Summary
Female colour polymorphism in coenagrionid damselflies is genetically determined for the 4 spp. so far studied. No size differences between (female) morphs have been reported. In another sp., size differences between morphs do exist but the genetic basis of the polymorphism has not been explored. In N. irene, 2 (female) morphs exist: one is similar to the male in both colour and pattern (the androchrome), whereas the other differs from the 6 (the more common gynochrome). No differences are shown in lengths of wing, femur or tarsus between androchromes and gynochromes, nor any differences in multivariate estimates of size or in wet mass corrected for these size estimates were found. The study controls for time of season, which is known to influence the size of emerging temperate damselflies. The results concur with results from other spp. where the genetic basis of colour polymorphism is known.
Methodology
Nehalennia irene is a small damselfly (body length 25-28 mm; CANNINGS & STUART, 1977) which is ubiquitous in most ofCanada (WALKER, 1953). It begins emergence in the first week of June at our study location, but the reproductive season can last tillmid-August. This species generally has a very short teneral period (< 24 hours)(WALKER, 1953), but even teneral females can be distinguished to chromatype based on pattern. Our working hypothesis is that male-like females may differ from typical females and demonstrate more male-like morphology in multivariate space. To this end, we measured wing lengths of similar numbers of males, male-like females and typical females from one study location over three days. We did this because size declines with season in many temperate damselflies (FORBES&BAKER, 1991;CORBET, 1999; JOHANSSON & ROWE, 1999) and we wanted to control for any such variation. Additionally, we measured femur and tarsal lengths and recorded wet mass, as detailed below. Finally, we also examined wing cell asymmetry for each wing pair. Fluctuating asymmetry, including wing cell asymmetry, has been associated with various metrics offitness and stressful conditions in other damselflies (e.g. HARVEY & WALSH, 1993;CORDOBA-AGUILAR, 1995;BONN etal., 1996; HARDERSON &WRATTEN, 1998; HARDERSON, 2000; but seeLEUNG & FORBES, 1997).
N. irene were collected from June 26 - June 28, 2001 at Indian Lake Bight located 1 km from the Queen’s University Biological Station near Chaffey’s Lock,Ontario, Canada (44°34‘N, 79° 15'W). Females were identified as either androchrome (with distinctive male-like triangular patch of pale blue on abdominal segment VIII and a pair of dark spots on segment IX) or gynochrome (without distinctive male-like characteristics on segments VIII and IX) (following FORBES et al. 1995).On each trip, individuals were collected using sweep nets and put in transport cages (30 x 30 x 40 cm). In the laboratory, damselflies were measured for wet mass (to the nearest 0.0001 g using Mettler AE100 Digital Scale). Wing length from the nodus to the pterostigma on the right forewing was also measured (to the nearest 0.01 mm; Mitutoyo digital calipers), as was cell asymmetry of all wings (based on wing-cell counts between the nodus and pterostigmataon the left and right forewings andhindwings). Finally, the right metathoracic leg ofeach damselfly was mounted and preserved in Glycerol on a microscope slide. The last tarsal segment and the femur were measured using a micrometer (to the nearest0.0001 mm; Graticules LTD, Tonbridge, Kent England) on a Leitz Laborlux K microscope (at 25 x and 10 x, respectively). In some instances, the last tarsal segment could not be measured, as tarsal morphologywas abnormal (see FORBES & BAKER, 1989).