Shift from a herbivorous to a carnivorous diet during the larval development of some Rhyacophila species (Trichoptera)

Rhyacophila larvae are aquatic free living Trichoptera, which are commonly considered as carnivorous predators by stream ecologists. We carried out gut contents analyses of each of the five larval instars of four Rhyacophila species, and we found that all larvae started their development (from the first to the first three instars) with a strictly phytophagous diet (diatoms and filamentous algae). The larvae became strictly carnivorous since instar III, IV or V, depending on the considered species. Both young herbivorous and older carnivorous larvae occurred together in the stream, during 10 and 7 months in the two species R. meridionalis and R. occidentalis respectively. Therefore, the functional role of these 'predators' within macroinvertebrate communities should be interpreted more conservatively. Some implications for common ecological approaches are discussed.