May Fly Entomology Pt. 6

The Swimmers

The Darting Swimmers Families Baetidae, Siphloridae and Metretopodidae.

The two main species found in S.E. MN are two forms of the ever popular BWO. We have the Baetis and the Plauditus punctiventris. These are both found in the Baetidae family when identifying these nymphs from the other two families size is the best indicator. Siphloridae and Metretopodidae fall into the Large Swimmers while Baetidae fall into the much smaller size category. All have gills on the abdomen sections 1-7. One more overall comment about the Swimmers, they are the most varied group in the mayflies and emerge in cases all year round based on water temperature. One will find swimmers in almost every stream.

Little Blue-winged Olive (Plauditus punctiventris) 20-22 Pale yellow olive Late June-Oct.

In Hatches 2 this is listed as Pseudocloeon anoka but has changed since publication. These are still swimmers and so they will have similar nymphal traits to the other swimmers but one quick way to distinguish these from other Baetis is the tail. Plauditus punctiventris has only two tails while Baetis has three, also the abdomen is thicker on these nymphs. These nymphs are 4-5mm in length, I’m noting this size for my own reference. These nymphs will most likely crawl on grass or other items to get out of the water during a hatch. They are less likely to swim to the surface and then hatch. Fish downstream of extensive weed beds.

Blue-winged Olive (Baetis) 16-22 Grayish olive to dark olive March-May & Sept.-Oct.

Baetis BWO

These nymphs will be found in underwater foliage. The slender bodies as well as legs make them ideal for swimming through aquatic plant life. The baetis nymphs have three tails like most other but the center tail is much shorter than the outer two. Note that the baetis will emerger early in the season and late in the season. One thing I’m finding is that the exact species of Baetis is not listed and as there are several this will be a task for me this coming year to determine which exact species inhabit the waters around me. The greatest numbers of these nymphs are found in alkaline waters with a ph. of 7.5 or greater. Hatching will occur between 11am and 4pm in March-April when water temp exceeds mid 60’s. Typically one will see these on overcast days and as a general rule overcast provides some of the best BWO fishing.

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