The Callisto story follows that of Phaethon in Metamorphoses 2.
A few language notes: First, the spelling of her name should be Kallisto (Greek for "most beautiful"). See Theoi for more about her - note that among other things, she was the daughter of Lycaon (or Lykaon). We remember him from Book One as the early king of Arcadia who tested the divinity of Zeus by serving him a dismembered child for dinner (in some versions the child is Arcas), and was turned into a wolf for his impiety.
At one point, Ovid refers to Callisto as "Parrhasis" - "one from Parrhasia" - which was a region in southern Arcadia, in the Peloponnese. Only one suggestive detail is associated with Parrhasia: it was said by some to be the place where Rhea, consort of Kronos, bore Zeus. Here's a description from the Greek poet Callimachus:
Callimachus, Hymn 1 to Zeus 10 ff (trans. Mair) (Greek poet C3rd B.C.) :
"In Parrhasia [in Arkadia] it was that Rheia bare thee [Zeus], where was a hill sheltered with thickest brush. Thence is the place holy, and nor fourfooted thing that hath need of Eileithyia nor any woman approacheth thereto, but the Apidanians call it the primeval childbed of Rheia. There when thy mother had laid thee down from her mighty lap, straightway she sought a stream of water, wherewith she might purge her of the soilure of birth and wash thy body therein. But mighty Ladon flowed not yet, nor Erymanthos, clearest of rivers; waterless was all Arkadia . . . And holden in distress the lady Rheia said, `Dear Gaia (Earth), give birth thou also! Thy birthpangs are light.' So spake the goddess, and lifting her great arm she smote the mountain with her staff; and it was greatly rent in twin for her and poured forth a mighty flood. Therein, O Lord, she cleansed thy body."
Another, briefer reference:
"Woodland Parrhasia [in Arkadia], where is still to be found the place untrodden in which primeval goddess Rheia was brought to bed [and gave birth to Zeus]." - Nonnus, Dionysiaca 13.29
Here we might sense a clue as to why Ovid took care to allude to the place name Parrhasia in telling the tale of Callisto: her homeland was sacred to the Great Titan Mother of the Olympians. It was in this rocky, barren place that Rhea, having watched as Kronos devoured all of her other children, chose to bear Zeus. She gave him to nymphs to be raised in secret, handing Kronos the famous swaddled stone to eat instead. Callimachus's description gives us a scene rich in high drama and desperate measures, involving the Titan Goddess's potent maternal instincts.
Parrhasia then is the birthplace of Zeus, and the sacred place where the "father of gods and men" was saved from being devoured by his father. And as we noted, it is associated with Lykaon, who tried to get Zeus to devour a child. This is a doubly sacred place, charged with divine birth, protective maternity, and sacrilegious paternal cannibalism. And, note the absence of water, and how the Goddess solved that problem in order to cleanse baby Zeus.
All of which at least echoes Ovid's tale of Callisto, who is called "Parrhasis" at the very moment Diana suggests they all take a swim:
Ut loca laudavit, summas pede contigit undas:
his quoque laudatis “procul est” ait “arbiter omnis;
nuda superfusis tingamus corpora lymphis.”
460Parrhasis erubuit. Cunctae velamina ponunt:
una moras quaerit. Dubitanti vestis adempta est;
qua posita nudo patuit cum corpore crimen.
Attonitae manibusque uterum celare volenti
“i procul hinc” dixit “nec sacros pollue fontes”
465Cynthia; deque suo iussit secedere coetu.
What is Ovid up to? For one thing he's gone from Phaethon's searing story of solar fire to a story of lunar secretiveness, aversion to sexuality and childbirth, and shame. Callisto is severely punished -- first by Diana, who exiles her (in other versions, it's Diana who turns her into a bear), and then by Hera, who is jealous of her bearing a child of Zeus. Both of her secrets are revealed (first, that she's pregnant, then, with whose offspring), and this leads to her being put, strangely, in the position of Rhea in Parrhasia. Callisto can never bathe - either in Diana's hidden pools, or, after her transformation into the Great Bear, in the vast Ocean. She's doubly cursed.
Ovid seems to be playing with the consequences of losing balance, of overshooting one or another temperate zone. This is externalized and literalized in the story of Phaethon. It's internalized and emotionalized in the culmination of Callisto's story -- the hysteria of Hera's lament:
‘You ask me why I, the queen of the gods, have left my home in the heavens to be here? Another has taken my place in the sky! I tell a lie, if you do not see, when night falls and the world darkens, newly exalted stars to wound me, set in the sky, where the remotest and shortest orbit circles the uttermost pole. Why should anyone wish to avoid wounding Juno or dread my enmity if I only benefit those I harm? Oh what a great achievement! Oh what marvellous powers I have! I stopped her being human and she becomes a goddess! This is the punishment I inflict on the guilty! This is my wonderful sovereignty! (tr. Kline)
From the paternal, solar myth in which Helios mourns the failure of his overly daring son to succeed him, Ovid's narrative has swung to the plight of a woman suffering the wrath of two goddesses: Diana, the lunar goddess of virginity and the hunt, and Hera, goddess of childbirth, who fears her own usurpation.
It little matters that Callisto neither meant to be pregnant nor to have her impregnator be Zeus. Her exile into the polar sky leaves her, like Io, seduced, transformed, and hounded by Diana and Hera into the restless night.