To me, the operative word is “immensos”; the only snake with “immensos orbes” likely known to Virgil is likely Python molurus (Indian python) known of from India by stories carried by long-range trade.
And perhaps by python skins, or items made from python skin, finding their way from India to Italy by trade. I can imagine Virgil seeing something imported made from python skin and being thankful that snakes THAT size were not found in Italy.
Georgics 2 line 136- 139 sets up the argument that no other country can rival the glories of Italy (laudibus Italiae certent) . After this general introduction Virgil starts his list of mythical prodigies which are not, he claims, to be found in Italy. The following paen to Italy is of course idealised and also partly mythic with its reference to Clitumnus and to “perpetual spring” etc. Then we come to the snake: “sweeping huge coils along the ground, does the scaly snake with his vast train wind himself into a spiral”. I think that given the context this has to be interpreted in a mythic way. Aeneid 2 204 has similar language “…immensis orbibus angues” in the description of the snakes sent to destroy Laocoön.
I have not time to follow this further but it seems to me that Virgil rather than talking about a “real” snake in this passage from the Georgics is evoking a well known literary topos, probably Hellenistic (Apollonius) in origin. Virgil is always I think to be read as an intertext, that is one that is in dialogue with other texts.
Your idea that the inspiration was from pythons is simply unprovable. Apparently (according to Austin) Pliny (NH vii 37) had heard of a snake 120 feet long, killed by Regulus in Africa. The fact that huge snakes were known about I dont think really helps. Virgil is more engaged with other texts and not the “real” world.
Richard Thomas (a representative of the Harvard school of criticism) is very fond of detecting deliberate falsehoods in the Georgics, and on this passage (not bk.4 but bk.2.153f.) speaks of Vergil as telling obvious lies.
For snakes in hellenistic literature, Nicander is your man. The great passage on poisonous snakes in Georgics bk.3 (414-39), also partly recycled in the Aeneid, has very strong intertextual relations with Nicander’s Theriaca.
Nicander also wrote a Georgics, whose importance for Vergil’s poem of the same name I believe is very much underestimated.
The previous message wrote: “Richard Thomas (a representative of the Harvard school of criticism) is very fond of detecting deliberate falsehoods in the Georgics, and on this passage (not bk.4 but bk.2.153f.) speaks of Vergil as telling obvious lies.”.
What were those untruths? We must distinguish from the result of popular error such as likely was common among country folk, including among Virgil’s farm’s workmen. Somewhere in the Georgics is a statement that a snake made itself venomous by eating poisonous plants :: I have come across beliefs as strange among country folk in England that existed long after Virgil’s time, for example an idea that some birds that disappear overwinter, hibernate underwater like frogs; one of the Anglo-Saxon riddles in the Exeter Book depends on that belief; nowadays we all know that they fly away south and migrate away.
And, Virgil’s and Aratos’s yearly timings of events by the first visible early morning risings of particular stars, have between their times and ours been gradually thrown out of register by around 2000 years of precession of the equinoxes.
The Calabrian water snake described in the Georgics, seems to me to be Natrix natrix, the grass snake. It is not venomous.
Many things in the Georgics look realistic to me, such as farm work, and damage caused by mice and other rodents, and livestock diseases in those days long before modern veterinary medicine.
And his description of life in a severe winter around the Sea of Azov in Georgics 3:349 etseq looks realistic (and made me shiver).