The fine structure of capture nets of larval Philopotamidae (Trichoptera), with special emphasis on Dolophilodes distinctus

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Abstract
Philopotamid larvae spin, in flowing waters, capture nets that have the smallest mesh opening sizes recorded among trichopterans. Sac-like nets of final-instar Dolophilodes distinctus are up to 6 cm long and 8 to 15 cm in circumference. Both larval nets and mesh opening sizes increase with instar, those of final instars being about 0.5 × 5.5 μm. Based on scanning electron microscope examination, a probable net construction sequence is proposed. The structure of the silk-gland opening suggests that a larva can simultaneously spin about 70 of the smallest (0.15 μm diameter) silk strands. Mature larvae may have about 100 million meshes per capture net and have silk strands totaling over 1.0 km in length, but the small diameter of these strands represents a small volume of silk. Food of D. distinctus consists primarily of fine particulate organic matter (FPOM), with associated microflora, and some diatoms. Larvae possess specialized feeding structures for collection of FPOM from the net. Nets of a Wormaldia sp. examined consisted of several layers of elongate rectangular meshes (about 0.4 × 3.7 μm). Underlying net layers are arranged diagonally, resulting in further reductions in effective mesh size to dimensions of about 0.4 × 0.4 μm. Larval nets of Chimarra spp. examined have both elongate rectangular mesh-opening shapes and nets apparently consisting of randomly arranged silk strands.