chemical defense glands in the long-jointed beetles (Tenebrionidae: Lagriinae). We also use 2D geometric morphometrics to quantify the evolutionary consequences of chemical defenses on elytral shape, as well as body and elytron size. We reveal multiple transitions between character states of chemical defense mechanisms across the evolution of the subfamily Lagriinae, and we demonstrate that those lagriine taxa without chemical defenses have smaller and more variable body and elytron sizes than those that have retained chemical defenses. These results establish that although many lineages of Lagriinae beetles have chemical defenses, possessing them may inflict an evolutionary constraint on elytron and body size across macroevolutionary time.