Figure 8.8 Forces maintaining the bubble gill of Potamodytes. a: Side view of the beetle with its bubble drawn out behind it. Dotted gray lines indicate the streamlines of flow. b: Measured pressures in a Potamodytes bubble go negative when velocities increase above about 0.5 m s-1. [After Stride (1955)]
ward and inward toward the center of the beetle's body. Water flows past the bubble faster than it would in the absence of the limbs. The added acceleration magnifies the suction pressure keeping the bubble inflated.
Potamodytes also uses structural features of its environment to enhance plastron respiration. Most streambeds are not smooth: especially in swiftly flowing streams, where silt that might settle in between pebbles and rocks will have been swept away, the bottom can be quite rough. Potamodytes frequently position themselves just downstream from pebbles or other obstructions to flow, as if trying to find shelter
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