This insect is often quoted as being Bibio johannis but is, in reality, a number of very similar species. From the angler's perspective this is a non-issue. All that matters is that the name refers to a small, black terrestrial - end of story.
The various species that we call the black gnat are generally much smaller than either the hawthorn or heather fly and may be imitated on hooks ranging from a size 14 down to an 18.
A simple but very effective imitation can be tied using a pinch of black dubbing and a couple of pale grey CdC plumes. When tying CdC on small hooks it can sometimes be difficult to fix the feathers securely very close to the eye, without creating an overly large head. To combat this, the wing is set slightly further back down the hook, so that more locking turns of thread may be added. To disguise these thread-turns a small dubbed thorax is added. The fur is then teased out with Velcro to increase the number of water-grabbing fibres, which when smeared with Gink help the fly to float.
Hook Size 14-18
Body Black SLF
Wing Pale grey CdC
Thorax Black SLF
1. Run the thread on at the eye and wind down to bend. Dub on a small pinch of black fur and wind along shank to form the body.
2. Stop once fur covers three-quarters of shank. Select two, small pale grey CdC plumes. Make sure they’re fluffy.
3. Place the two plumes together so their tips are level. Now, position them on top of hook and catch in with a couple of thread turns.
4. Secure wing with thread turns before trimming waste feather ends. Dub on a second pinch of fur to form the thorax.
5. Cast off thread at the eye with a whip finish. Using a dubbing needle or a piece of Velcro tease out the fur around the thorax.
6. The thorax is teased out to represent the insect’s legs. When Gink is applied it should only be to the thorax.