Notice the brilliant white abdomen—the origin of its common name. The abdomen is wider and flatter than in other species, to show it off to best effect; the underside of the abdomen is black. The white color is the result of pruinosity, which is also visible near the base of the wings. Speaking of the wings, the large black patches are another prominent field mark for this species—as if any additional field marks were needed.
This species generally perches on the ground, when bare areas are available. Here's another example:
Males chase one another back and forth along the shoreline incessantly, and in a very particular way. The pursued male depresses his abdomen to shows its white surface to the pursuer; the pursuer raises its abdomen to show the white surface to the male it's chasing.
Females of this species have small dark spots in the wings, similar to those in female Eight-spotted Skimmers and female Twelve-spotted Skimmers. A key difference, besides the number of black spots in the wings, is that the side stripes along the edge of the abdomen are straight and yellow in the Eight-spotted and Twelve-spotted Skimmers, whereas all but one are white and curve inward at the forward end in the Common Whitetail. This can be seen clearly in the photo of a female Common Whitetail below.
For comparison, here's a female Eight-spotted Skimmer, with her straight, yellow side stripes:
The abdomens of Common Whitetails are so stout and thick that it is difficult for them to maintain the wheel position; therefore, mating is very brief. In addition, it's hard for them to remain attached in tandem, and hence the female lays eggs by herself as the male hovers above her to ward off any intruders.
All in all, the Common Whitetail is a beautiful dragonfly species with lots of interesting behavior to observe. Read more about this and other species in my new field guide, Common Dragonflies and Damselflies of the Pacific Coast.
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