Portrait of a Whirligig Beetle (Gyrinus substriatus).
The most interesting insect compound eyes are those of whirligig beetles (Family Gyrinidae). You’ve probably seen groups of these lively little metallic beetles on ponds and slow streams. They have two sets of compound eyes. One pair of eyes is on top of the head to see what’s going on above, i.e. to watch for predators. The other pair of eyes is located below the water surface to look out for smaller aquatic insects on which they prey on themselves.
The antennae are also divided which is very unusual among beetles: the short and plump part is placed about at water level while the hairy lower part remains permanently submerged. The long bristles are used to detect vibrations of struggling insects that have fallen in and float with the current.