Monday, October 1, 2012

A Flying Swimmer

Rhantus sp. Predatory Diving Beetle
A wet, wet, wet summer has given way to a pleasingly dry Autumn here in Ireland, with some of the southern counties experiencing their driest September in 26 years. This has lead to some water bodies which remained swollen throughout the year to finally experience something of a drought. Some of the more temporary examples have began to dry up and with this disappearing of certain bodies, so the predatory diving beetles, the Dytiscidae, take to the wing in search of new hunting grounds. Voracious hunters, they are found in a variety of fresh water habitats such as streams, ditches, canals and lakes, although usually in shallow water (1). Their prey are quite varied, consisting of a wide number of invertebrate larvae as well as tadpoles and worm species (2), and are excellent swimmers with large back legs perfectly adapted to pursue these prey in the water. These are lined with hairs that spread out when the leg is drawn back (in a rowing motion) and then lie flat when the leg is pushed forward, reducing drag (3). Drag is further reduced by the shape of the of the beetle's body which is streamlined for rapid movement through the water. This is tempered somewhat with the need for stability, giving Dytiscids excellent maneuverability.
Rhantus sp. Predatory Beetle
Its something of a shock therefore to see these masters of the waterways barreling through the air with all the grace of a carelessly tossed stone. Moving to new territories, Dytiscids fly with a refreshing directness, generally only halted by the water they come seeking (the individual pictured just managed to avoid me on its approach flight). The fact that many species are of considerable size (up to 4 cm long) makes seeing them a little alarming. This propensity towards large size is linked to reduction of drag in the water, with larger bodies animals experiencing much less drag than their smaller relatives (3). Ireland is home to quite a number of Dytiscidae, with 86 species being recorded here (4).

  1. McGavin, 2005. Insects and Spiders of Britain and Europe pp. 86-87
  2. Aditya and Saha, 2006. Limnologica - Ecology and Management of Inland Waters 36 pp. 251-257
  3. Nachtigall, 2009. In Encyclopedia of Insects pp. 972-975
  4. Ferriss et al., 2009. Irish Biodiversity: a taxonomic inventory of fauna p. 98

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