The evolution and functional morphology of trap-jaw ants

Key innovations are traits that allow organisms to interact with their environment in novel ways and are thought to facilitate adaptive radiation. By providing access to previously untapped resources, key innovations allow organisms to move into new ecological niches and can promote morphological diversification and speciation. I am interested in the evolution of form and function of one particular morphological innovation in the diversification of “trap-jaw” ants: power-amplified mandibles used for prey capture, nest defense, and individual escape from predators. Insects are the most diverse and numerically abundant animal group on the planet. One feature that contributed to their evolutionary success was the diversification their mouthparts. From an ancestral mandibulate condition (still found in many extant taxa), insect mouthparts have diversified into many specialized forms such as the piercing-sucking mouthparts of true bugs and various parasites, the sponging mouthparts of flies, and the extendible proboscis of butterflies and moths. This diversity has allowed insects to occupy a variety of dietary niches, including predation, herbivory, liquid feeding, and parasitism. An understanding of the relationship between structure and function of insect mouthparts is, therefore, critical for understanding their ecological success. My dissertation consists of four chapters and investigates the evolution and functional morphology the highly specialized mouthparts of trap-jaw ants. In Chapter 1, I review the current literature on trap-jaw ant taxonomy, phylogenetics, and biomechanics. The trap-jaw morphology has independently evolved at least four times in the ant family Formicidae, and, in this chapter, I highlight the areas of convergence among the four trap-jaw ant lineages. The most well studied lineage of trap-jaw ants are found in the subfamily Ponerinae, and consist of the sister genera Anochetus and Odontomachus. In Chapter 2, I present my findings from the first comprehensive worldwide phylogeny for these two trap-jaw ant genera. Using molecular sequence from four nuclear and one mitochondrial gene, I establish a phylogenetic framework for approximately half of the currently described species. Specifically, I confirm that the two genera are monophyletic sister groups, and found support for seven monophyletic clades. These trap-jaw ants diversified approximately 30 million years ago predominately in Southeast Asia, with multiple dispersal events to Australasia, the Afrotropics, and South America. Size often determines the output of animal performance systems, and examples of these scaling relationships are common throughout nature. What is unclear is if scaling relationships in musculoskeletal systems are shared within and between species. To answer this question, I examined morphological and performance scaling relationships between different sized trap-jaw ants and within a polymorphic species. I found that among species of Anochetus and Odontomachus, there is a strong and significant negative relationship between speed and body size, with larger and having longer snap durations and lower peak speeds. Contrasting with interspecific scaling relationships, the speed of mandible strikes within the polymorphic species Odontomachus turneri did not show any relationship with body size. Instead the peak kinetic energy of mandibles within and among Odontomachus species scaled with body size, suggesting that there may be stabilizing selection acting on mandible speed, but that strike energy may be determined by body size constraints. In Chapter 4, I examine the biomechanics, morphology and kinematics of the trap-jaw ant, Myrmoteras barbouri. A member of the ant subfamily Formicinae, Myrmoteras trap-jaw ants have received relatively little attention compared to other trap-jaw ant lineages and the mechanism of their spring-loaded mandibles have previously been unstudied. Using high-speed videography, I measured mandible strikes that occur in less than 1 millisecond and peak speeds of 2.6 x 104 rad·s-1. These speeds are faster than can be explained by direct muscle contraction, and confirm that Myrmoteras jaws are spring-loaded. The spring that stores the potential energy required for the strikes is a modification of the occipital margin, which bends during mandible loading. Compared with other trap-jaw ants, Myrmoteras jaws reach similar peak velocities, but accelerate over a much longer period of time, which is likely a reflection of their unique mandible mechanism