PHENOTYPIC PLASTICITY IN A SUSPENSION-FEEDING INSECT, SIMULIUM-LUNDSTROMI (DIPTERA, SIMULIIDAE), IN RESPONSE TO CURRENT VELOCITY

The morphology of the labral fans of blackfly larvae is intimately int errelated with the lotic habitats in which they live. Marked seasonal variations in the primary fan morphology of Simulium lundstromi were f ound in a natural population. In separate experiments, larvae of the o verwintering and summer generations were reared at two different curre nt velocities. For both seasons, larvae living in ''slow'' velocity (8 cm s(-1)) flumes developed larger primary fans with wider distance be tween 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 balanc ing particle encounter rates with malfunction of the fans caused by th e effects of high drag. However, larvae of both generations had low su rvival rates and long development times at slow current velocities com pared with those in faster flow, because the morphological modificatio ns could not compensate completely for the reduced delivery rate of fo od particles resulting from the low current velocity. The observed pla sticity of fan morphology in response to variation in current velocity is consistent with the natural conditions found in the stream where t he larvae lived. This suggests that phenotypic plasticity in blackfly larvae has evolved in response to the intrinsically heterogeneous natu re of running water environments.