Why do caddisfly wings have a hairy appearance?

Caddisflies belong to the order Trichoptera, a name derived from the Greek words trichos (hair) and ptera (wing). Their wings appear hairy because they are either covered in fine hairs or possess hairs specifically along their wing veins.123

These mothlike insects typically fold their wings in a rooflike manner over their abdomens when at rest. While the presence of these hairs is a defining taxonomic feature of the order, the wings themselves are membranous.231