From Nookipedia, the Animal Crossing wiki
The Diving Beetle (ゲンゴロウ, Gengorō), also known as the Water Beetle in Doubutsu no Mori e+, is a common insect found in rivers and ponds, swimming and diving. They are worth 800 Bells. It appears during the months of May through September, and was introduced in Doubutsu no Mori e+. It is common to find when it is raining, but can be caught in all weather. The Diving Beetle is found underwater, with a dark shadow like fish, and above water, showing their green bodies. To catch them, they need to be above the water. When caught, the "Bugs and Fish" window says "These are said to have short but strong pincers. Ouch!".
Donating to the museum
In City Folk
"While they're marginally less odious as adults...dive beetles are horrific in their larval stage! These wretched youths are twice the size of the adults and wield a large poison barb. Foul villainy! Then again, I suppose the adults aren't much better. They're both completely objectionable, really..."
It can be found in the bottom-left hand enclosure in the insect exhibition, in the small pond.
In New Leaf
Upon being donated, the beetle can be found in the first room of the bug exhibit swimming in the pond.
"Diving beetles swim using thick, hairy hind legs and clean the water by eating dead insects. They store a supply of air under their wings to breathe underwater and surface to replenish as needed. When they're caught by predators, they release a foul-smelling bluish fluid from their heads in defense."
Adult beetles have streamlined, oval, or football-shaped flattened bodies that are usually 12-25 millimeters (1/8-1 inch) long. Most species are brown to black but some have distinctive patterns of spots, lines, or mottling on the wing covers. They have elongate hairlike antennae. Larvae are not frequently seen and have a long thorax and long legs. The head bears conspicuous large sickle-shaped mandibles without teeth.
Predaceous diving beetles are easily confused with water scavenger beetles (Coleoptera: Hydrophilidae). The latter surface for air head first and have a ridge or keel on the underside that runs down the thorax and extends into a point.