2806 Environmental Toxicology and Chemistry, Vol. 22, No. 11, pp. 2806–2812, 2003  2003 SETAC Printed in the USA 0730-7268/04 $12.00 + .00 TEMPERATURE INFLUENCES ON WATER PERMEABILITY AND CHLORPYRIFOS UPTAKE IN AQUATIC INSECTS WITH DIFFERING RESPIRATORY STRATEGIES DAVID B. BUCHWALTER,* JEFFREY J. JENKINS, and LAWRENCE R. CURTIS Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, Oregon 97331, USA ( Received 22 July 2002; Accepted 15 May 2003) Abstract—Aquatic insects have evolved diverse respiratory strategies that range from breathing atmospheric air to breathing dissolved oxygen. These strategies result in vast morphological differences among taxa in terms of exchange epithelial surface areas that are in direct contact with the surrounding water that, in turn, affect physiological processes. This paper examines the effects of acute temperature shifts on water permeability and chlorpyrifos uptake in aquatic insects with different respiratory strategies. While considerable differences existed in water permeability among the species tested, acute temperature shifts raised water influx rates similarly in air-breathing and gill-bearing taxa. This contrasts significantly with temperature-shift effects on chlorpyrifos uptake. Temperature shifts of 4.5°C increased 14 C-chlorpyrifos accumulation rates in the gill-bearing mayfly Cinygma sp. and in the air-breathing hemipteran Sigara washingtonensis. However, the temperature-induced increase in 14 C-chlorpyrifos uptake after 8 h of exposure was 2.75-fold higher in Cinygma than in Sigara. Uptake of 14 C-chlorpyrifos was uniformly higher in Cinygma than in Sigara in all experiments. These findings suggest that organisms with relatively large exchange epithelial surface areas are potentially more vulnerable to both osmoregulatory distress as well as contaminant accumulation. Temperature increases appear more likely to impact organisms that have relatively large exchange epithelial surface areas, both as an individual stressor and in combination with additional stressors such as contaminants. Keywords—Aquatic insects Temperature Respiratory strategy Permeability Chlorpyrifos INTRODUCTION Aquatic insects play fundamental roles in freshwater eco- systems. They are important food sources for fish and birds and play significant roles in nutrient cycling and organic ma- terials processing [1]. Because of their ecological importance and diversity, aquatic insects are used extensively to evaluate ecosystem health and water quality through field biomonitor- ing [2,3] and to a lesser extent through laboratory bioassays. Ecologists have observed that certain taxa tend to be extirpated from systems with degraded water quality. Similarly, toxicol- ogists have observed marked sensitivity differences among aquatic insect species [4,5]. Despite the widespread utilization of insects to evaluate environmental quality, life history characteristics that relate to differences in species’ responses to environmental stressors have received surprisingly little attention [6]. To date, func- tional feeding morphology [7] has been the primary life history characteristic explored in relation to degraded ecological con- ditions and has been most useful in assessing streams with severely altered riparian habitats. However, few diagnostic tools currently exist to evaluate insect community-level re- sponses to water chemistry–associated stressors such as tem- perature or chemical contamination. Other life history and physiological characteristics are important determinants of species responses to degraded water chemistry conditions and merit investigation. The small size of aquatic insects results in an extremely high surface-to-volume ratio, which in turn requires that the integument provide a substantial barrier to water and ions. * To whom correspondences may be addressed (buchwalt@usgs.gov). The current address of David B. Buchwalter is U.S. Geological Survey, 345 Middlefield Road, MS 465, Menlo Park, CA 94025. Osmotic gradients favor the passive loss of ions as well as the influx of water [8–10]. This osmoregulatory situation is gen- erally countered by barriers of waterproofing lipids, waxes, and proteins on the integument and/or by compensatory ac- tivity of chloride cells on the body surface (Fig. 1). Insects are extremely heterogeneous with respect to the integument both in terms of structure and function. This is, in part, because of their interesting evolutionary history as secondarily aquatic organisms, arising from numerous invasions of freshwater hab- itats [11]. Integument differences among taxa partially result from a wide variety of respiratory strategies that range from breathing atmospheric air to breathing dissolved oxygen in water through exchange epithelia [12]. In this paper, we use the term ‘‘exchange epithelium’’ specifically to describe a thin layer of cells that are effectively in immediate physiological contact with the surrounding water. Insect respiratory characteristics have received surprisingly little attention, particularly since insects are used so frequently to assess water quality [13]. Studies have demonstrated the importance of biological barriers in terms of contaminant up- take mechanisms and fluxes in aquatic insects [14–16]. Recent findings demonstrate substantial differences among species in terms of water permeability and chlorpyrifos (an organophos- phate insecticide) uptake rates. Highly water-permeable insects (dissolved oxygen breathers) have higher chlorpyrifos uptake rates than slightly water-permeable insects (air breathers) [17]. These differences are likely based on vast differences in phys- iological interaction or connectivity with the surrounding water via exchange epithelial surfaces as well as body size. It would be useful to further evaluate the importance of exchange ep- ithelial surfaces in determining temperature-modulated chang- es in both water influx and chlorpyrifos accumulation. Elevated temperatures pose major problems to aquatic or-