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The problem of interpreting the small systematic displacements of solar absorption lines towards longer wavelengths relative to the corresponding laboratory wave-lengths – the so-called solar red shifts – has ranked as one of the most controversial problems in solar physics ever since its discovery by Jewell in 1896. The observational and theoretical difficulties which confronted the pioneer workers in this field are reviewed in Chapter I of this thesis. The year 1920 marks the beginning of a new phase in the development of the problem, since this was when Saha introduced his Ionization Theory which formed a completely new conception of the physical conditions prevailing in the solar atmosphere. At the same time, the announcement that an eclipse experiment made in 1919 appeared to confirm Einstein's prediction regarding the value of the light deflection, encouraged the belief that the gravitational red shift was implicit in the observed values of the solar red shifts; consequently, the latter were taken as resulting from a superposition of this predicted displacement upon the Doppler effects of radial currents in the solar atmosphere. The validity of this relativity-radial current interpretation is examined in Chapter II on the basis of observational data presently at our disposal. Our survey serves to show that, although many features of the solar red shifts can be explained by assuming that the steady state of the solar atmosphere is being maintained by a microscopic circulation associated with the solar granulation, the well-established observational fact that the absolute (Sun-arc) displacements are generally in excess of the relativity value at the edge of the disk (where the Doppler effects should vanish) is in contradiction to the conventional interpretation. This difficulty was fully appreciated by Professor Freundlich, who held the opinion that it might arise as a result of attempting to fit the observational data into a framework to which they did not rightly belong. It was this attitude which stimulated Freundlich (1954) to propose his revolutionary hypothesis that the red shifts observed in stellar spectra were produced as a result of some unknown interaction mechanism whereby light loses energy as it travels through space. Since this view did not appear to be compatible with the existence of the gravitational red shift, it was considered to be of the utmost importance to analyse the solar red shift data – which were much more reliable than those based on stellar observation – without taking it for granted that this effect existed, and determine whether the observed value supported Freundlich's interpretation. This was the original object of the present research, begun in September 1955. The major results of the initial two years' work carried out by the writer in collaboration with Professor Freundlich at the Department of Astronomy of St Andrews University are contained in the three reprints from the Annales d'Astrophysique which are submitted along with this thesis: they appear to confirm the validity of Freundlich's hypothesis, at the same time revealing no indication of the predicted gravitational red shift. Nevertheless, it was recognised that these conclusions were necessarily based upon a relatively small amount of observational material, and efforts were made by the writer to collect additional data with a view to investigating in more detail the observational properties of the solar red shifts, with particular reference to their dependency upon wavelength and excitation potential. Chapters III and IV are concerned with a description of the observational programme carried out with this intent at the Osservatorio Astrofisico di Arcetri, Italy, and the Universitäts-Sternwarte, Göttingen during the three-year period between October 1957 and September 1960. Much has happened during this time, however, which has cause us to reconsider the problem of the solar red shifts in an entirely different light. The present viewpoint is expressed in Chapter V.

M.S.

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.

Oscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.

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