The Light Cahill mayflies are part of the Heptageniidae mayfly family. Although they are quite common on many northern Wisconsin trout streams, identifying their exact genus and species can be fairly difficult. Light Cahills used to be categorized under the Stenonema genus. Today, entomologists have split them into two newer genera Stenacron and Maccaffertium. Many older fly fishing books refer to Light Cahills as Stenonema, which can be confusing.
Light Cahill nymphs are part of the "clinging group" of mayfly nymphs. They have flatten heads and abdomens, and stocky legs. The nymphs can be found in many different parts of a freestone stream, not just the faster rock stretches. The Light Cahill nymphs look very much like the March Brown nymphs (Maccaffertium vicarium), except they are smaller in size. A hare's ear nymph pattern can be a good imitation for imitating the nymphs.
The Light Cahill duns generally hatch sporadically all day long making them less of a target for hungry trout. Although, isolated heavy emergence may occur in some of the species. When they do emerge in the open waters they generally drift on the surface for quite some time while drying their wings. This can make them an easy meal for hungry trout. Often the duns are emerging along with other caddisfly and mayfly species that are more concentrated and make a more desirable meal for the trout. These complex hatches can be difficult to determine which insect and what stage the trout are feeding on. The classic Light Cahill dry fly pattern or a creamy, white comparadun can be a good pattern for imitating the sporadic hatching Light Cahill duns.
Spinner falls are generally occur at nightfall and can be very concentrated. When the Light Cahill spinners finally drop onto the water a spent wing Light Cahill pattern can be deadly. Look for the spinner falls over the riffled waters and faster runs.
The two photos below I guess are Maccaffertium modestum female duns from a northern Wisconsin trout stream. Captured in June. They are about a size #12.
Below is a Stenacron interpunctatum, Yellow Sally. It has yellowish wings and an orangish colored body and is about a size #12 dryfly.
The nymph maybe a Maccaffertium sp. Notice the wide flattened head and abdomen.
The comparadun makes a very effective Light Cahill pattern in size #12 and #14.