Commentary Purg XXIX 4-6

Why are these two groups of nymphs made part of the simile, when Dante and Matelda are moving near one another and in the same direction?  Benvenuto (DDP Benvenuto.Purg.XXIX.4-9) glosses the tercet simply.  In the old days some nymphs wanted to leave the shade for the sun, while others desired to leave the sun for the shade.  Beginning with Tommaseo (DDP Tommaseo.Purg.XIX.4-6), some commentators have suggested a relationship with Virgil's two bands of nymphs (Georg. IV.383), one hundred guarding the woods, another one hundred guarding the streams ('centum quae silvas, centum quae flumina servant').  Neither the gloss nor the citation, however, answers the question, above, that has bothered many commentators, none more than Porena (DDP Porena.Purg.XXIX.4-7), who posits a lost classical source to explain our puzzlement but has not convinced others of this hypothesis.  It would seem that Dante wishes to express only the thought that, just as in the distant (classical) past nymphs would move purposefully from one place to another in the forest, so did he and Matelda move from where they had been standing to go somewhere else.  However, the brightness that they shall soon find would seem intrinsically to associate them with those nymphs who move from shade to sunshine.