Cinygmula subaequalis: do the nymphs differ by gender?

One of the flatheaded mayfly nymphs that easy to ID, even just using a loupe in the field, is Cinygmula subaequalis (subaequalis is the only species in the Southeast).   We need only note the two bumps at the sides of the head, which are the maxillary palpi.

But the color patterns I've seen on the nymphs that I've found aren't always the same.  Some, like the one in the photo at the top of the page, are essentially brown, others like this one

have a distinct pale area on the frons (the forehead). 

Last Sunday, I found two nymphs at the Rapidan River, one of each type.

It occurred to me that this might be a gender distinction, something we commonly see with small minnow mayflies, so I ran this by Steve Beaty.  He wasn't convinced, feeling that those with the pale areas on the top of the head might simply be nymphs that have just recently molted.  But I still wonder about this for one very good reason: the distinction is one that's consistent.  That is to say, where there are pale areas on one of the nymphs in the photos above, there are pale areas on the other nymph as well.  Have a look.

There's the spot on the head -- which is shaped the same on both of our nymphs, a second spot on the mesonotum, and the first tergite is pale.  (So too are terga 9 and 10, but that is also true of our solid colored nymphs.)  This is something I wouldn't expect if this were simply a matter of molting.

Time will tell, but I'll keep an eye out for these features on other nymphs that I find.
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Some other nice pics at the Rapidan River last week.

1) A fairly mature Ephemerella subvaria.

2) The stonefly, Agnetina capitata.

And 3), a rather spectacular pronggilled mayfly, Neoleptophlebia assimilis.

If you're a fly fisherman, pretty clear that the Blue Quills are hatching.

An odd pair of spiny crawlers at the Doyles River yesterday

It's the time of year when we see a lot of spiny crawlers in all of our streams, for the most part, genus Ephemerella.  E. invaria nymphs are present in appreciable numbers from February to April; E. dorothea nymphs show up in HUGE numbers from April to May to June.  I saw a lot of them yesterday at the Doyles River including this striking pair, male and female.

It wasn't until I got home and downloaded my photos that I noticed just how striking they were.  What's unusual is the orange spots on the sides of terga 5 and 6 and at the top edge of the mesonota.

Thinking I might have found a species I'd not seen before, I did the microscope work, using Beaty's "The Ephemeroptera of North Carolina" as my guide.  In the end, they keyed out to E. invaria, or rather, "E. invaria group".  Let me review the critical features.

Nymphs 6-13 mm; pale transverse stripe between eyes, may be interrupted medially; tarsal claws with 5-10 denticles; abdominal terga with short, sharp, paired submedian tubercles on segments 2-9, rarely on 2, sometimes barely discernible on segments 3 and 8, small on 4-7, rarely on 9; posterolateral projections on abdominal segments 3 or 4-9 (variable to absent on 3); may have dot-dash pattern on pale ventral surface and speckling on last few segments.  ("The Ephemeroptera of North Carolina," p. 52)

1) Our male nymph measured 6 mm, the female -- the larger and lighter of the two -- measured 8.

2) The pale stripe between the eyes is clearly visible on both of our nymphs; they don't seem to be "medially interrupted."

3) I could see 5 denticles on the tarsal claws, with a dark spot behind them (more denticles?).

4) On the female nymph (didn't check the male), there are paired, sharp pointed tubercles on segments 3-9.  In this photo, I've pointed out those on segments 4 and 5.  (Note, the "tubercle" is just that part of the pale spot that projects beyond the rear edge of the terga.)

5) There are posterolateral projections on terga 4-9, and yes, the last few ventral segments are speckled, and the dot-dash ventral pattern shows up very clearly.

So, Ephemerella invaria nymphs for sure.  But two things to keep in mind.  Beaty notes that "This is the most variable Ephemerella species in terms of size, color pattern, and size of tubercles," number one, and two, E. invaria is now considered to be a "group," which includes the species E. inconstans and E. rotunda.  No way to know which member of the "group" I found without finding some adults.
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The other taxa that is crowding our streams at the moment is Isoperla montana, the nymphs that hatch as "Yellow Sallies" for you fly fishermen.  They were also prominent yesterday.