Skating and diving:: Changes in functional morphology of the setal and microtrichial cover during ontogenesis on Aquarius paludum Fabricius (Hetoroptera, Gerridae)

We examined the morphology of setae and microtrichia in Aquarius paludum during larval development using a scanning electron microscope. We then conducted immersion experiments with larvae and adults in oxygenated and deoxygenated water. The adult water strider body is covered by a pilose double layer consisting of upper long setae (30-80 mu m) and lower filiform microtrichia (5-9 mu m). Only setae are present on the legs. Microtrichia on the larval body are very short: 0.5-0.6 mu m in first and second instars, and 0.8-1.7 mu m in third to fifth instars. Larval body setae are approximately as long as those of adults (25-50 mu m), but are much less dense at 1,800-5,750 setae per mm(2) versus 15,000-20,000 setae per mm(2) in adults. The density of setae on the legs remains relatively constant throughout development (larvae: 15,000-20,000 setae per mm(2); adults: 20,000-26,000 setae per mm(2)). Immersion experiments demonstrated that young instars may use cuticular respiration. First- and second-instar larvae survived underwater for several hours without a visible air supply, although they did not survive in deoxygenated water. We posit that the short body microtrichia have a waterproofing function in larvae, whereas they create a compressible air bubble in adults. In adults, waterproofing is accomplished by the setae. The density and length of setae on the legs of larvae was nearly the same as that on the body and legs of adults and is presumably optimized for waterproofing. Thus, a change in morphometrical parameters can result in a large functional change in the same structure. We discuss this interpretation in both ecological and physiological contexts.