Dissertation

The Behavioural, Chemical, and Morphological Basis of Caste Regulation in the Worker Caste of Ants

Lillico-Ouachour, Angelica

2017

Overview

The generation and regulation of castes is critical for the success of a eusocial insect colony. The hyperdiverse ant genus Pheidole has a worker caste divided into two subcastes: small-bodied minor workers and big-headed soldiers. The physical distinction between subcastes enables minor workers and soldiers to specialize on different tasks. Minor workers perform the majority of day-to-day tasks, while soldiers preferentially perform particular foraging and defensive tasks. To partition tasks efficiently, colonies regulate the proportion of soldiers and minor workers in the nest in response to a number of factors that influence individual development. My thesis is divided into 3 chapters centered on the worker subcaste and its regulation in Pheidole. In Chapter 1, I summarize the ecological, social, and developmental interactions that both regulate subcaste ratios in a colony and facilitate their evolution. This review highlights the significant advances made over the last 100 years and the areas that need more consideration. One such area is to understand how social interactions regulate individual development through pheromones. When soldier numbers are too high in a Pheidole colony, a 'soldier inhibitory pheromone' released by adult soldiers is thought to inhibit larvae from developing into soldiers. In Chapter 2, I identified a behavioural mechanism by which soldiers transmit the soldier inhibitory pheromone to larvae and uncovered potential chemical candidates for this pheromone that are compatible with the behavioural model. I found that soldiers transmit the soldier inhibitory pheromone by selectively and passively contacting larvae. As many contact pheromones in insects are cuticular hydrocarbons, I analyzed cuticular hydrocarbon differences between subcastes. I found that soldiers have more saturated hydrocarbons (alkanes) on their cuticle than minor workers. Based on this finding, I propose that alkanes are good chemical candidates for the soldier inhibitory pheromone. Lastly, in Chapter 3, I tested whether cephalic glands which produce many semiochemicals also differed between subcastes. I compared head structures, including glands, for minor workers and soldiers in 4 Pheidole species: Pheidole dentata, Pheidole hyatti, Pheidole moerens, and Pheidole spadonia. I found that although soldiers have more muscle than minor workers, they have smaller nervous systems and similarly sized cephalic glands. The finding that soldiers have similarly sized cephalic glands to minor workers is consistent with a hydrocarbon-derived soldier inhibitory pheromone because hydrocarbons are produced in individual secretory cells throughout the body. Taken together, the research summarized in my thesis integrates behavioural, chemical, and morphological perspectives to improve our understanding of how complex worker caste systems are regulated.

• Publisher: ProQuest Dissertations & Theses
• Subject: Entomology / Esophagus / Food / Nervous system
• ISBN: 9798582585053