Aw "shucks"! Stonefly Remains at the Rapidan River

Yet another sign of late spring/early summer -- stonefly shucks on stream side rocks (and leaves!).  We also find them on rocks in midstream, rocks whose tops are exposed.  When stoneflies mature and "emerge" as terrestrial insects, they crawl up on rocks, split open their shells, dry off their wings, and fly off to mate, lay eggs, and die.  The cases I saw today at the Rapidan River were -- I'm fairly certain -- those of Perlodid stoneflies: they weren't very big, maybe 1/2" or so, and almost all Perlodids by now have hatched.  Common stoneflies -- Perlids -- on the other hand, are in the process of hatching right now.  But Perlid shucks are typically 1" or so long.  Here are some Perlid shucks that I brought back from Montana two years ago.

If you look closely at the shuck on the left (click to enlarge) you can still see how the case split open when the insect emerged -- the exoskeleton is split open from the base of the head back through both sets of wing pads.

Now, there are still stoneflies in our streams, some Perlids will hatch throughout the summer.  And, remember that Perlid stoneflies (common stoneflies) can take 2-3 years to mature.  So, in addition to the mature nymphs yet to hatch, there are presumably 2-year old nymphs around and 1-year old nymphs as well.  In fact I've been seeing a lot of what I assume are 1-year old nymphs -- but they're too small to photograph at the moment.  Here's a nice common stonefly that I found today -- genus Paragnetina -- that may still hatch this summer, but it could also be a 2-year old with 1 year to go (the wing pads are not well developed).

I also found a Perlodid stonefly, one that will surely be hatching before very long (notice the dark wing pads).  This is an Isoperla holochlora -- a species that likes these cold, mountain streams, and one that tends to be a "late bloomer".

But all in all, it was a slow day of looking for bugs since so much has recently hatched.  I did find, in addition to the stoneflies pictured above (and those pictured below), some flatheaded mayflies -- Maccaffertium (pretty big), Leucrocuta, and Epeorus vitreus; a few Brushlegged mayflies; lots of common netspinners (genus Hydropsyche); some "Humpless case-maker" caddisflies (Brachycentridae); scads of baby Peltoperlid (Roach-like) stoneflies; and a large number of small Giant stoneflies.  But for the really interesting insects of summer -- small minnow mayflies, damselflies, dragonflies, genus Macrostemum common netspinners, Tricos, etc. -- I think we have to ignore the "pure" waters of mountain streams and look further along down in the valleys.  Time for me to think about spending more time at the Rivanna.

Three other good photos from today's trek: 1) a Glossosomatid (Saddle-case maker) caddisfly that actually crawled out from under its case;  2) the Perlodid stonefly riding a Perlid stonefly while the Glossosomatid looks at its reflection in the petri dish; and 3) another shot of a Perlodid shuck.

Genus ID of a Small Minnow Mayfly: Heterocloeon

In the entry I posted yesterday, I said that I would ID the small minnow nymph in the photo above as genus Heterocloeon.   I thought I might add some notes on this issue, just to show readers how miserable genus ID can be for an amateur with a minimum of professional training -- especially when we're working on a small minnow mayfly.

First, here are two more photos of the nymph in question.

and,

Now, let's work through our two dichotomous keys -- 1) Barbara L. Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, p. 33, and 2) R.W. Merritt, K.W. Cummins, and M.B. Berg, ed., An Introduction to the Aquatic Insects of North America, Fourth Edition, p. 192.

Peckarsky:
38a.  Two well-developed cerci; median caudal filament absent or less than 1/8 as long as cerci....39
38b. Three well-developed caudal filaments; although median caudal filament may be shorter and thinner than cerci, it is much longer than 1/8 length of cerci....41

Cerci mean "tails" to us; the "median caudal filament" would be a third, middle tail.  Since there is no median caudal filament visible, we clearly select 38a. and go to 39.

39a. Center region of gills with a large pigmented area (Fig. 69); procoxae with a single filamentous gill on inner margin....Heterocloeon
39b. Center region of gills without a large pigmented area; procoxae lacking a gill....40

"40" would take us to a choice between Acentrella and one type of Baetis (two-tailers).  But as I noted yesterday, the "center region of the gills" do indeed seem to have a large pigmented area.  Here's a good look at the photo.

But what about the second requirement listed -- the "procoxal gill"?    No picture of this is provided.
So, I have to guess what to look for and where to look.  But this ought to mean that there is some sort of gill showing on the inner edge of the front leg where it's attached to the body.  Here's what I found with the microscope.  (You may want to click on the photo to getter a closer view.)

I can't say for sure that I've found the "gill" -- but I'm willing to call this Heterocloeon based on the pigmented gills.

But let's look at our other key: Merritt, Cummins, and Berg, starting with couplet 36.

36. Terminal filament well developed; hind wing pads present...Baetis (in part)
36'. Terminal filament reduced; hind wing pads absent or present....37

Well, we know the terminal filament (= median caudal filament) is reduced, so we move on to the next couplet.

37.  Tarsal claws with two rows of denticles or a row of denticles and a secondary ridge that may be serrate or not (these claw characters require viewing at 400X): terminal filament minute to a visible stub not longer than tenth tergite; procoxae with or without gills....Heterocloeon
37'. Tarsal claws with a single row of denticles; terminal filament reduced, but longer than tenth tergite; procoxae without gills.

Now, I can't magnify to "400X" with my microscope (my maximum is "90X"), so I have no way to determine anything about the "rows of denticles" on the tarsal claws.  Also, note that in this key, "procoxal gills" are not a requirement for making a call of Heterocloeon!  (It is not unusual for keys to contradict one another.)  So, all I can do is look at the "terminal filament" (i.e. what would be a "middle tail").

The "10th tergite" is the top of the last abdominal segment, and I'd say the "terminal filament" is, indeed, not longer than the 10th tergite.

Ah joy!  Well, we do the best we can when we try to determine genus ID.  I think I'll play golf for a couple of days.

Small Minnows, Small Minnows, Small Minnows! Off to the Moormans

Not a small minnow mayfly of course -- but we have to start with this nymph.  A "Broad-winged" damselfly (Calopterygidae) -- the first one I've seen this season.  Some are brown; some are bright green, but like other aquatic insects, they seem to turn dark brown and dark green as they mature.  The trait we use to ID this particular family is the nature of the antennae:  "First antennal segment as long as, or longer than, [the] remaining segments combined" (Peckarsky, p. 45).  If you click on the photo to enlarge it, this feature ought to be clear.  Another look at this lovely insect.

It is not my ambition to identify Odonata to the level of genus -- but, for those of you who work on these things, this would seem to be Hetaerina (the prementum cleft does not extend beyond the bottoms of the palpal lobes.)  (See Peckarsky, pp. 46-47.)

Now, on to the theme of the entry -- there were small minnow mayflies all over the bottoms of rocks in the Moormans (near Free Union).  All different colors and sizes, and if I'm right, while most were genus Acentrella, there was a Heterocloeon among them, and one tiny Baetis.

Let's look at the "odd" ones first.

Why do I think this one is genus Heterocloeon?  Heterocloeon is one of the three genera (the others are Acentrella and one type of Baetis) with two tails.  But the critical thing is to look at the gills.  According to Peckarsky (p. 33), the center region of the gills has a large pigmented area.  Here's a microscope view.

This is the largest small minnow nymph that I found today -- also the darkest in color.

On the Baetis nymph -- it was a tiny one.  But, it had three tails (or, following Merritt, Cummins, and Berg,  p. 192) the "Terminal filament [was] well developed," and "hind wing pads were present."  First, let's look at the nymph, then as a microscope view that shows the tiny hind wing pad.

But most of the small minnows I found today were, I think, genus Acentrella, based on the tests we posed in the entry posted just yesterday: 1) two tails; 2) no visible hind wing pads (with these, at least); 3) a dense row of long silky setae (hair) on the backs of the legs; and 4) no "well developed medially projecting corner" on segment 2 of the labial palps" (Merritt, Cummins, and Berg, p. 192).  I did not look at the labial palps on every nymph that I brought home -- I only checked two.   But believe me, the dense row of hair on the legs was present on every nymph.  What follows are some of the best photos I got of the Acentrella nymphs.   I had one that was very, very small -- it's the one with the dark wing pads -- i.e. it's close to hatching.

...and one more

Revising my Views on Small Minnow Mayfly (Baetidae) Genera

I found this beautiful small minnow mayfly in the Doyles River this morning.  (The orange/red pigment is for real, by the way, those aren't eggs stuck to the body.)  I will show you some other good photos from today's trip later on.  But first,  I have to talk about something very important.

As you know, if you've been reading along in a regular way, I found a lot of small minnow mayflies this winter (February and March), all with two tails, and I argued that they were Baetis in terms of genus based on the presence of metathoracic wing pads (see the entries for 2/14, 3/2, and 3/16).  In the Peckarsky key (Freshwater Macroinvertebrates, pp. 34-36) the presence of metathoracic wing pads on a two-tailed Baetid distinguishes the genus Baetis from Acentrella.

Bad news.  I've now been told by a professional entomologist, a specialist on aquatic macroinvertebrates, that all Baetis small minnows in the eastern part of the country have three tails!  He also feels there is a very good chance that the nymphs I found in the winter were Acentrella in terms of the genus.  I worked a lot on this last night and this morning, and I now believe he was right:
the nymphs I found in the winter were probably genus Acentrella.  (So too is the nymph I found this morning -- the one in the photo at the top of the page.)

Let me make my case.  First of all, I'd refer you to a web publication entitled "Small Minnow Mayflies: Nymph key to genera" (go to, http://www.gunnisoninsects.org/ephemeroptera/key_baetidae_nymph.html).  where Acentrella as a genus is defined by a "Dense row of long fine setae on the back (or dorsum) of the legs."  It continues, "Everyone else [ i.e. all other genera have]...no rows of setae on the legs."  Here is a close-up of the legs of one of the nymphs from the winter that I had preserved.  (Please click on the photo to enlarge it.)

And, here is a close-up of the legs of the nymph at the top of the page.

That might seal the deal -- but I think it's important to see what the "Bible" -- in terms of dichotomous keys for aquatic macroinvertebrate entomologists -- has to say on this issue as well.  The "Bible" is:
R.W. Merritt, K.W. Cummins, and M.B. Berg, ed., An Introduction to the Aquatic Insects of North America, Fourth Edition (Dubuque, Iowa: Kendall/Hunt Publishing Company, 2008).  Having reduced our search to "two-tailed" Baetids, and eliminated the genus Heterocloeon from contention, we move to couplet 38:

38.  Hind wing pads present (although sometimes reduced to a small thread-like flap.... (go to) 40
38'. Hind wing pads absent.... (go to 39)

On the two-tailed small minnow nymphs I've been finding this summer, the hind (= metathoracic) wing pads have been "absent":  on the two-tailed small minnow nymphs that I found in the winter, they were small but "present".  So, let's refer to these as Summer Nymphs and Winter Nymphs and follow both directions above.

Summer Nymphs (hind wing pads absent) -- we go to couplet 39.

39. Femora, tibiae, and tarsi without a row of long setae on outer margin....Plauditus
39'. Femora, tibiae, and tarsi with a row of long setae on outer margins..... Acentrella (in part)

So, from the photo of the legs on the nymph that I found this morning, we'd have to go with Acentrella for the Summer group.

Winter Nymphs (hind wing pads present) -- we go to couplet 40.

40. Segment 2 of labial palps with well developed medially projecting corner....Baetis (in part)
40'. Segment 2 of labial palps without well developed medially projecting corner....Acentrella

Let me show you what "well developed medially projecting corners" look like.  This is a close-up of the labial palps of the "three-tailed" Baetis I found earlier this summer at Buck Mt. Creek.

That corner really sticks out like a shelf.  Now, here's a close-up of the labial palps from one of the "two-tailed" small minnows that I found this winter.

There is a corner -- but it's not well developed, no "shelf".  Our Winter Nymphs, I'd have to conclude at this point, were genus Acentrella, not genus Baetis as I have claimed in previous entries.  So, the two-tailed small minnows we found in the winter were apparently Acentrella, and the two-tailed small minnow nymphs I'm finding this summer are also Acentrella.  At this point in time, however, I feel strongly that I'm looking at two different species of Acentrella.  Remember, the "Winter" nymphs had wing pads showing; the Summer nymphs do not.  And, the nymphs I've found so far this summer are much smaller than the nymphs I found in the winter -- and some of the summer nymphs I've found have been pretty mature.

I hope I have this straight now.  But I am, after all, an amateur at this.  And given the complexities involved with genus ID in this particular insect family (Baetidae), this may be something we amateurs should leave to the pros.
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Two other photos to look at from my trip to the Doyles this morning.  First is yet another beautiful Perlesta common stonefly.

But second -- and I'm really excited about this -- is a fully mature flatheaded nymph, genus Leucrocuta.
I've never seen one before.  Two photos:

And, if you look really closely at the second photo, you can actually see that there's fibrilliform behind gill number 6, but not on gill number 7.  Here's a microscope photo pointing that out.

Fly Fishing, Flatheaded Mayflies, Major Species and Hatches

We can find flatheaded mayflies (family Heptageniidae) in most of our streams just about every month of the year -- if not every month of the year.  And they're easy to find because they're "clingers," i.e. they feed by clinging to rocks and eating the algae.  So when you're looking for them, simply turn over some rocks.

As I've mentioned in previous entries, I've found -- to date -- six genera of flatheaded mayflies in this part of Virginia: Maccaffertium (formerly Stenonema), Stenacron, Epeorus (both pleuralis and vitreus), Rhithrogena, Heptagenia, and Leucrocuta.  Maccaffertium nymphs seem to be here most of the year, all of the rest seem to be limited to certain seasons.  But as a fly fisherman who grew up looking forward to the "March Brown" (Stenonema vicarium) hatch every spring and the Light Cahill (Stenonema Ithica) hatch in the summer, I've started to wonder how the nymphs that I'm finding fit into the major hatches that are important to fly fisherman.

In this entry, I'd like to pull together data from three different sources, and we'll see what we see: I can't guarantee this will answer all of my questions.  Still, it will be nice for me -- and presumably others -- to have this data all together in one location.  My three sources of data will be:
1) My own records of the genera I've found and when I've found them.  When did I first see nymphs of certain genera?  By when were they mature?  (Naturally, dates vary from stream to stream.)
2) For the major hatches known to fly fishermen, and the flatheaded species known to produce them, I'll rely on the masterful study of mayflies -- Malcolm Knopp and Rober Cormier, Mayflies: An Angler's Study of Trout Water Ephemeroptera (Helena, MT: Greycliff Publishing Co., 1997).
3) And for information on the number of flatheaded species -- by genus -- that we find in this part of the country, I'll be using the "Tolerance Values for Benthic Macroinvertebrates," developed by the North Carolina Division of Water Quality.  This can be found at:  http://portal.ncdenr.org/web/wq/ess/bau.  (Note: This is a detailed and technical entry; some of you might want to skip it.)

1. Let's take an easy one first: the genus Rhithrogena.

a) My first sighting of a Rhithrogena nymph was on April 25 (Buck Mt. Creek), and my last sighting was on May 9 (the nymph in the picture above).  I'm sure there were nymphs in this stream and others before and after those dates, but these are the only days that I found them.

b) Knopp and Cormier (p. 137) list four Rhithrogena species that are important out West, producing the "Western March Browns," with hatch dates running -- in general -- from Mid-March through Mid-May.
They do not see this genus as important for fly fishermen in the East.  They state, "Scattered populations of Rhithrogena species also inhabit certain East and Midwest freestone rivers but are not known to produce very reliable hatches." (p. 137)  The only place I've seen nymphs of this genus is Buck Mt. Creek, and even there I've seen only a few.

c) There are 5 Rhithrogena species documented by the NC Division of Water Quality.  None of these corresponds to the important species out West (which are R. morrisoni, R. hageni, R. undulata, and R. futilis.)  Three of the Eastern species in the NC list have a tolerance value of 0.0; the other two are undetermined.
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2. Maccaffertium (known to fly fisherman by its former name -- Stenonema) -- see the photo at the top of the page.

a) This is the big one.  The first time I spotted a Maccaffertium nymph this year was on February 11th at Lickinghole Creek in Crozet.  The one in the photo at the top of this page was found on March 28th at Powells Creek in Crozet (close to being mature).  Then I found one with black wing pads just last week at the Moormans (see entry for 6/15), but I continue to find pretty big Maccaffertium nymphs that are still not mature.  This one was found Powells Creek on 6/21.

Were any of the nymphs I found early on in the spring Maccaffertium/Stenonema vicarium (the "March Brown")?    Will the large nymphs I'm finding now grow into "Light Cahills" (Maccaffertium/Stenonema ithaca)?  Sure wish I could answer those questions.

b) Knopp and Cormier (p. 176) list 7 Stenonema (i.e. Maccaffertium)  species that produce key hatches here in the East: S. vicarium (1.5), S. femoratum (6.9), S. modestum (5.7), S. ithaca (3.0), S.mediopunctatum (4.2), S. mexicanum integrum (4.7), and S. pulchellum (undetermined).  All of these species are present in our area based on the list published in North Carolina (tolerance values are listed in parentheses).  These species produce the following hatches: S. vicarium -- "American March Brown" (mid-May through mid-July); S. ithaca -- "Gray Fox" and "Light Cahill" (mid-June through mid-August); S. pulchellum, S. modestum, S. mexicanum integrum, S. femoratum,  and S. mediopunctatum -- the "Cream Cahill" (end of May through mid-September).  (Knopp and Cormier, pp. 176-185.)

c) Additional Maccaffertium (= Stenonema) species reported for our area by the NC Division of Water Quality are: M. carlsoni (2.1), M. exiguum (3.8), M. lenati (2.5), M. meririvulanun (0.5), M. pudicum (2.1) and M. terminatum (4.4).
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3. Stenacron. 

Earliest sighting and latest sighting.   3/18: Whippoorwill Branch

6/16: Mechunk Creek

a) In the 2-3 years I've been looking for insects, I've found very few Stenacron nymphs.  This year I found one at Whippoorwill Branch, one at Ballinger Creek (on 5/14), one at the Moormans (6/15), and this fully mature nymph at the Mechunk.  I think they're rare in our streams.

b) Knopp and Cormier (pp. 186-187) say that of the seven species in North America only Stenacron interpunctatum is important for fly fisherman.  It too hatches as a "Light Cahill," the hatch peaking "between mid-June and mid-July".

c)  The North Carolina Division of Water Quality lists 4 species found in this region: S. carolina (1.3), S. interpunctatum (6.4), S. pallidum (2.8), and S. spp (unidentified species; undetermined TV).
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4. Epeorus.  The North Carolina list of tolerance values has 5 species of Epeorus mayflies.  I've only seen two in our area -- Epeorus pleuralis (TV of 1.5) and Epeorus vitreus (TV of 1.2) -- and I'll treat them separately.  Knopp and Cormier (p. 159) provide hatch information for 5 species (but not the same 5 recognized in North Carolina, with the exception of pleuralis and vitreus).

4A. Epeorus pleuralis.

a) Well known to fly fishermen, this produces the "Quill Gordon" (also "Slate Duns")  hatch in the spring.  This year, my first sighting of Epeorus pleuralis was on January 2nd.  This is one of the photos I took.

My last sighting -- at least the last sighting I can document with my photos -- was on 4/20.  But I had earlier (3/24) found this mature nymph at the Rapidan River.

b) Knopp and Cormier (p. 159) give a hatch range of mid-April to mid-June for Epeorus pleuralis.  But a lot of Eastern hatch data is based on when things occur in New England.  So, the hatch might start at the beginning of April here in VA.

c) The North Carolina tolerance value for Epeorus pleuralis, as I've already noted, is 1.5.

4B. Epeorus vitreus ("Gray-winged Yellow Quill")

a) My first photo of an E. vitreus nymph was taken on 5/11, my lastest (below) on 6/10.  Both nymphs were found in Buck Mt. Creek.

I'm finding E. vitreus in some of the same streams in which I found E. pleuralis in the winter.  But, I have not found them in the small, rocky tribs to the Moormans that were earlier loaded with E. pleuralis nymphs.

b) Knopp and Cormier (p. 161) point out that "...E. pleuralis and E. vitreus may be separated by differences in gill structure, habitat preference, the shape of the insect's femoral flange, and the presence of paired spots marking the tergites [dorsal abdominal segments] of E. vitreus nymphs."  The paired spots on E. vitreus nymphs can be seen in the photo above, and if you compare the gills of the E. pleuralis and E. vitreus nymphs, you can see the large vein patterns in the latter.  As for the femoral flange, the E. vitreus flange comes to a sharp point.  This we can see in the microscope photo below.

They give a hatch range for E. vitreus in the East of mid-May through mid-August.

c) E. vitreus has a tolerance value of 1.2.

5. Heptagenia and 6. Leucrocuta.   The "Pale Evening Dun" and "Gray Fox".

b) Let me go out of order on this one, since these two genera produce the same adults -- from the fly fishing point of view.  Knopp and Cormier (p. 149) begin their discussion by noting "This complex contains the important mayflies of the closely related genera Heptagenia, Leucrocuta and Nixe that populate the slow- to moderate-flowing reaches of cool, unpolluted trout streams and rivers across North America."  They continue, "Entomologists have catalogued twenty-one species of this complex from East-Midwest waters, where Leucrocuta hebe is the principal species to produce fishable hatches; on eastern waters, H. pulla and L. aphrodite are the major species producing fishable hatches."  On dates of emergence they state, "Leucrocuta aphrodite is the first of the eastern species of this complex to emerge in early May and produces minor sporadic hatches for several week.  Following about two weeks later, Heptagenia pulla begins its emergence, producing hatches of minor importance through the period of mid-May to August.  On East and Midwest waters, Leucrocuta hebe is an important hatch from July to early October."

a) This summer, I have not yet found any Heptagenia nymphs -- believe me, I'm looking!  I found a good number of them in August last year -- and they were pretty mature -- in the Moormans River.  Thus, the only picture that I can show you is of a preserved nymph from last year.  I certainly don't know if this is H. pulla, but this does look pretty well ready to hatch.

On Leucrocuta nymphs, I found one as early as January 6, and I have other photos from 3/21, 5/9, 6/15, and 6/18.  Significant numbers of Leucrocuta nymphs are now present on rocks: before June, my sightings were more sporadic.  Could it be that the nymphs I saw in winter and early spring were L. aphrodite, while the ones I'm finding now are L. hebe, the species that hatches from July to October?
Here are my photos from 1/6 and 6/18 -- both nymphs are from Buck Mt. Creek.

c) The North Carolina list of tolerance values documents four Heptagenia species for this area and three Leucrocutas.  They are: Heptagenia julia (TV undetermined), Heptagenia marginalis (2.2), Heptagenia pulla (2.2), and Heptagenia spp (1.9); then, Leucrocuta aphrodite (2.9), Leucrocuta hebe (TV undetermined), and Leucrocuta spp (2.0).

The other flatheaded genus that we find in this "complex" -- Nixe -- is one that I've found in Montana, but I've never found one in our streams.  There is one Nixe species -- Nixe spp -- in the North Carolina list of tolerance values, but the TV is undetermined.

Why Are Summer Insects So Small?

I will get to my topic -- but first things first.  I am sooo excited about finding this stonefly!  This is a common stonefly, genus Neoperla.  This is the first time I've seen one that's approaching maturity, and the first time I've gotten live photos.    And the stream I went to today -- Powells Creek in Crozet -- is the only place I have seen this genus of common stonefly.  How do we know that it's Neoperla?  It's an easy one: we look at the head.  There are only two ocelli -- all other Perlids (common stoneflies) have three that form a triangle at the back of the head.  On a Neoperla nymph, there is no "anterior" ocellus.  Here's a close-up.

An ocellus -- and I'm quoting Peckarsky (p. 424) is "A small visual organ with only one lens.  In many arthropods, ocelli occur in addition to compound eyes."

Another look --

Two other nice photos and then I'll get down to work.

1) Flatheaded mayfly: genus Maccaffertium.

and 2) (surprise, surprise), a common stonefly, genus Perlesta.  (Readers must be sick of seeing this insect by now!)

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So, why are summer insects so small?  I've been thinking of doing an entry on "The Summer Doldrums" since we find fewer insects during the summer in most of our streams, and the insects we find are often tiny.  We get spoiled by the nice, big stoneflies and mayflies that we find in the winter and spring.

Evidence.  1) I'm finding two genera of small minnow mayflies in all of my streams -- a "three-tailed" Baetis, and a two-tailed Acentrella.  I found both genera today at the Powells.  All the small minnow nymphs that I'm finding are much smaller than the "two-tailed" Baetis that I found in the winter.  Consequently, they're hard to pick up without harming them, and I'm having trouble getting good photos.  2) The genus of flatheaded mayfly that I see all over the place in the summer is Leucrocuta: these nymphs are tiny.  3) I know that in a month or two I'll be seeing some "Small Square-gilled" mayflies (family: Caenidae): they're very small.  4) And I know that at the end of the summer when I hit the Rivanna I'll find some "Little Stout Crawlers" (Tricos): I don't have to tell fly fishermen that this mayfly is small.  5) True, I'm still finding some decent sized stoneflies (common stoneflies: genus Perlesta, and now genus Neoperla).  But Perlids (common stoneflies) mostly hatch in late June and early July.  After that, most of the stoneflies I'm going to see will be small.

What's the explanation?  Well, of course we have a new generation of mayflies and stoneflies, even some caddisflies, so naturally they will be small.  But there's another factor that we could easily overlook.  In the summer, there is less dissolved oxygen in the water for the insects that live in the streams.  Flows have dropped -- sometimes significantly, and water temps are much higher than they were in the winter and spring: all of this means a decrease in oxygen levels.

As a result -- and I think this is really interesting -- aquatic insects have evolved and adapted to this condition by having the smallest families, genera, and species show up in the summer.   The smaller the body, the less oxygen needed to keep it alive!   The Acentrella small minnow mayflies I'm seeing right now are "tiny": I know for a fact that another species of Acentrella shows up in our streams in the fall, and they're much bigger-- easy to see and to pick up.

I think I'm right in saying as well that the families, genera, and species of mayflies that we see in the summer (not sure about stoneflies and caddis) are those with the higher tolerance values, i.e. the ones that can survive in lower oxygen levels.

This information comes from a book that I read, and I wish I could remember the source.  I can't.  So, my apologies to the author for not giving credit where credit is due.  Also, if there are professional entomologists reading this entry who wish to comment on this -- please do.

                                                  (Powells Creek in the summer.)