INVESTIGATION OF THE WITHIN-SAMPLE AND BETWEEN-SAMPLE COLOR PATTERN DIFFERENTIATION IN HYDROPSYCHE CONTUBERNALIS (TRICHOPTERA)

The caddisfly Hydropsyche contubernalis (Trichoptera; Hydropsychidae) occurs in rivers and large streams throughout Europe. The aquatic lan ae of this species are highly variable in their head capsule colour pa tterns. This paper describes the differentiation of such a pattern wit hin and between 13 samples from thr ee different West European river b asins. The main aim is to investigate whether differences in colour pa ttern exist between river basins. Because the head capsule colour patt erns of H. contubernalis do not have clear detectable morphs, an image analysis technique is presented to quantify the pattern in the form o f mean colour curves expressed as grey levels. These curves are descri bed by mathematical spline functions, and the differences between thes e functions, which formalize the original data set of colour curves, a re analysed by multivariate Between-Group Principal Component Analysis . Our results show that distinct head capsule colour patterns occur in the Loire and Rhone, individuals of the Rhone being darker than indiv iduals of the Loire. However these results also suggest that important differences in such a colour pattern can be detected at smaller scale s: along the whole length of the Loire; within a 10 km stretch of the Doubs, and also within a single site on the Loire (Amboise). Temporal differences at the same site between different H. contubernalis genera tions were also recorded. The mechanisms responsible for such differen tiation patterns are discussed but still remain unclear. The overall s tatistical procedure detailed in this paper could potentially be used in numerous other fields of evolutionary biology. (C) 1997 The Linnean Society of London.