And Yes, The Black Fly Larvae Have "Colonized" a Lot of Our Streams

They're actually kind of cute when seen individually.  But that's not how we normally see them.  At the moment, when you look into some of our streams -- e.g. the Doyles River and Buck Mt. Creek -- this is what you're likely to see on the tops of large rocks.

Or sometimes you find them on leaves when you're sorting through leaf packs.

Not very appealing.  Large colonies of black fly larvae (family: Simuliidae) are common in a lot of our streams -- often very good streams -- in the winter.  It's just their season.   The black fly genus I find in the winter (roughly November through March) is Prosimulium (general tolerance value: 4.5); the genus I find the rest of the year is Simulium (general tolerance value: 4.9).   The defining characteristics of Prosimulium larvae are the dark tips of the antennae (click on the photo to enlarge),

and the fact that the "cervical sclerites" (the little black dots at which the arrows are pointed) are enclosed by the postocciput.

(On Simulium larvae, the arms of the postocciput are further apart, leaving the sclerites baldly exposed at the back edge of the head.)

Black fly larvae like to colonize rocks that are in the main flow of the current.  You'll often find them on rocks over which the current spills at the head of a riffle.  This makes very good sense.  Black flies are "filter collectors," in terms of their Functional Feeding Group classification, using their head hairs -- I like to think of them as their "whiskers" --  to "filter/glean" nutrients out of the water: their main source of food.

The large numbers of black fly larvae in our streams at this time of year present stream monitors with a genuine problem.   With their high tolerance values -- Virginia DEQ uses a single value of 6.0 for all black fly genera -- when large numbers of them show up in a sample because someone rubbed the "wrong" rocks, a stream's "score" can plummet.  (I once ended up with 700 larvae from a 1 second net!)   But large numbers of black flies in winter samples do not indicate unhealthy streams.  They're just there at this time of year,  so sampling methods must be devised that make adjustments for this situation.  I'm not proposing a "revised" method: I'm happy that those kinds of matters are dealt with by others!

So, I've done my bit on black flies for the winter -- and now I intend to return to my focus on mayflies, caddisflies, and stoneflies!