Variation, Development, and Evolution of Head Structures in Ants

Abstract

Morphological diversification arises through the interaction of environmental, evolutionary, and developmental processes that determine a trait's size, shape, and scaling relationship. In ants, these processes are further structured by caste systems where discrete phenotypes emerge. This thesis investigates how allometry, developmental plasticity, and patterns of integration and modularity contribute to the diversification of caste-specific head morphology (antenna and mandibles) in the hyperdiverse ant genus Pheidole. Chapter 2 identifies a previously undescribed morphological pattern in antennal scape length, where scape-to-body scaling is decoupled between worker and soldier castes but scales within each caste. Comparative analysis across the genus reveals the pattern is widespread, while comparative analysis across ants reveals this pattern's possible evolutionary origin. Experimental manipulations of hormonal, pheromonal, and inter-organ signalling pathways demonstrate their role in regulating trait-to-body and their ability to access unrealized developmental potentials to generate novel phenotypes, including nanoworkers and nanosoldiers. Chapter 3 examines scaling within the antenna, demonstrating that individual segments follow varying scaling relationships with body size. Where intermediate segments vary with overall body size, and proximal and distal segments deviate, indicating modular organization of the antenna. Experimental manipulations of hormonal, pheromonal, and inter-organ signalling pathways demonstrate their role in regulating the modular components of the antenna. Chapter 4 investigates mandible morphology in terms of size and shape. Workers and soldiers have distinct size and shape, where conserved developmental timing suggests differences in patterning pathways, and inter-organ signalling of a soldier-specific organ is important soldier size but not shape. This indicates that size and shape for mandibles are governing by partially independent developmental pathways. Together, this thesis shows that morphological evolution in Pheidole is governed not only by changes in trait size and shape, but also the altering of scaling relationships through caste evolution and developmental regulation, highlighting how allometry, modularity, and integration interact to generate morphological diversity.