Friday, October 14, 2011

Small Minnow Mayflies: Tracking the Annual Cycle

It may be premature to attempt a schedule of what we are likely to see and when in our part of Virginia in terms of species of small minnow mayflies (Baetidae).   Still, I think it unlikely that I'll be seeing anything new as we move into late fall and early winter.  So, I thought I might do a "wrap up" of the species of small minnow mayflies that I've found in the seasons of the year.  However, let me note right away, that there are two species that are around for more than one season.  So, my periods will be: 1) Winter/Spring (February, March, April, May);  2) Summer ( June, July, August);  3) Fall (September, October, November); and 4) Multi-seasonal (May through October).

I. Winter/Spring (February, March, April, May).: Heterocloeon amplum
This is the only small minnow species that I saw during this period.  My first photos are from February 14th, and those are of a pretty small nymph.

I saw lots of these in March, with some towards the middle of that month having black wing pads.  And I have a few photos from April 1st.

March 1st:

March 15th:

I found H. amplum nymphs in the Doyles River, the Lynch River, and Buck Mt. Creek, and possibly -- I can't recall for sure -- Powells Creek in Crozet.  This is the largest small minnow mayfly we see: 7-9mm.

II. Summer  (June, July, August): Baetis pluto, Heterocloeon curiosum, and Baetis flavistriga.  (A. nadineae and B. intercalaris are also around in the summer, but they are "multi-seasonal" insects.)

1. Baetis pluto
I only found two nymphs of this species, so my evidence would suggest that this is not a major species that we find in our streams.  I found the first nymph pictured below in Powells Creek on May 11th and the second in Buck Mt. creek on June 10th.

I would guess that this species hatches in June and July.  The dorsal pattern is distinctive, with tergum 5 being pale, while tergites 6 and 7 are very dark.  Also, the middle caudal filament is considerably shorter than the two lateral filaments.

2. Heterocloeon curiosum
I found these exclusively in the summer months of July and August, and most of the specimens found were in the Rivanna River.  However, the first nymph I found -- on July 11th -- had black wing pads, so clearly there were H. curiosum nymphs to be found in June had I been looking in the Rivanna.  Below, specimens found on July 11th and August 8th.  One of the distinguishing features -- the gray pigmented area in the center of the gills --  is visible without the aid of a microscope.  The other key features can only be seen with magnification -- "procoxal" gills.

3. Baetis flavistriga
I have only one photo of a B. flavistriga nymph.  I know I found others, but I only realized what they were when I saw them under my scope.  This species is apparently common and tolerant, though I can't say I saw a lot of them in our streams.  This photo was taken on August 10th at the Moormans River.

One of the distinguishing features may be visible in this photo (but this nymph was very small) -- the two pale, kidney-shaped dots on either side of the abdominal terga.  They're easier to see in this microscope photo.

IV. Fall (September, October, November): Acentrella turbida
I think of this as a "Fall" species since this is when I normally see it, and I see it in significant numbers in some of our streams.  But as I noted in a previous entry, A. turbida might be "bi-brooded," producing two generations a year.  I found some nymphs in the Rapidan River in early May that, looking back, look an awful like A. turbida nymphs.  First, a few pictures I've taken this fall.  The first two are from 9/29, the third from 10/9 in which the nymph is riding the back of a Giant stonefly.

The next photo was taken on 5/9 at the Rapidan River, and note how it closely resembles the nymph in the second photo above.

A. turbida nymphs have two features which help with identification: 1) the thorax is very broad, and 2) the legs are very hairy.  The "hairy legs" are best seen in a microscope view.

I typically find A. turbida nymphs in the fall in the Doyles River, the Lynch River, and the Moormans River.

III. Multi-seasonal (May through October): Acentrella nadineaeBaetis intercalaris
These two species were common throughout this period, found in significant numbers, and found in many streams.  Given the length of the period during which I've been finding these species, it is quite possible that I've been seeing more than one brood: i.e. these species may produce more than one generation a year.

1. Acentrella nadineae
The first photo I have of a nymph of this species was taken on May 30th, the latest photo I have was taken on September 3rd.  But I found a small A. nadineae nymph on 9/29 of which I do not have a photo.  The three photos below date from June 23rd, June 30th, and August 4th.  The nymphs I found on 5/30 and 6/23 were fairly mature -- note that the nymph I found on 8/4 was very small.

Two of the key identifying traits for nymphs of this species are visible in all three of the photos: 1) they have splashes of red on the thorax and abdominal terga, and 2) there is a wide band of gray pigment towards the front of each gill which runs from the body out to the edge of the gill ("anteromedial").

2. Baetis intercalaris
I found a B. intercalaris nymph at Powells Creek on May 11th, and it was already fairly mature: at the time, I could not identify the species.  I saw this species commonly throughout the summer and now into the fall, and I've seen it in a lot of our streams.  The photos below date from June 24th and October 5th.

The nymph found on 10/5 is still not fully mature, so B. intercalaris and A. nadineae nymphs may still be hatching out in November.  One of the distinguishing traits of this species is clear in both of the photos: there are pale slashes that look like "parentheses" marks on the leading edge of each abdominal tergum.  The fully mature B. intercalaris nymph in the photo below was found in Powells Creek on August 2nd.


Note: "Tergum" refers to the dorsal side of an individual abdominal segment: "terga" is the plural form of "tergum".  "Tergite," seems to be used interchangeably with "tergum".  I defined those terms incorrectly in a previous entry.

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