Aquatic insects obtain oxygen either from air dissolved in water or directly from air via surfacing. Diving beetles (Coleoptera: Dytiscidae) rely on a subelytral air store, which has to be renewed periodically at the water's surface. [Correction added after publication 28 July 2007: in the preceding sentence ‘Diving beetles Ilybius montanus (Coleoptera: Dytiscidae)’ was corrected to ‘Diving beetles (Coleoptera: Dytiscidae)’]. The way in which diving behaviour responds to changes in the insect's environment has not been previously explored, however. Here, we investigate the diving behaviour of the widespread European species Ilybius montanus (Stephens). We go on to examine how different aspects of the diving behaviour of individual insects change with temperature and water acidity, using both hydrochloric and sulphuric acids. Individual surfacing frequency was significantly higher at 20.5°C than at 14.5°C, which resulted from a shortening of the mean dive duration at higher temperatures, no associated change being seen in mean time spent exchanging at the surface. The observed reductions in dive duration may have resulted from increased metabolic rate at higher temperatures, while the constancy of surface time suggests that this represents a trade-off between gas exchange demands and selection to minimize time spent at the surface, perhaps to avoid predation. Although acidity did not affect surfacing frequency or the mean duration of individual dive parameters, it resulted in a breakdown of an individual's ability to regulate diving behaviour with temperature. This effect was most pronounced when experimental water was acidified using sulphuric acid, indicating that anion, as well as acidity itself, is of importance. As the dive response could be broken down into readily quantifiable components, and this behaviour showed marked responses to the factors considered here, we propose that diving activity could serve as an integrative organismal biomarker in these and other surface-exchanging aquatic insects.
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