Monday, September 4, 2017

Exploring species ID for Brushleggged mayflies (Isonychiidae)

Brushlegged mayfly nymphs are fairly easy to find -- if not prolific -- in many of the streams that I visit: the Rivanna River, Buck Mt. Creek, the Rapidan River, and so on.  I've taken photos of many of them over the years but no longer bother since they all look the same and species ID isn't encouraged.  Leave them at genus: Isonychia.

But emboldened by the species descriptions in our new key (Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, pp. 126-129), I've decided to look more closely at what I'm finding this year.  All the same species?  Species that vary by stream?  Can I decide on the species with any exactness?  That's why I was excited this morning to find this nymph at Buck Mt. Creek.  It looks nothing like the brushlegged nymphs that I commonly see, which look like this.

Clearly visible here is a wide, pale stripe that runs from the head to the tails.  Nothing like that on the nymph that I found today.

Intriguing.  Let's see what we can find out from our key.  Couplet one.

239  Filamentous forecoxal gills each with single filament..........240
239' Filamentous forecoxal gills each with cluster of filaments.........244

Ours are single.

(Before I proceed, let me note that if I find any nymphs with a cluster of filaments on each of the gills -- and I hope that I do -- it may be difficult for me to make any headway.  Identifications in that case require seeing setae on the median ridges of gills -- I don't have the magnification required to carry that out.)

240  Abdominal sterna 6-8 with marking similar to any of Figs 2.386-2.389 ............  241
240' Abdominal sterna 6-8 not exactly matching any of the above variations...........242

Let's look at the sterna.

There clearly are markings on 6-8 (on 5 and 9 as well for that matter), and it looks something like this: (. .) -- a parentheses with two small dots in-between.  That pattern is a very good match for variation 2.388 (p. 127).   On to 241.

241 Abdominal sterna 6-8 with markings similar to any of Figs. 2.386-2.389; abdominal terga yellowish brown to dark reddish brown (with or without lighter mid-longitudinal stripe) .....Isonychia georgiae
241' Abdominal sterna 6-8 with markings similar to Fig. 2.389; abdominal terga light orange-brown with indistinct mid-longitudinal stripe (stripe interrupted by reddish-brown posterior margins of terga 2-9)........ Isonychia obscura

Tough call on the color of the terga since they're really a mix of yellow and brown.  Still, given the match with illustration 2.388,  I'm tempted to propose an ID of Isonychia georgiae.

Well, it's a start.  Don't expect any dogmatic assertions on species ID for the brushlegged nymphs that I see.  Still fun to see what we come up with.  If the other nymph that I showed you --

had the single filament forecoxal gills, by the way, it would key out to Isonychia serrata ("Abdominal terga with wide mid-longitudinal stripe on terga 1-9").  Pretty much a slam dunk on that one.

No comments:

Post a Comment