This rather disturbing event in the novel occurs in Chapter One, and is accompanied by a motif that occurs during the rest of the novel and is shown to represent the various forces that are ranged against this family and their attempts to live their lives with dignity and respect. Note the way that the arrival of the scorpion triggers off a kind of music in Kino's mind:
In his mind a new song had come, the Song of Evil, the music of the enemy, of any foe of the family, a savage, secret, dangerous melody, and underneath, the Song of the Family cried plaintively.
It is this Song of Evil that begins when the scorpion is spotted, and its threat to the life of their baby boy indicates the way that it threatens the way of life of Kino and his family. You will want to pay careful attention to the way in which this Song of Evil becomes attached to the pearl itself as the novel develops and the various ways in which it brings evil upon the family.