If it shall appear from the evidence that a defendant did the act charged as constituting the offense, but at the time of committing the act he was insane, the jury shall render a special verdict to the effect that the defendant is not guilty by reason of insanity, but if the jury does not believe from the evidence that the defendant committed the act or if it believes from the evidence that he is not guilty upon any ground other than his alleged insanity, it must return a general verdict of not guilty; otherwise, it must return a verdict of conviction.