Monday, October 17, 2011

The Size of the Emerald Damselfly

Female Emerald Damselfly, Lestes sponsa
In the race to be the top of the heap in biological terms, organisms can follow two paths. They can take their time about it all, growing nice and big and strong, but missing out every time a potential mate comes by and they are not developed enough to take advantage of the situation. Or they can rush into things, becoming sexually mature quickly but growing to be small in size, and possibly unattractive to potential mates. What makes this choice all the more difficult is the prevalent environmental conditions that can, and often do, hinder the organsisms development. Availability of resources, threats from predators and the vagaries of the seasons mean that the chosen path is soon skewed. This has been elegantly demonstrated in a series of studies on the Emerald Damselfly, Lestes sponsa (1). This attractive, bottle-green odonate overwinters in the egg stage, has a brief larval period in spring, then emerges and reproduces in late June, the latest Irish damselfly to do so. It is common and widespread through out Ireland, commonly found near small lakes and acid ponds. L. sponsa individuals were measured for foraging activity, development rate and mass at emergence when exposed to the presence/absence of predators and the perceived onset of winter, and the abundance/scarcity of food and, again, the perceived onset of winter. In the presence of predators, all three perameters measured were reduced, while individuals under time constraints showed increased foraging and development, but decreased mass at emergence. Larvae that had an abundance of food showed high levels of all three parameters, while those that had time constraints in high levels of food showed lower mass at emergence, as was to be expected. What was not expected, however, was larvae at low food levels under time constraints showed very slow development and that largest mass at emergence. This is thought to be a result of this last group delaying emergence until the following season. Such life history plasticity is a feature of the life of L. sponsa.
Female Emerald Damselfly, Lestes sponsa
  1. Johansson et al., 2001. Ecology 82 pp. 1857-1869

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