This thesis describes investigations of how chemicals present on the cuticle of ants impact three important features of social living in insects: hygiene and disease resistance; navigation; and interspecies chemical signalling. Eusociality brings many benefits, but also has the potential to make insect colonies vulnerable to disease. In Chapter 2 of this thesis I investigate the role of the antimicrobial agent micromolide, in the Yellow meadow ant, Lasius flavus. Micromolide is found to be present on the cuticle of L. flavus workers, and is also found to be deposited onto a substrate by walking ants, revealing a possible mechanism for maintenance of sanitary nest conditions. Chapter 3 of this thesis focuses on navigation in L. flavus, specifically route-memory formation and the possibility of home-range markings providing a chemical cue via which ants can navigate from a food source to the nest. It was found that allowing ants to follow a pheromone trail to food increased the number of navigational errors made by returning ants, and that home-range markings did not provide effective guidance to ants returning to the nest. In Chapter 4, I report on a project undertaken during field work in Brazil into how cuticular hydrocarbons (CHCs) of the ant Camponotus arborious can act as kairomones when detected by Nasutitermes corniger, a common termite species. Experiments showed that N. corniger is less likely to repair experimentally opened tunnels in the presence of C. arborious CHCs, with 4 of 7 colonies tested blocking up tunnels, rather than rebuilding over CHC marked areas. Finally, Chapter 5 of this thesis discusses potential future projects, following on from the work presented in Chapters 2, 3 and 4.

Group living is a widely adopted strategy by many organisms and given the advantages offered by a social lifestyle, such as increased protection from predators or increased ability for resource exploitation, a wide variety of animals have adopted a social lifestyle. Arguably none have done this more successfully than the social insects. Indeed their efficient division of labour is often cited as a key attribute for the remarkable ecological and evolutionary success of these societies. Within the social insects the most obvious division of labour is reproductive, in which one or a few individuals monopolise reproduction while the majority of essentially sterile workers carry out the remaining tasks essential for colony survival. In almost all social insects, in particular ants, the age of a worker will predispose it to certain tasks, and in some social insects the workers vary in size such that task is associated with worker morphology. In this thesis I explore the proximate and ultimate causes of worker and reproductive division of labour in ant societies, which span a range of social complexities. I predominantly focus on both the highly derived leaf-cutting ants – a so-called ‘pinnacle' of evolution within the social insects, with a complex division of labour and a strong worker caste system – and in the more basal primitive societies of the queenless ponerine dinosaur ants, which can offer an insight in to the evolution of division of labour at the earliest stages of social lifestyles. This work demonstrates the environmental and genetic determinants of division of labour in group-living societies outside of the classical honey bee model system. This is important as it helps us to better understand the broader processes shaping behaviour and phenotype in the animal kingdom.

In social insects, foraging is often cooperative, and so requires considerable organisation. In most ants, organisation is a bottom-up process where decisions taken by individuals result in emergent colony level patterns. Individuals base their decisions on their internal state, their past experience, and their environment. By depositing trail pheromones, for example, ants can alter the environment, and thus affect the behaviour of their nestmates. The development of emergent patterns depends on both how individuals affect the environment, and how they react to changes in the environment. Chapters 4 – 9 investigate the role of trail pheromones and route memory in the ant Lasius niger. Route memories can form rapidly and be followed accurately, and when route memories and trail pheromones contradict each other, ants overwhelmingly follow route memories (chapter 4). Route memories and trail pheromones can also interact synergistically, allowing ants to forage faster without sacrificing accuracy (chapter 5). Home range markings also interact with other information sources to affect ant behaviour (chapter 6). Trail pheromones assist experienced ants when facing complex, difficult-to-learn routes (chapter 7). When facing complicated routes, ants deposit more pheromone to assist in navigation and learning (chapter 7). Deposition of trail pheromones is suppressed by ants leaving a marked path (chapter 5), strong pheromone trails (chapter 7) and trail crowding (chapter 8). Colony level ‘decisions' can be driven by factors other than trail pheromones, such as overcrowding at a food source (chapter 9). Chapter 10 reviews the many roles of trail pheromones in ants. Chapters 11 – 14 focus on the organisation of cooperative food retrieval. Pheidole oxyops workers arrange themselves non-randomly around items to increase transport speeds (chapter 11). Groups of ants will rotate food items to reduce drag (chapter 12). Chapters 13 and 14 encompass the ecology of cooperative transport, and how it has shaped trail pheromone recruitment in P. oxyops and Paratrechina longicornis. Lastly, chapter 15 provide a comprehensive review of cooperative transport in ants and elsewhere.

Parasites represent a considerable and ubiquitous threat to organisms, and studies of host-parasite interactions can demonstrate important insights into key biological processes. Identification and quantification of host defences and their role in parasite resistance is an important part of understanding these effects. Additionally, life-history traits can have significant effects on host-parasite interactions. For example, living in groups has many benefits, but also may have associated costs in terms of increased parasite transmission. Thus group-living animals may be predicted to invest heavily in disease resistance strategies, though which may depend on each species' parasite pressure. Social insects, and ants in particular, are an ideal model with which to test these evolutionary and ecological hypotheses, as they possess an array of mechanisms to defend themselves against disease and have highly diverse life-histories. However, previous studies into disease resistance tend to have been performed on single species, often looking at just single measures of investment of defence. In this thesis I explore the comparative importance of disease resistance in different ant species. I show that ants possess a variety of defence mechanisms to protect themselves against the threat of parasites and demonstrate how investment into these important defences can vary between individuals and species, and may depend on context, type of parasite, and life-history of the host. Work such as this, demonstrating the costs of individual components of disease resistance in multiple species, is important in developing our understanding of how changes in parasite pressures can influence host biology and how organisms can survive in a world abundant with parasites.

No description available.