The hoverflies (Diptera:Syrphidae) represent an apparently paradoxical visual Batesian mimicry complex, with what appear to be "poor" Mimics outnumbering their more accomplished counterparts. The purpose of this thesis is to determine how far conventional mimicry theory is capable of explaining the apparent paradoxes of mimicry in the hoverflies. It becomes obvious that determining the mimetic status of the supposedly poor Mimics is not a trivial task. Conventional experimental tests of mimicry, using captive predators, seem incapable of predicting the degree of protection enjoyed by a Mimic in the field. The research therefore concentrates on developing some novel empirical approaches to the study of mimicry. This includes developing a method of image analysis which yields an objective, single-value measure of the similarity between Model and Mimic patterns. This index of similarity is used to produce unique descriptions of the structure of mimetic communities in terms of Mimic frequency and similarity to the supposed Model. These profiles indicate that there is an objective basis to the perceived paradox, and suggest that there is not a simple relationship between the actual and perceived similarity of two patterns. The perceived similarity of Model and Mimic will be a key determinant of mimetic success. The index of similarity is also used as a basis for direct comparison of the supposedly mimetic hoverflies with a more established example of mimicry in the butterflies. This exercise demonstrates that an index of pattern similarity enables a unique comparative analysis of mimicry. It is proposed that an index of similarity also provides a unique opportunity to test our theoretical understanding of mimicry, if it is used in conjunction with a mathematical model that possesses some specific attributes. A suitable prototype model is developed and demonstrated. The thesis concludes with an indication that the novel empirical approaches developed here, have been adopted elsewhere. This latter work indicates that those hoverfly species which are apparently "poor" Mimics, may be exploiting some constraint in predator perceptual and cognitive systems to achieve mimetic protection, despite a relatively low degree of actual similarity to the Model species.

Hoverflies (Diptera: Syrphidae) vary widely in their mimetic associations, comprising wasp-mimetic, bee-mimetic and non-mimetic species. Social wasp mimics are dominated by 'imperfect mimics' which outnumber their supposed models (Hymenoptera: Vespidae) by large factors. The purpose of this thesis is to determine to what degree Batesian mimicry can account for these paradoxes, and to test alternative hypotheses for the evolution of the yellow-and-black patterns. There is little evidence of an effect of wasp abundance on 'imperfect mimic' abundance across 23 years of trapping data, as predicted if mimics are protected from predators through their resemblance to wasps. The seasonal asynchrony and high abundance of 'imperfect mimics' relative to their models is also notable, as well as the possible significance of wasp predation on hoverflies. Predictions concerning the function of the colour patterns of 'imperfect mimics' are tested using the association between similarity to the model and flight agility (indirectly measured assuming a trade-off between reproductive potential and flight agility). There is no strong indication that mimetic protection is the primary function of the colour patterns, but the evidence concurs with an aposematic function, signalling to predators the unprofitability of attempting capture. These conclusions are tentatively supported by direct measures of flight agility, though the small differences among species are difficult to pick up. The data on reproductive morphology of hoverflies show considerable variation across species, especially in males. The existence of giant testes in some species suggests that methods of dealing with sperm competition in hoverflies are diverse and deserve further study. The high ratio of 'imperfect mimics' to both models and good wasp mimics is also partly explained by habitat disturbance; undisturbed habitats show significantly less 'imperfect mimics' as a proportion of the hoverfly population. Current relative abundance in the UK may therefore be very different to when the colour patterns evolved.