Investigation of the within- and between-sample colour pattern differentiation in Hydropsyche contubernalis (Trichoptera)

The caddisfly Hydropsyche contubernalis (Trichoptera; Hydropsychidae) occurs in rivers and large streams throughout Europe. The aquatic larvae of this species are highly variable in their head capsule colour patterns. This paper describes the differentiation of such a pattern within and between 13 samples from three different West European river basins. The main aim is to investigate whether differences in colour pattern exist between river basins. Because the head capsule colour patterns of H. contubernalis do not have clear detectable morphs, an image analysis technique is presented to quantify the pattern in the form of mean colour curves expressed as grey levels. These curves are described by mathematical spline functions, and the differences between these functions, which formalize the original data set of colour curves, are analysed by multivariate Between-Group Principal Component Analysis. Our results show that distinct head capsule colour patterns occur in the Loire and Rhône, individuals of the Rhône being darker than individuals of the Loire. However these results also suggest that important differences in such a colour pattern can be detected at smaller scales: along the whole length of the Loire; within a 10km stretch of the Doubs, and also within a single site on the Loire (Amboise). Temporal differences at the same site between different H. contubernalis generations were also recorded. The mechanisms responsible for such differentiation patterns are discussed but still remain unclear. The overall statistical procedure detailed in this paper could potentially be used in numerous other fields of evolutionary biology.