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.