They're simply called Giant Water Bugs. They're the largest of the "true bugs," live in ponds and shallows, and eat most anything smaller than them.
I used to catch the occasional one when hunting for salamanders as a kid. I'll never forget when one of them crawled out of the water in the bucket I kept it in, dried itself on a rock, and then
flew away. I had no idea those things could fly until then. I can't imagine how nasty it would be to have one smack you on the face while cycling or walking down the street.
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