Risky gleaning behaviour by Ischnura elegans (Vander L.) (Zygoptera: Coenagrionidae)

While in southern Spain on 26 August 1995, we witnessed what we believe to be an unusual event concerning the coenagrionid I. elegans. We were walking down the valley of the Canuelo River close to the small town of Jimena de la Frontera and had stopped at about 10.30 local time for a rest beside a pool of the barely-flowing river. (On the 1:50,000 scale Jimena de la Frontera map. No. 14-46, the map reference of the site, an abandoned mill, is approximately 790 344). Our attention was attracted by a female : I. elegans which was perched on a small branched twig projecting from the water. The twig supported a small spiders’s web which was almost horizontal and a few centimetres above the water surface. An unidentified spider was in residence, resting with legs extended anteriorly and posteriorly, partly on the twig and partly on its web. The spider was about 12 cm from the damselfly. Several times in the course of 15-20 min very small Diptera became trapped on the web. Three times the Ischnura responded by taking off and deftly plucking a small fly from the web and returning to its resting position on the twig. On one occasion it attempted to remove a small fly from the web, but failed. On each of these gleaning flights the spider showed no response. However, at one point the spider moved its legs and this caused the Ischnura to leave its perch very briefly before returning to its original resting position. Each time the Ischnura removed or attempted to remove an insect from the web it remained clear of the sticky threads and never became entangled. We report this gleaning behaviour because we are unaware of any similar activity involving coenagrionids and spiders.