Abstract
The number of primary rays in the labral fan of filter-feeding black fly (Diptera: Simuliidae) larvae has been used by some authors to aid in species identification. We examined the number of primary labral fan rays in field-collected last-instar larvae of Simulium rostratum and Simulium decorum. Significant inter- and intra-specific differences were found both spatially and temporally. Simulium rostratum and S. decorum larvae were also reared to last instar (from eggs or first- or second-instar larvae) under three feeding regimes to test the effect of seston load on the number of primary labral fan rays. In both species the fan-ray number decreased when the food supply was increased. We conclude that the primary fan ray number should be used cautiously in species diagnosis because it is a phenotypically plastic trait that varies with seston load. A potential feedback mechanism to explain the observed differences in fan-ray number is hypothesised.
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