Thursday, April 11, 2013

The Ameletid mayflies in our small mountain streams

I found this fairly mature Ameletid last Friday: I left it out of my report since the photos I got weren't all that good.  Here is another look.

They're not as sharp as I like my photos to be.  Still, I thought this might be a different species than the one that I normally see, so I wanted documentation.  I was right: this is Ameletus cryptostimulus; the one I normally see -- one that is common in some of our streams -- is Ameletus lineatus.  I can't say for sure that these are the only two species we have in this region, but they seem to be the only two species documented to date in North Carolina.  (See Steven Beaty, "The Ephemeroptera of North Carolina," p. 1.)

[Ameletids, by the way, are not very important to fly fishermen, but the hatch they produce is the "Brown Duns."  For a full discussion of the species fly fishermen normally see, go to pp. 92-93 in Knopp and Cormier's Mayflies.]

Let's look at the features we use to ID the two species we see in our streams.  As I normally do, I'll begin by citing Steven Beaty's detailed descriptions.

I. Ameletus cryptostimulus

A. cryptostimulus  -- inner setal row of mandible less than 1.5x the length of the setal gap; labrum tan with darker medial mark similar to "Ω"; tarsi with a dark apical band; posterior spinules present on abdominal terga 5 or 6-10; submedian curved marks on tergite 7 followed by a median spot.  Known from Southwest VA and in checklist.  Recorded from GSMNP (Great Smoky Mountains National Park).  

In a recent update to this description (unpublished), Beaty goes into much greater detail, and he changes his description of the labrum: "labrum tan with darker medial subtriangular mark." (personal communication)

1. I have not measured the setal row of the mandible.

2. The labrum does indeed have a darker "subtriangular" mark.

3.  The dark apical bands on the tarsi are easy to see.  (Note: there are no dark basal bands.)

4.  There are posterior spinules present on terga 6-10, none on 5, and in this photo we can also see the median spot on tergite 7.

In his more recent description of A. cryptostimulus, he adds, on terga markings, "each abdominal tergite with two pairs of pale spots, one submedial pair and a pair anterolateral to those, also with a small pair of dark brown submedial spots on each segment.

And he makes two other points that we can document with our sample: 1) "ventral surface of abdomen pale with segments 9 and 10 at least darkened laterally; caudal filaments basally brown and with a dark brown medial band followed by a pale band and tipped finally with brown."  I can only see dark lateral marks on tergite 10 in the nymph that I found; the tails are a match.

That's probably more information than most of us care to know (!) -- but there it is: A. cryptostimulus.   This species is not at all common.

II. Ameletus lineatus

Beaty note that this species is "common in the Mountains and Piedmont."

A. lineatus -- inner setal row of mandible approximately 4.8 times the length of the setal gap; tarsi with dark basal and apical bands; posterior abdominal terga 1 or 2-10 with spinules; sterna with median and lateral dark longitudinal stripes, median stripe obscure or absent on sternites 1-4; caudal filaments with a dark median band interrupted by 4 to 5 very narrow pale bands.

Let me note at the outset that I do not have a nymph in my collection that will allow us to see all of these features.  All insects "leach out" when preserved in alcohol, and Ameletids lose all of their colors in a very short period of time.

1. Again, I have not tried to measure the setal rows and gaps.

2. The dark apical and basal bands on the tarsi are very clear in the photo above.

3. I can no longer make out the spinules on the terga on any of the nymphs I have preserved, nor can I verify with a photo the lateral stripes on the sterna (though I can see them).

4. But I can see the pale bands in the caudal filaments.

5. They seem to show up even better on this small nymph that I found on 2/3/11.

Again, Beaty gives us a more detailed description of the abdominal markings in his update on Ameletus.  "abdomen dorsally with 3 pale spots on 2-7 with a large pair of submedial pale spots on 2-3 and 6-7;  an anterolateral pair of pale spots sometimes fused with the submedian spots; tergites 8-10 mostly dark with some pale etchings; segments 1-9 with dark comma-like submedian markings visible particularly when pale spots are fused."  Phew!  I think you can pick out most of those features in the following photo.  Certainly, segments 2-3 and 6-7 are very unique, and 8-10 are much darker than the preceding terga.

A. lineatus.  North Carolina assigns a TV of 2.4 to A. lineatus; too little is known of A. cryptostimulus to determine a tolerance value.

No comments:

Post a Comment