Phenotypic Plasticity in a Suspension-Feeding Insect, Simulium lundstromi (Diptera: Simuliidae), in Response to Current Velocity

Abstract

The morphology of the labral fans of blackfly larvae is intimately interrelated with the lotic habitats in which they live. Marked seasonal variations in the primary fan morphology of Simulium lundstromi were found in a natural population. In separate experiments, larvae of the overwintering and summer generations were reared at two different current velocities. For both seasons, larvae living in slow velocity (8 cm s -1 ) flumes developed larger primary fans with wider distance between individual rays, and longer stems than those exposed to fast current velocities (31-35 cm s -1 ). These morphological adaptations apparently serve to enable feeding at different flow regimes by balancing particle encounter rates with malfunction of the fans caused by the effects of high drag. However, larvae of both generations had low survival rates and long development times at slow current velocities compared with those in faster flow, because the morphological modifications could not compensate completely for the reduced delivery rate of food particles resulting from the low current velocity. The observed plasticity of fan morphology in response to variation in current velocity is consistent with the natural conditions found in the stream where the larvae lived. This suggests that phenotypic plasticity in blackfly larvae has evolved in response to the intrinsically heterogeneous nature of running water environments.