In the (THE FLY SHOP) catalog it shows both and the midgeling looks to have a little tuft of white for a tail and wings and a collar of ostrich herl. It looks to me like the migdeling is the emerger stage of the zebra midge. the size for the midgeling is #18 and the zebra midges are #'s 16, 18, 20 so the midgeling is not a smaller fly.

Hope that helps