Benzer used forward genetics to investigate the genetic basis of various behaviors such as phototaxis, circadian rhythms, and learning by inducing mutations in a Drosophila population and then screening individuals for altered phenotypes of interest. To better identify mutants, Benzer developed novel apparatuses such as the countercurrent device, which was designed to separate flies according to the magnitude and direction of their phototactic response. Benzer identified mutants for a wide variety of characteristics: vision (nonphototactic, negative phototactic, and eyes absent), locomotion (sluggish, uncoordinated), stress sensitivity (freaked-out), sexual function (savoir-faire, fruitless), nerve and muscle function (photoreceptor degeneration, drop-dead), and learning and memory (rutabaga, dunce).