Sunday, September 9, 2012

Perching for Prey

Four Spotted Chaser, Libellula quadrimaculata
Other than the ubiquitous Common Darter (Sympetrum striolatum), probably the most common dragonfly encountered in Ireland is the Four Spotted Chaser (Libellula quadrimaculata). This is due to its incredible aggressive behaviour. Both males and females move with an almost frantic speed around their territory and will challenge and fight any other odonates that encroach (1). The individuals pictured were observed around a c. 20 x 10 m pond in a dune slack which was 1 m at its very deepest. The pond had cluster of low growing willow (1 m above the waterline at its highest) at its center, ideal for the dragonflies to perch. L. quadrimaculata was observed to patrol the pond at high speed and challenge both the S. striolatum and Blue Emperors (Anax imperator) that were also present at the pond. A. imperator is considerably larger than L. quadrimaculata, but this did not deter it from the challenge. L. quadrimaculata was also seen to patrol the areas bordering the pond and to chase prey. One individual was seen to catch and eat a Comon Blue butterfly (Polyommatus icarus), taking the prey to the grasses in the surrounding dunes to consume it, as is illustrated in the photograph. After patrolling, L. quadrimaculata was seen to perch either among the willow in the middle of the pond or among the grasses in the surrounding dunes.
Four Spotted Chaser, Libellula quadrimaculata, feeding on a Common Blue butterfly, Polyommatus icarus
When perching, L. quadrimaculata held their bodies at a seemingly awkward angle (often 45°) in relation to the perch, even though resting their abdomens against the perch was often an option. The reason for this is not related to the animals body , but to its eyes. The compound eye of odonates, and many other insects, can be divided into dorsal and ventral areas where the structure of the fovea varies (2). In many odonate species, the different areas of the eye are quite easily distinguished from each other, with the dorsal side being much lighter in colour due to the presence of screening pigments (3). The dorsal fovea exhibits a high spatial resolution as well as a high degree of sensitivity to short wave resolution. This makes it ideally suited to the detection of prey flying against a blue sky. Therefore L. quadrimaculata perches in such a way that its dorsal fovea is exposed to the maximum amount of blue sky (4).
  1. Nelson and Thompson, 2004. The Natural History of Ireland's Dragonflies pp. 259-267
  2. Kral, 2002. European Journal of Entomology 99 pp. 1-4
  3. Labhart and Nilsson, 1995. Journal of Comparative Physiology 176 pp. 437-453
  4. Sauseng et al., 2003. European Journal of Entomology 100 pp. 475-479

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