Looking down on this fast-flowing stream in Shakerag Hollow we see wavy lines on the submerged rocks. Moss? Algae? No, ...
...these are clusters of blackfly larvae (Fanily Simuliidae). Each larva is attached to the rock with claspers on its rear end; silky threads provide additional anchorage. They prefer the fastest flowing parts of the stream, so these attachments are strong.
With a flash photo, we can see the larvae leaning with the current. They thrive in the oxygenated water of unpolluted streams.
Two fan-like feeding appendages crown each animal. The fans are used to sieve bacteria, algae, and small pieces of plant material from the flowing water. In good conditions, every few seconds the fans sweep food into the animal's mouth.
After a few weeks, the larvae pupate underwater (still attached to the rock in flowing water), then emerge as the infamous flying adults. Males feed on nectar, but females want blood. Unlike the swarms that emerge in northern states, blackflies are seldom numerous here.
David George Haskell 
Cool! Something new to look for!
Why I once loved studying biology–worlds within worlds within worlds.