Thursday, June 24, 2010

The Trials of the Azure Damselfly

The Azure Damselfly, Coenagrion puella

Darwin, in The Descent of Man (1871), stated the development of a character under sexual selection may be limited by an opposing force of natural selection. This is beautifully illustrated in a study by Banks and Thompson (1985, Animal Behaviour 33 pp. 1175-1183) on mating habits in the Azure Damselfly, Coenagrion puella.

C. puella is a medium sized damselfly with a body length of 23-30 mm. It is sexually dimorphic, the males being larger than the females and have less black marking on the abdomen. They are associated with slow moving water areas although individuals may be found well away from water in suitable areas such as wet meadows (Gibbons, 1999 Insects of Britain and Ireland pp. 21). Males distinguish females from other similar species (C. pulchellum, and to a lesser extent Enallagma cyathigera) mainly by color, color pattern, body shape, and flight style. Males will pursue and attempt to grasp any female that comes within sight, and if successful in achieving tandem, will attempt immediately to initiate copulation (Tennessen, 2009 “Odonata: Dragonflies and Damselflies” in Encyclopedia of Insects pp. 721-729). This occurs when the male clasps the female's prothorax with his anal appendages. The female then curls her abdomen upwards to bring her genitalia into contact with the male's accessory genitalia under his second abdominal segment.

Banks and Thompson found that a male's mating success is determined, not by size, but by the number of days it spends at a breeding site, which is in turn determined by the life span of the male. They also found that weather conditions had a major effect on mating success as C. puella only engages in reproductive activity on warm, sunny days. This evidence illustrates that variance in male reproductive success is neither evidence for sexual selection, nor a measure of its intensity.

The Azure Damselfly, Coenagrion puella

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