Hairs on the whirligig beetle's mid and hind legs prevent loss of speed by extending and folding during swimming strokes.

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The mid legs and hind legs of the whirligig beetle contain numerous microscopic hairs (called laminae) that increase surface area while swimming, resulting in greater thrust and power. These laminae are dynamic, extending and folding throughout the beetle's swimming motion to maximize efficiency.
 
During the power stroke when the beetle swings its legs back against the water, the laminae extend, allowing for a greater push. During the recovery stroke when the beetle pulls its legs back against the water to its starting position, the laminae fold, thereby reducing drag. This enables the beetle to maximize speed during the power stroke while minimizing loss of speed during the recovery stroke.

This strategy was contributed by Rachel Major
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References

"During the power phase, the middle and hind legs have a contact area about 40 times greater than during the recovery phase [3], [15]." (Zhang et al. 2012:2)
 
"....the effective area of the hind legs with the laminae extended...was estimated to be 1.92 mm [squared]. This represents a 77% increase in propulsive area compared to the hind leg without the laminae extended. The area for the middle legs with the laminae extended...was 1.19 mm [squared], representing an 80% increase in propulsive area, when compared to the folded state. Since the laminae are folded in the recovery phase of the beat and only extended in the power phase, this leads to a reduced-drag recovery stroke aiding in the propulsive efficiency." (Zhang et al. 2012:4)

Journal article
Experimental Studies and Dynamics Modeling Analysis of the Swimming and Diving of Whirligig Beetles (Coleoptera: Gyrinidae)PLoS Comput BiolNovember 29, 2012
Zhonghua Xu, Scott C. Lenaghan, Benjamin E. Reese, Xinghua Jia, Mingjun Zhang
Editor/s: Philip R. LeDuc

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