Comparative Morphology of the Extrinsic and Intrinsic Leg Musculature in Dictyoptera (Insecta: Blattodea, Mantodea)

Abstract

Insect legs, as primarily locomotory devices, can show a tremendous variety of morphological modifications providing a multitude of usages. The prehensile raptorial forelegs of praying mantises (Mantodea) are a prominent example of true multifunctionality since they are used for walking while being efficient prey-capturing and grasping devices. Although being mostly generalist arthropod predators, various morphological adaptations due to different environmental conditions occur across Mantodea. Recently, the general mantodean morphology, and particularly their raptorial forelegs, received an increased interest. Yet, knowledge about the evolutionary transition from walking to prey-grasping legs is still scarce. From evolutionary and functional perspectives, the question arises: what changes were necessary to achieve the strongly modified raptorial forelegs—while keeping walking ability—and how does the foreleg morphology differ from the remaining four walking legs? In this context, we investigated the musculature of the raptorial forelegs in seven phylogenetically distant mantodeans, including pterothoracic legs in four of them, using high-resolution microcomputed tomography and dissection. To understand the results from an evolutionary perspective, we additionally examined all three pairs of unmodified walking legs of the closest sister group—Blattodea. We updated the knowledge of blattodean morphology, revealing differences in cuticle structures of the coxal articulation of the first pair of legs between the two orders and a shared musculature set-up in all pairs of legs among later-branching mantodeans. Interestingly, the early branching species Metallyticus splendidus and Chaeteessa sp. show several muscular characteristics, otherwise found exclusively in one or the other order, with a few procoxal muscles showing an intermediate state between the two orders. Studying the evolutionary transition from a walking leg to a raptorial leg will help to understand the character evolution of this highly specialized biomechanical system from a purely locomotory appendage to a multi-functional device with all related amenities and constraints.