Insects face dangers that can damage their locomotor appendages. Any hope of successful reproduction requires compensation for loss of an appendage. Damselflies that lose one of their hind wings are not crippled. They are still capable of flight, flying in a straight line and maintaining a proper flight attitude. Loss of a single wing means the lift and propulsion on one side is potentially much greater than the other. Insects can fly with only 3 wings because they are capable of generating far more power for flight than is required to ascend and remain airborne.
Damselflies, unlike most insects, have direct flight muscles that can operate each wing independently. When a hindwing is missing, the forewing on the same side beats with a higher amplitude, a change that increases the power output and the remaining hind wing reduces its beat amplitude, reducing its contribution. This, along with other changes equalizes the power output so the insect is not turning to its wingless side. All wings beat at a slightly higher frequency to compensate for the missing wing.
How do damselflies know how to compensate? Insect flight systems have a multitude of sensors that can provide feedback to the system and initiate changes in the movement of the flight muscles. The system is very robust and able to stabilize flight under a variety of conditions, including wing loss. When all wing are present, the flight stabilization is important to transient changes in conditions such as wind gusts and collisions. Studies of insect flight stabilization systems may provide important insights for human built flying machines.