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UPDATE May 2014: On May 09, 2014 I found two patrolling male White-tailed Sylphs in Del Rio, Texas. I gote a few decent flight pics plus lots of crappy ones; I kept a couple of not-sharp images to use as comparisons to this individual. I feel that they lend more weight to the argument that this 2008 record was inded a White-tailed Sylph. I have added the images at strategic points to provide close comparisons with the photos of the 2008 bug:

This male presumed Macrothemis pseudimitans was seen feeding briefly at tree-top height at Santa Ana NWR, Hidalgo county, Texas on October 30, 2008. There is no record for any spatulate-tipped Macrothemis in the LRGV (nearest record is of M. imitans/leucozona (Ivory-striped Sylph) from Frio county, south of San Antonio and also from Tamaulipas, Mexico southwards).
M. pseudimitans (White-tailed Sylph) is a taxon known from Tamaulipas, Mexico southwards (with one record from Arizona in September 2007; UPDATE - a 2nd from Arizona in Oct 2012); the only other contenders are M. inequiunguis (Three-striped Sylph) known from north and west of San Antonio, Texas, plus Nuevo Leon, Mexico, southwards, and M. hemichlora, also known from Tamaulipas, Mexico southwards. Both these latter two taxa have two or more complete (or almost complete) pale stripes on the side of the thorax, and inequiunguis has a visibly narrower spatule at the end of the abdomen:

I only got two photos before it disappeared...

original size:

enlarged:

UPDATE, May 2014: these are pics of the two May 2014 White-tailed Sylphs from Del Rio: I have added a small amount of motion-blur, and you can see how the thoracic pattern would match that of the 2008 bug were one to change the angle of the body to match:

enlarged more; note that the thorax seems to have four large white patches separated by dark lines (the size of these white patches may be exaggerated by motion-blur):

Compare the image above to the two below of a M. pseudimitans; the 2nd version of the image has been motion-blurred to show how the white area can look "fatter"; also note that the much shallower camera perspective on the TX individual will foreshorten (= make fatter) any longitudinal features:



original:

rotated enlargement: UPDATE, May 2014: to the right are a pic of one of the May 2014 White-tailed Sylphs from Del Rio (above) and a pic of an Ivory-striped Sylph (below); note all the structural similarities/differences with the 2008 bug, allowing for a slight difference in body angle:



larger:



Note the short, converging cerci, and the exact shape of the "spatule":




In the comparison below of a specimen pseudimitans (top; thanks to Dennis Paulson) with the Texas individual (bottom), note the similarity in the shape of S6 (its relative narrowness distally), the general rate of curve through S7 and S8, the rapid narrowing on S9, the thickness of S10, and the in-proportion short triangle formed by the appendages:





In the comparison below of various leucozona (top three; fifth and sixth pic) with the Texas individual fourth pic), note the difference in the shape of S6 (its relative broadness distally), the gentler rate of curve through S7 and S8, the less-rapid narrowing on S9, the thinness of S10, and the out-of-proportion long (almost parallel-sided) triangle formed by the appendages:








Not only are the cerci of pseudimitans shorter and more-convergent than either leucozona or hemichlora, segments 7 and 8 seem longer, too. I measured profile images of the three taxa and compared them to the TX individual thus:
Referring to the large leucozona image above, I created a ratio of X : Y (i.e. "tip of cerci to base of S7" compared to "length of cerci"): UPDATE MAY 2014: added more data in green:

twelve leucozona: 4.47; 5.09; 4.69; 4.47; 4.43; 4.57; 4.96; 4.87; 5.05; 4.69; 5.00; 4.64
three hemichlora: 5.19; 4.90; 4.90
five pseudimitans: 6.18; 6.03; 6.40; 6.35; 6.24 - the 2 from Del Rio in May 2014

The Texas individual
(using a "longest-plausible" cerci length):

6.35

- the ratio for the TX individual is more than 20% larger than the largest value for leucozona or hemichlora, while being inside - at the upper end - of the range for pseudimitans.