A female Widow Skimmer dragonfly at Prettyboy Reservoir, Baltimore County, Maryland, USA. © 2017 S. D. Stewart
Note: While I initially thought this was a female, widow skimmers are sexually dimorphic, meaning that even though the mature males and females look different, the immature males look similar to the females (this is also not uncommon in birds). A good way to separate the sexes is by their terminal appendages, as nicely illustrated in this post by Walter Sanford. The female has two, while the male has three. In order to determine this, one needs a clear close-up view. Unfortunately the resolution of my images is not quite high enough to determine the sex for certain. When I have the image magnified, it looks to me like there is possibly an epiproct present, but because of the angle of the shot I can’t be sure.
UPDATE: Walter Sanford stopped by and identified it as a female (see comments). Thanks, Walter!
s. d. stewart 
Tom's Nature-up-close Photography and Mindfulness Blog
/ July 28, 2017Great shot! :)
Anonymous
/ July 29, 2017Thanks for citing my blog! This individual is a female.
waltersanford
/ July 29, 2017This individual is a female. The “nubbin” at the tip of the abdomen is actually the S10, the last of 10 abdominal segments, rather than an epiproct.
birds fly
/ July 29, 2017Ah, good to know. Thanks very much, Walter!
birds fly
/ July 30, 2017Additional image showing possible epiproct.
birds fly
/ July 30, 2017Walter reports that what I thought could have been an epiproct is more likely to be dragonfly excrement. He even has a pertinent photo for comparison on his excellent blog. Thanks again, Walter, for all your help here. Much appreciated.