Colloquially called Drake Mackerel, Timpanoga hecuba is a late summer/autumn mayfly in the Yellowstone area.  Owing to their large size and body conformation, they’re often referred to as Green Drakes, but in fact they’re a separate (though related) species.  Rivers such as the Lamar, Slough Creek, Soda Butte Creek and the Snake harbor fine populations of this fly.  The general time frame for emergence is late August into early October.  Populations fluctuate dramatically year-to-year; 2018 is proving to be a good year.  

Pictured below are snapshots of hecuba nymphal shucks and a stillborn dun.  The nymphs are silt dwellers, with specialized opercula to protect their gills from river detritus (note the silt clinging to the shucks).  Duns are a size #12, with dark gray wings and bodies in various shades of tan and reddish-brown.  

Expect to see these flies emerging in the afternoon hours, roughly 1:00 – 4:00 p.m.; air and water temperatures are a bit too cold for them to emerge earlier in the day.  T. hecuba is the last large mayfly of the year to hatch and trout rarely pass them by, even when hatches are sparse.