<i>Lestes disjunctus</i> Selys and <i>L. forcipatus</i> Rambur (Odonata: Lestidae): Some Solutions for Identification

Abstract

Five species of the damselfly genus Lestes live in British Columbia, Canada, and of these, Lestes forcipatus Rambur and L. disjunctus Selys are the most similar and most difficult to separate morphologically. Females can be readily distinguished by the size of the ovipositor, but males are difficult to separate. In British Columbia, L. disjunctus is the more common, widespread and familiar species. Before 1998, L. forcipatus specimens were mistaken for those of L. disjunctus because the former is primarily an eastern North American species and because most Lestes species are usually identified using male characters. With the discovery that L. forcipatus is part of the western fauna, an evaluation of the relative status of the two species in British Columbia is necessary. The best method for separating the two species uses the length of the anterior lamina (part of the secondary genitalia) as a unique character or as part of ratios using other measurements. In addition, in at least western North America, L. forcipatus males are more pruinescent than those of L. disjunctus, especially on the thorax. Identification using the pruinescence pattern was tested in the field and is recommended as a simple and accurate method for western North America. Soaking Odonata specimens in acetone, a common technique used to preserve colours, damages surface pruinescence and should not be used to preserve mature, pruinescent adults, including those of Lestes species. To identify L. disjunctus and L. forcipatus males treated in acetone, it may be necessary to calculate ratios based on various character measurements. Future research should investigate spatial and temporal differences between the species, as well as modes of interspecific communication.