Occurrence and taxonomic significance of a palpal spine in larvae of Enallagma and other genera (Odonata: Coenagrionidae)


A small inconspicuous spine, first detected at the base of the distal-most seta on each labial palpus of early stadia of two species of Enallagma, is reported to occur in final-stadium (F-0) larvae of Palaearctic and Nearctic E. cyathigerum, in F-0 of 31 other Nearctic species of Enallagma and in F-0 of three other coenagrionid genera among 11 inspected for this character. The spine is absent from F-0 of eight other coenagrionid genera, including Coenagrion and Ischnura. It is also lacking from F-0 of the two species of Afrotropical Enallagma that we examined, a discovery that suggests the latter may not be closely related to Nearctic and Palaearctic species. In European populations such a spine occurs in the first few stadia of certain Coenagrion species but persists to F-0 only in E. cyathigerum. We re-emphasize the potential value of this spine as a means of distinguishing at least the last three stadia of E. cyathigerum from those of other coenagrionid genera in Europe, and very probably from Coenagrion and Ischnura everywhere.

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