The Fast Water Clingers Family Heptageniidae
These nymphs all have flat slender bodies, they also all have stronger legs to hold in fast flowing water. All the nymphs except the Epeorus genus have three tails, epeorus having only two. The mayflies that have substantial hatch numbers in S.E. MN are Macaffertium vicarium the March Brown and Macaffertium ithica the Light Cahill. I should note that the names of both of these species have changed from Stenonema vicarium and ithica to Macaffertium after further research demanded the shift in genus. Hatches 2 has them listed as Stenonema.
March Brown (Macaffertium vicarium) 10-12 Mottled tan and brown. June-Mid July
The March Brown is the larger of the two species that in habit Southeastern MN. According to my research the nymphs of vicarium are easily identified by the long tails in addition to the flattened body and strong legs. The tails on vicarium also splay at nearly 90 degrees. These nymphs are on average 16mm in length and are reddish brown. Vicarium move prior to emergence and one should examine rocks in ankle deep fast-moderate water for their presence. One should fish nymph patterns deep near the bottom as these nymphs either crawl or swim close to the bottom to find their emergence site.
Hatches 2 makes it a point to note that the duns of Vicarium have a wing that slants more than other varieties at a nearly 45 degree angle. Hatching occurs throughout the day from 10am to roughly 6pm during the proper conditions.
Light Cahill (Macaffertium ithaca) 12-16 Cream to tan. June-Mid July
There is no picture of an ithaca dun on troutnut.com. I was however, able to find an image of a nymph. It’s interesting to me that troutnut.com has so little information on this species. I will be excited to find samples of these in the wild.
Hatches 2 clearly states that the nymphs of both varieties here vicarium and ithaca need only be tied in medium and dark brown. Nymphs for ithaca should be tied in size 10-12. I’ve decided that when the mayfly series is over I will be posting a master list with sizes for nymphs and dries as well as color for all species related to S.E. MN.
Anglers should note that days in June where the weather is cooler due to overcast or rain can produce day long emergence, on hotter days they will be concentrated in the morning. I like the fact that Hatches 2 notes that during this time of the year water levels are usually going down and that you should be careful while wading.
I like the second image, making me want to try tying nymph with black thread down the hook shank then using perhaps lighter dubbing to simulate the dark center of the nymph.