Vanni curses God and at once a snake curls around his throat. Dante is disgusted with Vanni and wishes that his home city of Pistoia would burn to ashes. Other snakes wrap around Vanni and Dante thinks that he has seen no other spirit as defiant against God. Vanni runs away, and Dante sees a centaur being tortured by snakes and a dragon-like monster that Virgil identifies as Cacus. (Cacus was a mythological monster that dwelled near the site of ancient Rome and was supposedly killed by Hercules.)
Dante is filled with pious anger at Vanni's blasphemy. However, his curse is directed not at Vanni but at Vanni's home city in Italy. Once again there is a tension between grand concerns (blasphemy and evil) and very local ones, as Dante uses Vanni to denigrate the town of Pistoia. Cacus is another classical monster (described in Virgil's own Aeneid) that Dante places in his Christian underworld.
Three spirits come up to Dante and Virgil and ask who the two poets are. One of them calls for someone named Cianfa. Dante tells his readers that they may not believe him, but at that moment a six-legged worm appears and jumps on one of the spirits. The creature clings so tightly to the spirit that they merge into one figure, their bodies melting into each other. The other two spirits cry out for their companion, whom they call Agnello. The strange creature made from the union between Agnello and Cianfa (who was in the form of the six-legged worm) slithers off like a lizard.
This scene of bodily transformation is one of the more otherworldly episodes of Dante's poem.
Another lizard comes up to one of the other two spirits and strikes him in the stomach, but he doesn't react and simply yawns. A kind of smoke is emitted from the lizard's mouth and from the spirit's wounded stomach. The streams of smoke merge and the two change bodies: the lizard turns into a human body while the spirit morphs into a lizard. Dante says that this incredible transformation is more remarkable than anything told by the Roman poets Lucan and Ovid.
Dante takes this opportunity to vaunt his own literary achievement, as he claims to narrate things even more incredible than Lucan or Ovid. Ovid wrote an entire epic poem about bizarre transformations—The Metamorphoses—so this is quite a claim. It also represents Dante's placing his Christian epic above the admirable but pagan epics of his classical predecessors.
Dante describes, detail-by-detail, how the lizard's body transforms into a human's, and vice versa. The spirit that is now a lizard leaves, hissing, and the spirit who was just a lizard chases it, calling the lizard Buoso. The third spirit runs off, as well, and Dante recognizes him by his limp: he is Puccio Sciancato, a Florentine thief.
These bizarre transformations can be made sense of as a just punishment for thieves. They stole other people's property and now are themselves victims of theft, as others rob them of their most intimate possession: their bodies.