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

Journal of the Entomological Society of British Columbia, Dec 2004 by Simaika, John P, Cannings, Robert A

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.

Key Words: Odonata, Lestes forcipatus, Lestes disjunctus, identification, British Columbia, pruinescence, acetone, anterior lamina.

INTRODUCTION

Five species of the damselfly genus Lestes (Odonata: Zygoptera: Lestidae) occur in British Columbia (BC), Canada: L. congener Hagen (Spotted Spreadwing), L. disjunctus Selys (Northern Spreadwing), L. dryas Kirby (Emerald Spreadwing), L. forcipatus Rambur (Sweetflag Spreadwing), and L. nnguicnlatus Hagen (Lyretipped Spreadwing). L. disjunctus is the most common, widespread and familiar Lestes species in the province, and one of the most abundant odonates in Canada, ranging as far north as the Arctic treeline (Cannings 2002). It inhabits many types of standing water habitats with abundant aquatic vegetation and, in southern BC, adults fly from mid-June to mid-October (Cannings 2002).

L. forcipatns is generally much less common than L. disjunctus, although it is as abundant in some cold fen habitats, and both species often occur at the same site. L. forcipatits does not range as far north as L. disjunctus and, although not known from much of BC's north, it has been collected in the southeastern Yukon. In the western Canadian Cordillera, it is most common in sedge fens (Cannings 2002). Walker (1953) described L. forcipatus habitat in Ontario as "ponds, both temporary and permanent, marshy lakes, and slow, weedy streams". In BC L. forcipatus has been collected from mid-June to mid-September (Cannings 2002).

L. forcipatus was not reported in BC until 1998, when it was first collected in the Rocky Mountain Trench north of Golden and subsequently found in many other localities in the southeastern part of the province. However, it probably has long been a resident of the province; it was long overlooked because of its close resemblance to L. disjunctus (Ramsay and Cannings 2000). Before 1998, L. forcipatus was not known west of Saskatchewan (Walker 1953, Westfall and May 1996), and had just recently been found in Washington State, the first record west of Montana (Ramsay and Cannings 2000). The species is now known from seven counties in that state and one in Idaho (Paulson 2004). By 1999 L. forcipatus had been discovered at several other BC locations farther south and west, and by 2000 had been collected on Vancouver Island. Some of our old museum specimens of L. disjunctus from many regions of the province have been re-identified as L. forcipatus, indicating that museum collections across western Canada probably contain many misidentified specimens.

Males of L. disjunctus and L. forcipatus are difficult to separate, although numerous characters have been employed in identification (Walker 1953, Westfall and May 1996, Catling 2002, Donnelly 2003). The usual method of distinguishing the two species and confirming their presence at a location is through identification of the females. In L. forcipatus females the ovipositor valves reach the tips of the cerci; in L. disjunctus they do not (Walker 1953, Cannings 2002) (Fig. 1).

Lestes species are usually brown, black, metallic green or bronze above and mostly pale below; especially in males, the head, thorax, base and tip of abdomen become pruinescent bluish white with age. Pruinescence (pruinosity) is a waxy substance produced by the hypodermis in many groups of Odonata and excreted on the cuticular surface through porous canals (Gorb 1994). Pruinescence is implicated in thermal regulation in dragonflies (Garrison 1976, Paulson 1983) and is thought to play a role in species recognition and intraspecific communication - indeed, the patterns of pruinescence in males may be a result of sexual selection (Jacobs 1955, Corbet 1999). Therefore, pruinescence patterns might offer good species identification characters, especially in males.