1)
What is the significance of sending snakes?
1.
Malbim: Hashem says, you think that Dan is like a snake? It will be the opposite - I will send harsher snakes, and they will bite you yourselves, and not your horses!
2)
What are "Tzif'onim"?
1.
Rashi: After 70 years, [a snake] becomes a Tzif'on, and it seals its ear and does not hear one who charms it - "Pesen Cheresh Yatem Azno" (Tehilim 58:5)
2.
Radak citing Targum Yonasan: A nation that kills like Chormin (a species of) snakes.
