From: Cary
Received: Wednesday, June 15, 2005 5:30 PM

In Missouri, on a spring creek type flow, there was a very small (4 -6 mm) mayfly hatching. I only managed to capture a couple of them, but here goes .....2 tails, very pale dun wings, I'm putting a question mark on the hind wings, body was orangish brown on the dorsal side, and the ventral was yellow with a hint of orange. It looked to me like a "toy" sized  cross between a March Brown and a Sulphur Dun. Could you send me in a direction to search?

Okay, allow me to walk you through the analysis I have done to identify your mayfly.  Firstly, I entered your observations into my Adult Mayfly Identification page with the following result:

Adult Mayfly Identification

Number of tails = 2
Body length = 05 mm.
Fore wings = uniform color
Hind wings = not obvious (absent or minute)
Other distinguishing characters: ?
Emergence water type: ?
Emergence behavior: ?
Emergence light condition: ?
Geographic location: ?

The following mayfly genera satisfied entered descriptors:

 As all indications pointed to it being a Baetid, I looked up each of the above genera in Mayflies, An Angler’s Study of Trout Water Ephemeroptera by Malcolm Knopp and Robert Cormier, as that reference has good coverage of eastern mayflies.  They are described on page 76 as follows: 

The Gray-Winged Olive Quills

Family: Baetidae, Genera: (Centroptilum and Procloeon)

       Mayflies of this complex are uncommon inhabitants of trout waters, being found only in localized areas such as lake margins, weedy backwaters, and very slow flowing streams where the nymphs live exclusively among aquatic vegetation.  Included in this complex are the species Procloeon ingens, P. rubropictum, P. simplex, and Centroptilum triangulifer, which had been previously classed as members of genus Cloeon.  The species considered inhabit eastern waters, and emergence is usually during the summer season.

       The duns range from yellow to orange to a reddish brown, with pale gray wings and legs and two pale-colored tails.  The adults of this group lack hind wings and may be separated from other groups by the presence of single marginal intercalary veins on the trailing margin of the fore wing.  An inverted reddish brown Y may be observed on the dorsal region of the adult’s body segments two and three.

Back-referencing those four species in Hatches II by Al Caucci and Bob Nastasi required some additional thrashing, as they have all been re-classified since it’s publication.  The following table reflects those changes:

Unfortunately, there was no information on C. triangulifer.  However, we can eliminate C. ingens for reasons of both location and length.

The following appeared as a footnote to the information on Cloeon spinners:

The duns of this genus are generally pale.  Their bodies, light in color, range from yellowish to orange to reddish-brown.  The legs and tails are likewise pale.  There are no markings on their palish-gray wings.

Was elated to discover this, as it validated your observation of an orange coloration on the dun, which for a mayfly, is unusual in the extreme. Okay, so where does this leave us?

Family: Baetidae
Common name: Gray-Winged Olive (or more appropriately, Tiny Gray-Winged Olive)
Scientific name:  perhaps Procloeon rubropictum or P. simplex, or possibly Centroptilum triangulifer

Sincerely,
Roger

Created: 08/13/2005   Last modified: 08/25/2006    www.FlyfishingEntomology.com