Could J&J perhaps elaborate on their "likely" comment? I have been looking for related information for years.

(I tried to "Comment" on J&J's comment but got thrown into the general comments of Kevin's ID request - "Preview" is somewhat limited - perhaps in BG2...)

In researching this behavior myself, I came across some interesting links. Apparently, Pantala flavescens, the Globe Skimmer, is the only other species in this genus, and makes the longest insect migration known:

http://en.wikipedia.org/wiki/Globe_Skimmer

http://www.ted.com/talks/lang/eng/charles_anderson_discovers_dragonflies_that_cross_oceans.html

We seldom see single species groups when we're out. Looking up from an open field we will see Aeshna, Anax, Tramea, Libellula, Pantala, and others flying overhead. Even in the fall when huge numbers of Anax seem to gather at Lake Michigan and fly south, we see other species in the mix. We don't know if the species composition has been measured. Here in Illinois that would likely be the two Tims, Tim Vogt and Tim Cashett, who are our leading Odonate experts, who likely would have done the sampling. We may try to see if we can find anything by them. Here is an article about swarms by others.