Well done on body taper.
Things you need to work on.
Consistency. Get more consistency in the bulk of the thorax. The amount of peacock herl varies so sometimes you get a thin thorax and other times a bulky one. Look at the two flies to the right. The bodies and thoraxes should look identical.
Proportion. The proportion of thorax to abdomen varies in your flies. For a real nymph, the thorax and abdomen each are about 50% of the total body length. The thorax can be a bit shorter but 50/50 is that you should shoot for.
Take a look at real nymphs:
mayfly nymph images
For the three mayfly type nymphal patterns on the left, only the top fly comes close.
I would suggest that you now try tying a proper Copper John. Note the proportion of abdomen to thorax and bead in the fly below.
Add a wing case using Wapsi Thin Skin - Mottled Oak pattern. Then add the strip of pearlescent tinsel or flashabou, then cover with UV acrylic, Knotsense, or epoxy.
Even using a folded wing case of turkey tail, or pheasant tail fibers with the Flashabou and no epoxy will improve your flies.
Practice tying just one size and version of a fly until you master it. The vary the sizes. Tying variations makes it more difficult to master a pattern.
Uses the patterns below as your guide and note the proportions of the fly and materials
Charlie's FlyBox - Colorado's Best FlyShop and online Fly Tying Tutorials
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