Size and Shape Variation in Syrphus ribesii L. (Diptera, Syrphidae)

Syrphus ribesii is a common and highly successful hoverfly. I identify here size and shape differences within and between the sexes. Males have shorter, relatively broader wings, a bigger thorax, and a narrower abdomen than females. The labellum, a fleshy pad at the end of the proboscis, is involved in feeding, and its size is closely correlated with body size in males, but varies more independently in females. Labellum size is the most important contributor to shape variance in females: abdomen size is the corresponding variable in males. Male measurements show negative skewness and kurtosis both in individual variables and when compared with a multivariate-normal distribution. Females show no deviations from uni- or multivariatenormal distributions. Adaptive reasons for these differences are suggested: male size and shape may be adjusted for the ability to catch females by high-speed chases in cool weather early in the morning; female shape may be influenced mainly by feeding behaviour.