Falling off means it has been dead for some time, and that's good. Look closely for more individuals. If one doesn't fall off, it's alive. Remember, they are hermaphrodites, if only one is left and is ready to mate but can't find a daddy, she can do it herself and have a quadrillion babbies. It gets worse. If the branch is too crowded where a baby is born, the first stage of life is winged, and they can ride the winds to a new place.