Identity and synonymy
Aeshna crenata, a Euro–Siberian species, was thought by some scientists in the 19th century such as Hermann Hagen (in his 1875 Synopsis of the Odonata of America), to be synonymous with A. eremita, the two actually being one species extending from Labrador to Siberia. Philip Calvert endorsed this view in an 1894 paper titled "On the specific identity of Aeshna clepsydra Say and Ae. crenata Hagen (eremita Scudder)". Calvert considered the synonymy of A. crenata and A. eremita to be an established fact "known since 1875", but after a morphological analysis, he concluded that the two were in fact synonyms of a third, North American, species, Aeshna clepsydra. Comparing a set of features of specimens identified as A. eremita and A. clepsydra, Calvert found: "None of these differences are constant, and they vary independently of each other. I can find no characters that are invariable, and consequently regard the two specimens here tabulated as two extremes of one and the same species." He considered Aeshna clepsydra to the valid name of a species extending from Finland through Siberia to Illinois and Labrador. Edmund Walker was far less enthused. In his 1912 monograph, he wrote: "Calvert ... attempted to show the identity of Ae. eremita Scudd. with Ae. clepsydra Say. This study, ... although a careful piece of work, ... takes no cognizance of the important characters found in the accessory genitalia of the males and the thoracic colour-pattern. A detailed study of all these forms in much larger series leaves no room for doubt as to the specific distinctness of eremita and the comparison of this form with the types of crenata likewise shows that these two forms are also quite distinct."
Aeshna hudsonica, described in 1875 by Belgian scientist Edmond de Sélys Longchamps, is considered a subjective synonym of A. eremita. Sélys published an account in the Entomologist's Monthly Magazine of dragonfly specimens collected in Newfoundland by one John Milne, and described based on five specimens a new species similar to Aeshna juncea which he called Aeshna hudsonica. Edward Bruce Williamson, in a 1906 article on Newfoundland Odonata, dismissed A. hudsonica as "hardly distinct from A. juncea, even as a variety". In 1908, René Martin published the Collections Zoologiques du Baron Edmond de Selys-Longchamps: Catalogue systematique et descriptif in which he provided detailed illustrations of the male anal appendages of A. hudsonica, A. juncea, and A. clepsydra, listing A. eremita and A. crenata both as synonyms of the latter. Martin admitted that A. juncea and A. hudsonica appeared very similar, but listed a number of diagnostic differences in the colouration and the shape of the appendages. In 1910, however, Richard A. Muttkowski synonymized A. hudsonica with A. eremita without detailed explanation. Canadian entomologist Edmund Murton Walker found in his 1912 monograph on North American Aeshna that René Martin's diagrams showed "unmistakably that hudsonica and eremita are synonymous, the latter name having priority."
