1. Ontogenetic shifts in predator behaviour can affect the assessment of food-web structure and the development of predator–prey models. Previous studies have shown that the diel activity pattern and functional response differed between larval instars of the carnivorous caddis, Rhyacophila dorsalis. The present study examines switching by larvae of R. dorsalis presented with different proportions of two prey types; either small (length 2–4 mm) and large (5–8 mm) Chironomus larvae for second, third, fourth and fifth instars of R. dorsalis; or Baetis rhodani (9–12 mm) and large Chironomus larvae for fourth and fifth instars. Experiments were performed in stream tanks with one Rhyacophila larva per tank and 200 prey arranged in nine different combinations of the two prey types (20 : 180, 40 : 160, 60 : 140, 80 : 120, 100 : 100, 120 : 80, 140 : 60, 160 : 40 and 180 : 20). Prey were replaced as they were eaten. A model predicted the functional response in the absence of switching and provided a null hypothesis against which any tendency to switch could be tested.
2. There was no prey switching in the second and third instars, with both instars always showing a preference for small over large Chironomus larvae. Prey switching occurred in the fourth and fifth instars. As the relative abundance of one prey type increased in relation to the alternative, the proportion eaten of the former prey changed from less to more than expected from its availability, the relationship being described by an S-shaped curve. In the experiments with small and large Chironomus, the two instars switched to large larvae when their percentage of the total available prey exceeded 29% and 37% for fourth and fifth instars, respectively. In the experiments with Baetis and large Chironomus, both instars switched to Baetis larvae when their percentage of the total available prey exceeded 36%.
3. Non-switching in second and third instars was related to their feeding strategies, both instars preferring smaller prey items. When the fourth and fifth instars foraged actively at night, they preferred larger over small Chironomus larvae, but when they behaved as ambush predators at dusk, they captured the more active Baetis larvae in preference to the more sedentary large Chironomus larvae and only switched to the latter when they were >64% of the available prey.