Gyrinid beetles, as adults, scavenge on the surface of the water. (Their larvae, which look quite different, live under water and prey on other aquatic insects.) When disturbed, the beetles swim madly about in circles. If there are a lot of them together, their gyrations make the water hiss. They are difficult, but not impossible, to catch with a net.

Here's a picture of one, courtesy of eny3005.ifas.ufl.edu/lab1/oleoptera/Gyrinid.htm.

Only two of the Gyrinid's legs are visible in this picture - the forward pair that are specialized into paddles for moving rapidly in the water.

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