What they are
Caddis live most of their lives as larvae on the river bed, many of them in little cases of sand or twig, under the rocks of every run and riffle. When they hatch, the pupa cuts free and swims to the surface, and the adult that emerges looks like a small moth with its wings folded in a tent. The adults live for days, come back to the water to lay eggs, and skitter across the surface while they do it.
Trout eat all three stages, and they eat the larvae all year. That is why a scruffy nymph under a dry catches fish on a river with nothing hatching.
When and where
NSW DPIRD's Snowy Mountains guide lists the caddis dry among the flies for the region's rivers in the warmer months, and the picture this page is written from is the same: the evening riffle, spring to autumn, the fish moving up into the faster water as the light goes. On the Eucumbene that is the runs between Denison and Providence; under the timber on the tailwater below the wall the same insects live in the shaded bedrock pools.
The hatch is noisier than a mayfly hatch. Splashy, hurried rises in moving water, rather than quiet sips in a glide, are the sign.
How to fish it
At dusk, an Elk Hair Caddis in 12 to 14 through the riffles and the heads of runs, dead-drifted or skated a few inches at a time. By day, in the same water, a tungsten Hare's Ear fished as the dropper under a buoyant dry is the larva the fish are eating whether or not anything is hatching.
If fish are boiling in a riffle and refusing the dry, they are taking the pupa just under the surface. A Klinkhammer, with its body hanging in the film, is the fly for that moment.