Interactions between syrphid predators and their prey are poorly known. The adaptations of syrphids to aphid defences and the consequences for the evolution of life history traits in these predators especially are mostly unstudied. This is the first of two papers investigating the evolution of prey specialization in aphidophagous hoverflies. The study focuses on two questions: (1) Are differences in the body size of syrphid predators reflected in differences in the size of their prey? (2) Are differences in body size, body mass and development time of the syrphid predators correlated with the defence strategies of their aphid prey (e.g. mobility, toxicity)? Platycheirus clypeatus (Meigen, 1822), Platycheirus fulviventris (Macquart, 1829), Melanostoma mellinum (Linnaeus, 1758), and Melanostoma scalare (Fabricius, 1794), which differ considerably in their prey specialization, but are closely related, were chosen as model species. Life history data for these syrphids came from a laboratory study, and that for the aphids from a literature survey. These syrphid species can be arranged on a gradient of increasing prey specialization, from 32 prey species for the generalist M. mellinum and only 3 for the specialist P. fulviventris. Differences in prey specialization were even more evident when the defence ability of the various species of aphid prey was considered. For instance, the specialization on ant-attended aphids in M. scalare. Larvae exhibited a one or two weeks diapause which made the determination of developmental time imprecise. Body size of the predators was not reflected in that of their aphid prey. The postulated relationship between body size of the predator and the defence strategies of their prey was not supported by our data. A comparison of a wider range of syrphid species from different taxonomic groupings together with a phylogenetic correction procedure might reveal clearer trends. The second part of this paper (Dziock, in prep.) will investigate the correlation between prey specialization and reproductive strategies (i.e. clutch size, egg size and number) and will put the results into a broader framework.
Since the publication of papers by Polis et al. (1989, 1992), intraguild predation (IGP) has been considered in numerous systems and a growing number of papers have been published, demonstrating the ubiquity and the importance of this interaction. It appears that aphidophagous guilds constitute especially favourable systems for IGP. Temporal and spatial distributions of aphids promote interactions, such as IGP, among natural enemies. However, despite extensive laboratory and field studies, several questions remain to be answered: Which traits of the different protagonists of the system may promote or hamper the occurrence of IGP? How do ants tending aphids modulate the interaction? How should one consider the "sub-lethal" effects (and the defensive mechanisms) associated with the risk of IGP? May IGP disrupt or enhance aphid control? These different issues are discussed by focusing on the predatory species in the complex system of aphidophagous guilds. It appears that, first, IGP may constitute one of the main forces influencing the structure and dynamics of aphidophagous guilds, and therefore it has to be taken into account in research studies, and second, because of the ephemeral nature of aphid colonies and the great number of species belonging to aphidophagous guilds, understanding all the implications of the interaction, both direct and indirect, constitutes a very complex task.