Yup, those look like Hex- big enough to have commercial plates with 2 tails. Fishing should be great with every fish in the lake feeding on them- bass love 'em. The fishing should be great if fish can find your fly in the blizzard of real ones. The fishing should last well into dark over mucky bottoms.
Here's some info from troutnut:
Mayfly Species Hexagenia limbata (Hex) Hatch
You may want to consider a road trip up to Michigan in a week or two for some trout fishing on the Ausable
Another hatch you may run into on Lake Erie is the whitefly hatch, which should start late July and into August (also evenings into dark). A size 10-12 White Wulff should work for them. SM and LM Bass love them too. Both the Hex and white fly hatch are dense enough that they show up on weather radar--- and have been responsible for delaying the start of night baseball games.
There are some great Hex patterns out there, some pretty involved- extended bodies, articulated nymphs etc. Since time is of the essence, here’s some quick and dirty ones:
Hook: size 4, 6 and 8 3xl nymph hook
Weight: lead wraps
Tail: Dark Ginger wet fly hackle fibers or pheasant tail fibers
Body: Olive marabou, twisted and wrapped like peacock herl or picked out scruffy Medium Olive nymph dubbing (rabbit, haretron etc)
Wing case: Dark Brown Turkey wing quill slip, or tail fibers
Legs: Dark Ginger
Hex Compara/Sparkle Dun
Hook: Size 6-8 4x long streamer hook (Use the big hooks for females, smaller ones for males) I use the streamer hooks to avoid tying extended bodies.
Tail: Brown Mink tail fibers, fibbets or bristles from a nylon paint brush, elk or moose mane fibers tied split (or Tan Antron, Z-lon or sparkle yarn shuck)
Body: Creamy-Olive dry fly dubbing (Superfine, Fly Rite etc)
Wing: Arc wing of Medium gray deer hair (sold as “Coastal” or Comparadun” hair)
Hackle: Optional, but you could add a couple of wraps behind and in front of the wing with medium dun hackle if you have some big enough (spade hackle or Coq de Leon)
Hook: Size 6-8 4x long streamer hook (Use the big hooks for females, smaller ones for males)
Tail: Ginger Mink tail fibers or fibbets, or nylon bristles from a paint brush tied split
Body: Creamy-Yellow (for female) or Reddish Brown (for male) dry fly dubbing (Superfine, Fly Rite etc)
Wing: If you can find hackle large enough, Light dun and Ginger mixed tied in 360 degree collar to skitter or trim flat on stream to lie flush in film, or you could just tie it with some white Bill’s Body Braid or Poly spent wings.
Since this is often a night time deal, having a headlamp is a big help. There are some great ones out there Petzel etc, but they can be expensive. You can can a decent one for cheap, like a Rayovac 3-in-1 LED for 16 bucks. The thing I like about it is that it’s light, runs on 3 AAA batteries and has a red LED light as the first option on the clicker (doesn’t spook fish). I wear it as a neck light rather than a head lamp and only use it when needed.
Amazon.com: 3 in 1 LED Head-Lite Headlamp Flashlight: Home Improvement
Good luck, hope you get into them.