This spectacle of the spider and the fly seems indeed to be specially apt to exert a sexual influence. I have heard of a precisely similar case in a man of intellectual distinction, and another in a lady who acknowledged to a feeling of “exquisite pleasure,” on one occasion, at the mere sound of the death agony of a fly in a spider’s web.
 Quoted by Obici and Marchesini, Le Amicizie di Collegio, p. 245.
 It may be noted that we have already several times encountered this increase of excitement produced by arrest of movement. The effect is produced whether the arrest is witnessed or is actually experienced. “A man can increase a woman’s excitement,” a lady writes, “by forbidding her to respond in any way to his caresses. It is impossible to remain quite passive for more than a few seconds, but, during these few, excitement is considerably augmented.” In a similar way I have been told of a man of brilliant intellectual ability who very seldom has connection with a woman without getting her to compress with her hand the base of the urethral canal to such an extent as to impede the passage of the semen. On withdrawal of the hand copious emission occurs, but it is the shock of the arrest caused by the constriction which gives him supreme pleasure. He has practised this method for years without evil results.
 Fere, “Le Sadisme aux Courses de Taureaux,” Revue de medecine, August, 1900.
 Fere, L’Instinct sexuel, p. 255.
Why is Pain a Sexual Stimulant?—It is the Most Effective Method of Arousing Emotion—Anger and Fear the Most Powerful Emotions—Their Biological Significance in Courtship—Their General and Special Effects in Stimulating the Organism—Grief as a Sexual Stimulant—The Physiological Mechanism of Fatigue Renders Pain Pleasurable.
We have seen that the distinction between “sadism” and “masochism” cannot be maintained; not only was even De Sade himself something of a masochist and Sacher-Masoch something of a sadist, but between these two extreme groups of phenomena there is a central group in which the algolagnia is neither active nor passive. “Sadism” and “masochism” are simply convenient clinical terms for classes of manifestations which quite commonly occur in the same person. We have further found that—as might have been anticipated in view of the foregoing result—it is scarcely correct to use the word “cruelty” in connection with the phenomena we have been considering. The persons who experience these impulses usually show no love of cruelty outside the sphere of sexual emotion; they may even be very intolerant of cruelty. Even when their sexual impulses come into play they may still desire to secure the pleasure of the persons who arouse their sexual emotions, even though it may not be often true that those who desire to inflict pain at these moments identify themselves with the feelings of those on whom they inflict it. We have thus seen that when we take a comprehensive survey of all these phenomena a somewhat general formula will alone cover them. Our conclusion so far must be that under certain abnormal circumstances pain, more especially the mental representation of pain, acts as a powerful sexual stimulant.