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Update 4: Here is a link to Troy Hibbitt's sighting of a similar individual in July 2009. I feel this similar female stretches to likelihood of them both being abberant diapyra that happen to mimic apeora beyond the limits of credulity...

Update 3:
I've added a couple more potential ID features to the list below, based on further research; it is possible that with access to a larger sample set, more features might be determined.

Update 2:
I've added a few
new images (marked "NEW IMAGE:") that emphasize the mesepisternal and thoracic suture features. plus a couple of uncropped flight pics to show its foraging height:

I received feedback from various experts, and those who have expressed an identification have all gone for C. apeora - an apparently rare species known from Veracruz (and probably Nuevo Leon), Mexico southwards into Central America (and probably Cuba) that Dennis Paulson described in 1994 along with a similar taxon, C. diapyra, with a similar known range (see Odonatologica volume 23 pages 379 - 392). It seems that this individual is definitely one of these two taxa, and that all visible features favor apeora. Here are the relevant ID features, as I understand them from talking to Dennis and studying the few photos I've been sent of these two taxa:

1) Front of thorax: apeora has a brown stripe/mark of variable length on each mesepisternum (= the two thoracic plates behind the eyes that are otherwise all green, separated by a narrow brown carina); diapyra lacks any brown marks on the mesepisterna.
2) The frons (= top of head in front of the eyes): female apeora has a thickish-based black T-spot that does not change with age; female diapyra starts as a teneral with a black T-spot (possibly the central base is thinner than on apeora) but soon (how soon?) has this fade or change color such that it largely disappears, resulting in an almost uniform frons on adult females. What percentage of female diapyra of similar age to the Texas individual would show such a prominent thick base to the "T" mark?
3) Lateral thoracic sutures (= two main plate join-lines on the side of the body): apeora has a thin but obvious brown line along both sutures, with the anterior line being very slightly thicker in the dorsal half, while the posterior line is very slightly thicker on the ventral half; diapyra has virtually no color along these sutures, showing a hairline that is very slightly more prominent on the upper third of the posterior suture. Fresh teneral individuals of diapyra may have the suture lines slightly more visible, but probably not to the extent typical of apeora. What percentage of female diapyra of similar age to the Texas individual would show a suture line pattern exactly mimicing that of typical apeora?
4) Size of pterostigma (= length of dark marks near wingtips): apeora appears to have slightly longer pterostigma; on 3 available apeora wings, the FW pterostigma subtends 4 or 4-plus cells and the HW subtends 4 or almost-4 cells; on 3 diapyra wings the FW pterostigma subtends 3-plus cells and the HW subtends 3 or almost-3 cells; Dennis Paulson made a brief check of his specimens and found a female diapyra with 4 cells under the FW stigma and 3 cells under the HW stigma, so there is some overlap - but I wonder what percentage of female diapyra have the FW pterostigma subtend 4 cells and the HW subtend 4 cells, as on the Texas individual - ?
5) Veination between Cu1 and Cu2 (= number of split cells between these two hindwing veins): A small sample of each taxa indicated that apeora has more split-cells between these two veins than does diapyra; two female apeora specimen photos have 4 or 5 pairs of split cells; the Texas individual has 4 or 5 pairs of split cells in each hindwing, judging from the photos. Dennis Paulson sampled 24 female diapyra and across the 48 hindwings they averaged 2.333 pairs of split cells. One aberrant individual had 6 and 7 pairs; only one had 4 and 4, and only two had 4 and 3 pairs. Thus less than 2.1% of female diapyra in this sample had a split-cell count that matched or exceeded that of the Texas individual (which matched exactly the very few female apeora available to sample).
6) Cell density in the basal trailing sector of the hindwing (= number of cells in a discrete section of the hindwing below the triangle): Count the total number of cells between the wing margin, veins A3, Cu2, and a line perpendicular to the wing margin drawn to it from the most-distal point of the Triangle. I got 47 cells for two diapyra, 40 for one apeora. This seems counter-intuitive given that apeora is larger than diapyra, and seems to reflect a genuinely denser-veined sector in diapyra, despite the average smaller size. I count 39 cells in this same sector on one wing on the Texas individual.
ere is another way of expressing this density difference:- take any cell on the wing margin between vein A3 and the perpendicular line described above, then count the shortest number of cells it takes to reach Cu2. For any particular "row" in the sector, most of the time there will be one more cell in that count for diapyra than for apeora: for example, if you choose a margin cell close to the end of A3 and count inwards roughly along the outside of vein A3, you get six cells from the margin to Cu2 for the two sampled diapyra, and five cells for the one sampled apeora (and the Texas individual). For the majority (but not all) of the "rows" in this sector, the same is true - certainly there seem to be a number of "rows" that contain six cells in diapyra, while in the apeora (and the Texas individual) there are no "rows" with as many as six cells, in this sector. It would be helpful to know the equivalent total/"row" counts on Dennis' 24 female diapyra so that we can quantify the value of this feature.
7) Flight: apeora is Greek for "flying on high" - so-named by Dennis because of this taxon's habit of flying almost constantly, and at tree-top level or higher; diapyra typically flies much lower and is much easier to net than apeora. The Texas individual persistently fed at tree-top height for the entire period of observation, only going lower on two brief occasions - once to land.
8) Size: apeora is significantly larger than diapyra; apeora is roughly similar to Regal Darner C. ingens in size (the Texas individual was assessed to be between ingens and E. heros in size - the observer having seen a number of these forms in east Texas just a few days previously).

- there may be other subtle features that have yet to be worked out; any potential differences in the underside of S8 - S10 are not visible in any images of these taxa known to me.

I've changed the embedded text below to reflect this new situation:-

This female Coryphaeschna darner was photographed April 15, 2008 c.4 miles east of Santa Ana NWR on the north bank of the Rio Grande, Hidalgo county, Texas; it spent most if its time feeding in a c. 30 yard "beat" at tree-top height (or above), just twice briefly dropping down to above head height; it perched once - 20+ feet up - for c. 45 seconds; it was a large darner - I estimated its length to be between Swamp and Regal Darners (E. heros and C. ingens) - two species i'd seen in a feeding swarm the week before in east Texas:
The shape of the lower-base of the hindwings clearly make it a female...

... but the full-length petiolate cerci are very short-looking for this genus.

Note in the image below, the (arrowed) brown elongate mark in the center of the mesepisternum (green panel on the lateral-front part of the thorax); also note (arrowed) the brown lower end of the posterior thoracic suture:

NEW IMAGES: The two images below are from two seperate originals to the one above - each at a slightly different angle - yet all three clearly show the mesepisternal mark:

note that in the image below - at a different angle to the one above - the brown mesepisternal mark has been drastically foreshortened and has disappeared into a shadowed area; note above and below the prominence of the T-spot on the frons, and the thickness of the black base of the "T":

NEW IMAGE: A new version of the above photo that shows the mesepisternal mark (here at its least-visible angle and in shadow):

It is clear that the pattern of the thorax rules out C. ingens and teneral female C. adnexa (which have wider brown stripes, almost like those of C. ingens - per photos by Michael Veit and Tom Langshied):

- and a closer look reveals the brown mesepisternal mark visible above the end of the folded foreleg, plus the thin but obvious brown lines along the thoracic sutures:

NEW IMAGE: Better-showing the brown mesepisternal mark extending beyond the folded foreleg (also note the thoracic suture lines):

Note that the cerci, when "folded-back", do not reach the joint suture of S8/S7; compare this individual in the first two following images with the third image - a specimen C. apeora:

note also that the wings completely lack any hint of coloration:

above is the original; below a zoomed-version adjusted to reveal more detail in the shadows:

notice again how short the cerci look in the flight images:

above is the original; below a version adjusted to reveal more detail in the shadows:

NEW IMAGES: Here are two shots to show the typical flight height of this darner: