Silk is made by many species of insects and spiders, and used in many roles. Caddisfly larvae occur in freshwater lakes, streams, and rivers worldwide. Like their relatives, the caterpillar stage of moths and butterflies, they use silk. This species, Arctopsyche irrorata, uses silk to make a safety retreat and a capture-net. The net is spun from two glands – each of which is a chemical factory that makes two different protein compounds and which yield the paired strands. The strands get their tensile strength from the protein fibroin, but are coated with sericin, a glue-like protein that also glues the intersections together. The silk is a liquid until pulling forces during extrusion align the protein molecules, turning it into a solid. The completed seine is approximately 1cm in diameter, and is a mesh of repeated, equally sized rectangles. Mesh size is species-specific, and some physical properties correlate with the velocity of the water in their preferred section of river. Algae, diatoms, organic debris, and even small insects are strained from the water, which are collected into the mouth by special tooth-brush shaped hairs surrounding the mouth. Caddisfly larvae and other freshwater insects are often used to gauge water quality and pollution; these net-spinning caddisflies build aberrantly shaped nets when heavy metals pollute the water. Some species of stoneflies, another group of aquatic insects, can be seen patrolling the mesh and getting an opportunistic meal. This image was captured with a JEOL 5300 scanning electron microscope at 500X magnification.
Electron microscopy time provided by the Jordan Hall Imaging Facility; Darryl Krueger, Director.