Water Bug, common name for aquatic insects of several families, including water boatmen, back swimmers, and water scorpions. The name water bug is sometimes restricted to the giant water bugs. This group of tropical to temperate species includes the largest of aquatic bugs, some of them more than 11 cm (more than 4.5 in) in length. Giant water bugs are also strong fliers. Their piercing, sucking mouthparts can deliver severe wounds through injection of proteolytic (protein-breaking) enzymes. Water boatmen, less than 12 mm (less than 0.5 in) in length, have long, oarlike hind legs. Water scorpions, which are approximately 2.5 to 5.0 mm (about 1 to 2 in) long, have long whiplike "tails" that are actually two respiratory tubes. Their painful bite is not as dangerous as the sting of true scorpions.
Scientific classification: Water bugs belong to the order Hemiptera. Water boatmen make up the family Corixidae. Back swimmers make up the family Notonectidae. Water scorpions make up the family Nepidae. Giant water bugs make up the family Belostomatidae.