Size-dependent flight initiation by a lotic mayfly in response to a predatory fish

1. Laboratory experiments were performed to determine whether flight initiation by lotic mayfly larvae of Baetis tricaudatus exposed to the longnose dace ( Rhinichthys cataractae ) is affected by an individual's size and its physiological state.
2. We used a three factorial ANOVA design to test whether flight initiation distances (FIDs) were affected by Baetis size (small, large), starvation level (low, high), and the length of a predator-free foraging period (short, long).
3. FIDs were significantly affected by the interaction between Baetis size and starvation level, and the main level effects of starvation and Baetis size. FIDs of small larvae were unaffected by starvation level and the length of the predator-free feeding period, whereas starvation reduced FIDs of large larvae 2-fold.
4. Subsequent experiments showed that size-dependent differences in FIDs could not be attributed to size-specific differences in the economics of rates of food intake or gut evacuation. For both small and large larvae, food intake rates declined with increasing time spent foraging and both small and large larvae consumed the majority (70–75%) of food within the first 3 h of the 12-h feeding period. Similarly, gut evacuation declined with increasing starvation time and rates of evacuation did not differ between small and large larvae. These data suggest that size-dependent differences in flight initiation by Baetis larvae do not involve the energetics of food intake or processing.