Lake Eildon Floating Grubs - What Are They?

As a recovering bait angler its so easy to get on the water and start throwing around a few flies and asking why you didn't catch fish later. But its a total waste of time. Its far more efficient to work out what the fish are feeding on and use a fly that represents their food.

I have been frustrated by the hyper intense pre dawn rises that occur on the Howqua arm of Lake Eildon for a the last year. About an hour before dawn, when you can just see, the fish start rising with intenisty. And half an hour later at what the weather bureau calls first light the action slows down dramatically. 

If you are on a boat during this critical thirty minutes you have unlimited fish to cast to. A different fish rise within an easy 30 foot cast of the boat every 30 seconds. Many of them are only roach but I don't care as they are still a challenge to catch.

Until now I though these fish were rising to chironomids. Why? Because that's what every one said they were rising to. But after some very basic observation (look into the water fool!) I now know these fish are rising to grubs. The grubs are a very dark brown and about the length of a maggot. They have six legs and lie in a semi curled up position. They float right on top of the water - no part of them is beneath the surface. What the hell are they? And what fly do you need to represent them? They look like some Czech nymph patterns I have but these are way too chunky and they don't float above the surface.

If you venture out onto the lake mid morning the grubs have disappeared. Did they hatch into something else, become water logged and sink to the bottom or were they all gobbled up by fish?