Female dimorphism and physiological colour change in the damselfly Argia vivida Hagen (Odonata: Coenagrionidae)

Abstract

Female Argia vivida appear as two distinct colour morphs in each of two populations studied in British Columbia, Canada. Males and both female morphs also experience a temperature-related physiological colour change. Individuals are "dark phase" at ambient shade temperatures below approximately 20 °C and change to "bright phase" at temperatures above 20 – 24 °C, particularly when basking. Individuals of either colour phase will attempt to mate. Bright phase males reflect not only visible light but also ultraviolet light. Observations of marked individuals and experiments in which males were offered live females (singly or as a pair containing each morph) pinned to long grass stems indicated that males do not show a preference for either female colour morph. The two female morphs do not differ in size, nor do males that mate with each morph. A male removal experiment revealed no change in the relative number of females of each morph that mated each day, suggesting that the morphs do not differ in their ability to attract males or to avoid excessive matings. Several social, ecological, and genetic explanations for sustaining the female dimorphism in the population are discussed.