When they were alive, they choose to separate their souls from the bodies with their own hands. That is why now they have a paralyzed body; in which they have to suffer forever. They have a desperate desire of taking their lives away, but they cannot move, so they have to live and bleed all the eternity. The harpies just make the punishment more severe. The souls "are deprived of even the semblance of the human form. [ ] They are the image of the self-hatred, which dries up the very sap of energy" (Sayers, notes Canto XIII). The way the trees bleed is very important, because they evoke the constant pain that these souls suffer, but it is not enough to kill them. Maybe Dante decided to choose the form of a tree as the just cage for a person that had committed suicide, because they live for long period of times. In fact, after two hundred years they become petrified, but they still stand firmly.
Our next mode of retribution is the one applied to the sorcerers. They are found in the eight circle of Hell. This circle contains ten bowges or trenches, in which the sinners that had committed fraud and malicious acts are punished. The sorcerers or fortune tellers suffer their punishment in the fourth bowge. They have their bodies twisted, with their head like backwards as their whole body stayed forward.
"Their faces were reversed upon their shoulders.
so that they came on walking backward,.
since seeing forward was denied for them." (XX. 13-15).
The fault that these sinners committed in their past lives was that they pretended to see the future, and told lies to the people that believed in them. Dante gives a very special punishment to these sinners. Since in their past lives, they used to look to the future, now they have to look backwards. We also see other kinds of contortions, for instance, Tiresias is turn from a male into a female. He was a sorcerer that predicted what was going to be the end of Jason in Medea.
