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Using Wing Vein Coloration to Separate Argia agrioides (California Dancer) and A. nahuana (Aztec Dancer)
 
 










 
Argia agrioides (California Dancer) and Argia nahuana (Aztec Dancer)—both males and females, are virtually identical in coloration and require in-hand examination (or close, sharp photos) of structural differences to confirm identification. Recently, Dennis Paulson found that some male Argia agrioides can be identified by the pattern of markings on the second abdominal segment, but other males require a view of structural characteristics (see Dragonflies and Damselflies of the West, 2009, by Dennis Paulson). Females have some coloration differences too, but these are not definitive because of variation.

Recently I discovered what appears to be another helpful character for identifying both males and females of these species in the field or in good quality, well-exposed photos. On nahuana, the subcosta, radius anterior, radius posterior first branch, and cubitus are noticeably paler (light brown or golden) than other major veins. In particular, the pale subcosta and radius anterior contrast with the darker costa, and especially compared with the costa proximal to the nodus (sometimes more obvious, sometimes more subtle—possibly depending on the angle and lighting). On agrioides the wing veins are relatively uniform in tone without any contrastingly pale veins, or if there is any difference in tone, the costa is paler than the other veins.

Below is a Argia vivida (Vivid Dancer) wing with subcosta (Sc), radius anterior (RA), radius posterior first branch (RP1), and cubitus (Cu) highlighted in red. I happened to have this image available—I'll try to scan the wings of Argia nahuana sometime soon.
 
A close look at specimens of Argia nahuana reveals that there are some differences between the wings' outer and inner surfaces (on the closed wings). On the outer surface, it is the subcosta (Sc), radius posterior first branch (RP1), and cubitus (Cu) which tend to be most pale; on the inner surface, it is the radius anterior (RA) which tends to be most pale, but this can be quite visible through the wings on the opposite side (e.g. when looking at the left side of the closed wings, you may see the pale RA on the right wings).

I haven't had a chance to field test this difference between agrioides and nahuana, but I imagine that it is apparent through binoculars and macro lenses (with adequate lighting), and could help those who do not identify these damsels in-hand to sort out which species are encountered during an outing. It could help to target individuals of nahuana where they are not known to occur within the range of agrioides, and vice versa, where agrioides is not known to occur within the range of nahuana. I think it is always a good idea to confirm the identity by examining structural characteristics in-hand, particularly when individuals are found outside of their known range.

Below is a series of photos of males and females of both species—the two species alternated for easier comparison, with enlargements of the wings. Most of these were taken on Twentymile Creek, Lake County, Oregon, on 26 July 2010, except for the female Argia agrioides which was photographed on the Illinois River, Josephine County, Oregon, on 14 September 2008. The identities of all were confirmed in-hand. In addition, you can check out Ray Bruun's photos of Argia agrioides and Argia nahuana, and Greg Lasley's photos of Argia nahuana for comparison. No doubt there are other photo collections on the web to check out.

Note that there are no apparent contrastingly pale veins on the Arizona individuals of Argia nahuana on Greg Lasley's site. It is unknown if this is due to variation, angle, lighting, camera exposure/flash settings, image processing, or some combination. Greg was kind enough to send enlargements of the wings and there are no visible traces of pale veins in either. Greg does not recall whether those individuals were collected or confirmed in-hand, so the possibility that they are actually agrioides can't be ruled out.

A few of my specimens of nahuana have veins which are just barely paler than the costa. That may be due to postmortem changes, but this does seem to be the case in some live individuals too—look at the second male nahuana below. If this individual was photographed without the use of flash, it's possible that the veins would appear more uniform in tone. I suggest this take home message: if the subcosta, RA, and RP1 are visibly paler than the other major veins (the costa proximal of the nodus, as well as other veins near the posterior margin of the wing), you can assume that you have nahuana, but the appearance of uniformly toned veins is not necessarily indicative of agrioides and closer scrutiny is recommended to confirm the identity.

I'd be interested in hearing about your experience with the usefulness of this character. There is no doubt some variation in both species, but how much is not apparent at this time.

Thanks to Ken Tennessen, Tim Manolis, and Rosser Garrison for reviewing these photos as well as specimens available to them.
 
Argia nahuana, Aztec Dancer
Argia nahuana, Aztec Dancer, cropped
 
Argia agrioides, California Dancer
Argia agrioides, California Dancer, cropped
 
Argia nahuana, Aztec Dancer
Argia nahuana, Aztec Dancer, cropped
 
Argia agrioides, California Dancer
Argia agrioides, California Dancer, cropped
 
Argia nahuana, Aztec Dancer
Argia nahuana, Aztec Dancer, cropped
 
Argia nahuana, Aztec Dancer
Argia nahuana, Aztec Dancer, cropped
 
Argia agrioides, California Dancer
Argia agrioides, California Dancer, cropped
 
Argia nahuana, Aztec Dancer
Argia nahuana, Aztec Dancer, cropped