Phenotypic plasticity of larval retreat design in a net-spinning caddisfly

Larval Macrostemum carolina caddisflies construct silken catchnets within protective retreats, often on submerged trees and branches (i.e., snags). In the Savannah River, M. carolina larvae construct three distinct retreats that differ in the configuration of the water entrance hole: (1) at the end of a silken tube, (2) flush with the top of the retreat, and (3) backed by a ∼180-degree silken backstop. To identify the proximate mechanism mediating this retreat polymorphism, we removed larvae of known phenotype from their original retreats and brought them into the laboratory, allowing them to construct new retreats. We found that (1) larvae can construct more than one type of retreat, so variation in this behavior is not under strict genetic control; (2) larvae do not preferentially reconstruct their original retreat design, so these alternative behaviors apparently exhibit little heritability; and (3) larvae primarily construct each phenotype in a particular microhabitat (i.e., "tube" and "backstop" retreats are principally constructed on the downstream half of the snag, and "flush" retreats on the upstream--bottom quadrant). Therefore, the retreat polymorphism in M. carolina is phenotypically plastic and is controlled by microhabitat location or a correlated environmental variable. Most net-spinning caddisflies construct their nets in fairly specific microhabitats. However, behavioral plasticity allows M. carolina larvae to construct retreats all around a snag, thereby reducing potentially intense competition for space with other net-spinning caddisflies. Consequently, this may be the ultimate reason this polymorphism evolved. Copyright 2003.