Flying insects, like many other animals that rely on their sense of vision to guide behaviour, have a tendency to maintain a default orientation of their eyes relative to the environment. During flight, reflexes act to keep the head level and minimise retinal image shifts resulting from rotational steering manoeuvres, or from external perturbations such as wind gusts and turbulent air flow. Gaze stabilisation serves a number of functions, which include: i) simplifying the estimation of translational self-motion, ii) aligning the head-based sensory systems with the inertial vector which facilitates the transformation of sensory signals into motor commands, iii) supporting the tracking of moving targets, and iv) reducing motion blur in the visual input. This thesis reports studies on species-specific adaptations and general principles underlying multisensory gaze stabilisation in a number of different Dipteran flies. A variety of stimulation methods were explored, along with their suitability for a linear systems analysis of the gaze stabilisation system across species. Using results obtained from the well-characterised blowfly for comparison, novel experimental work was performed on the gaze stabilisation behaviour of robberflies, hoverflies and horseflies. Species from each family were shown to stabilise their heads in compensation for body rotations around the roll axis. The performance of the reflex was found to be species-specific and dependent on the sensory modalities involved. Experimental evidence suggests that in contrast to the other families, hoverflies appear to make use of the inertia of the head to maintain a level gaze, a novel finding that has previously been reported only for dragonflies. Finally, the integration of signals in the context of gaze stabilisation obtained by the two visual systems in blowflies - the ocelli and compound eyes - were explored in both behavioural and electrophysiological experiments. This research opens new lines of investigation by identifying behaviours that demonstrate different control strategies employed by the nervous systems of flying insects.

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In this thesis I use Drosophila melanogaster as a model organism to study the possible cognitive mechanisms controlling plastic behavioural responses to sperm competition. This plastic behaviour involves a male D. melanogaster responding to the presence of a rival male by increasing mating duration when housed with a female. I provide a general context to the work (Chapter 1) before examining my model in more temporal detail by investigating how the length of time males were exposed to a high sperm competition environment affected maintenance time of the plastic behaviour. I show that for males to accurately portray the sperm competition environment in their behaviour over a useful timescale they must possess accurate sensory systems. Without these, behaviour is still fully plastic, but change occurs at a slower speed than males with full sensory ability (Chapter 3). I then show that extended mating duration is controlled by a suite of well-known learning and memory genes highlighting the need for specific memory pathways to reflect ecological change (Chapter 4). However, those same genes do not change in their expression due to increased sperm competition, potentially pointing to some other mechanism of temporal change underlying the behavioural change (Chapter 5). Due to this reliance on learning and memory, I show that an increase in sperm competition can affect cognitive ability, and increase expression of synaptic genes over a longer time period (Chapter 6). Finally, I summarise my thesis findings and discuss how future research can build on the research presented to develop the field (Chapter 7). My research shows that learning and memory is paramount for males to react to changes in the sperm competition environment on a relevant timescale where behaviour and the environment have not become mismatched. In addition, I show that sperm competition pressures can cause an increase in male individual cognitive ability, posing the question of whether competition is one of the main drivers of non-mammalian cognitive ability.

Peptides and biogenic amines are important regulators of muscle activity in reproductive tissues, and therefore can play important roles in the reproductive success of insects. The role of neuropeptides was investigated in regulating contractions of the male accessory gland (MAG) and the ejaculatory duct (ED). The FlyAtlas tissue expression database indicated that DMS, dromyosuppressin (DMS) pro-hormone, is highly expressed in the MAGs of D. melanogaster. However, DMS, could not be found using HPLC and mass spectrometry, but was detected by immunohistochemistry which revealed extensive staining of neuronal processes on the surface of MAGs and ED of Drosophila melanogaster, D. yakuba, D. erecta, D. virilis, D. simulans, and the pest species D. suzukii. The GAL4-UAS expression system revealed DMS neuronal fibres descending from the abdominal ganglion onto the surface of the MAG, ED, and seminal vesicles. A pair of rectal cells and a single cell, that we have called the ejaculatory duct cell because of its projections to the ED, also contain DMS. The rectal and ED cells were isolated and subjected to MALDI-TOF mass spectrometry. Molecular ions of several neuropeptides were detected, and the identity of DMS and sNPF4-11 were confirmed by fragmentation sequencing. Both DMS and sNPF 4-11 reduced the frequency of contractions of the MAG and ED, suggesting that these peptides are involved in regulating ejaculation of seminal fluid and spermatozoa. Furthermore, the finding of DMS-R1 and DMSR2 expression in the epitracheal cells suggested that both receptors might be involved in adult ecdysis. The silencing of DMS expression using nSyb-GAL4/UASdicr2 > DMS-RNAi (108760), shows abnormal abdomen phenotype and mortality at day 5 post-emergence. Inhibitors of neuropeptide metabolism were shown to be larvicidal when fed to two species of mosquitoes (Anopheles gambiae and Aedes aegypti) identifying an enzyme target for the development of novel control chemicals.

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Nutrition is a vital determinant of lifespan, reproduction, health and ageing. Much has been done to investigate the lifespan consequences of short-term (proximate) nutritional manipulation, but much less is known about long-term (evolutionary) nutritional manipulation and nutritional mismatches. In this thesis I addressed this important omission, using the fruit fly, Drosophila melanogaster, model system. I empirically tested two evolutionary theories: the Thrifty Phenotype and Thrifty Genotype hypotheses, which predict the general life history consequences of nutritional mismatches over the lifetime or over evolutionary time, respectively. I also tested how the latter interacted with long-term nutritional selection regimes. Contrary to predictions, I showed that the costs of nutritional mismatches between developmental and adult diets were not universal, but instead dependent on the nature of the mismatch, sex and the components of life history measured (Chapter 2). Similarly, the costs of mismatches between evolved and proximate nutrition were dependent on evolved feeding regime, sex, life history component measured and proximate diet (Chapter 3). I discovered that there was enhanced sexual dimorphism for lifespan in nutritionally selected lines, which was associated with sex-specific life history patterns and a partial resolution of sexual conflict (Chapter 4). Transcriptome-wide analysis of these nutritionally selected lines revealed differential expression in genes with functions related to lifespan, post-mating responses, regulation and epigenetic modification (Chapter 5). Finally, I found that manipulation of another important component of altered lifestyles, activity level, had no effect on lifespan or reproduction (Chapter 6). Overall, my results make a novel contribution to the study of nutritional mismatches and long-term nutritional selection. The results also highlight the importance of simultaneously studying both sexes and several age-specific components of life history, in different proximate environments, to fully elucidate the fitness consequences of nutritional manipulation.