Tuesday, October 31, 2017

A fresh look at the Ameletidae (Painted minnow mayfly) that live in our streams

While I've been waiting for rain (and yes, we finally got some!), I was browsing through some of my photos and decided to take a fresh look a the issue of species ID for the Ameletidae nymphs that I've found over the years.  I'm more convinced than ever that I've found at two different species, possibly three.  And while I doubted the Ameletus cryptostimulus ID for the that nymph at the top of the page for awhile, that's no longer true.  Let's have a look.

1. Ameletus cryptostimulus

Small mountain streams, and they're fairly common in March and April in the Moormans tribs that I visit in Sugar Hollow.  Let me work through the description found in Beaty's "The Ephemeroptera of North Carolina," p. 1.  I will rely on dorsal features only, I don't have any specimens at the moment so I can't check things like mandibles, and ventral patterns.

cryptostimulus -- labrum tan with darker medial subtriangular mark; tarsi with a dark apical band only.

Check, and note that the apical bands on the tarsi are visible in all of our photos (though it just looks like a dark tip).

posterior spinules present on abdominal terga 5 or 6-10.

On 6-10 for sure and probably on 5.

each abdominal tergum with two pairs of pale spots; one submedian pair and a pair anterolateral to those, also with a small pair of dark brown submedial spots on each segment; abdominal tergum 2, and sometimes 7, mostly pale, terga 6-9 with an additional single, medial pale spot removed anteriorly.

Yep.  One more thing.

caudal filaments basally brown and with a dark brown medial band followed by a pale band and tipped finally with brown.  Recorded from GSMNP and small mountain streams.

This is the feature that gave me pause a while back.  Are the caudal filaments (tails) "basally brown"?  I now think they are, just not dramatically so in immature nymphs.  But look at them on the one mature nymph that I've found.

No doubt about it, though, true, it's only the lateral tails that are fully brown.  Note that we can also see on this nymph that terga 2 and 7 are paler than the rest.

Ameletus cryptostimulus -- I'm a believer.  Tolerance value -- not enough data to reach a conclusion.

2. Ameletus lineatus

tarsi with both dark basal and apical bands; posterior margin of abdominal terga 1 or 2-10 with spinules; abdomen dorsally with 2 large pale sublateral spots on 2-7 and with segments 2-3 and 6-7 with a submedian pale spot; terga 8-10 mostly dark with some pale etchings; segments 1-9 with dark comma-like submedian markings visible particularly when pale spots are partially fused.

The pale spots mentioned in the description are all visible in this photo.

 For the spinules, here's a close-up of the abdomen of a second nymph.

Have to look closely, but you can actually see them on segments 2-10.   We can see the "comma-like submedian markings" clearly on a younger nymph that I found in the spring in the Moormans river itself.

For the caudal filaments, Beaty says this.  "caudal filaments with dark median band interrupted every four segments by very narrow pale bands.  Common in the Mountains and Piedmont.  A parthenogenetic species. "  Bingo.  Ameletus lineatus.

One note of caution.  There is another species -- Ameletus ludens -- for which Beaty says "dorsal and sternal abdominal coloration similar to A. lineatus."  To determine the species for sure, we'd have to see the sternal pattern.  But those differences are small enough for Beaty to conclude that A. ludens is "possibly a northern variant of A. lineatus.  We find much the same in our new key.   On A. lineatus: "This species might be equivalent to A. ludens."  (Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 40)  If I find these nymphs in the spring, I'll be sure to look at the venter, but it sounds like we won't really come up with anything firm.

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