What is this strange bug we found on our dock?

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My husband and I discovered this enormous bug on our dock this fall. We were surprised at its size (bigger than some of the fallen spruce leaves) and its speed. What is it?
—Carrie & Ryan S., Harcourt Park, Ont.

That looks like Lethocerus americanus—common name: giant water bug—one of the largest insects in North America. Adults are five cm long, about the size of a domino (but some species, in other parts of the world, can get three times as large). They’re aquatic, and prefer to live in ponds, marshes, and streams with vegetation and slow-moving water. They’re drawn to light, so in the summer and early fall, you might see them near the cottage, under porch lights or exterior lamps. As it gets colder, they’ll often burrow into the mud to overwinter, or move to deeper water bodies and remain active under the ice.

Giant water bugs are considered beneficial because they eat other insects. The feeding process is not pretty: They grasp their prey with their two powerful front legs, and use a sharp beak to inject digestive juices into their victims’ bodies. The enzymes liquefy the prey’s innards, which the giant water bug can then suck up. The bugs also frequently prey on fish, tadpoles, snails, and crayfish—plus there’s at least one case of a Japanese species eating a baby turtle. (Google it.) They’re not dangerous, but you probably don’t want to pick one up: They have a painful bite.