The day before the July Palio, it was good to be a Caterpillar. Il Bruco, the neighborhood association that takes that insect as its emblem, was the favorite in Siena's bareback horse race, which pits the city's 17 wards — or contrade — against one another each year on July 2 and August 16. The Caterpillar had won three Palios in 10 years. They had just gotten first choice in the random drawing of horses. And they had the legendary Luigi "Trecciolino" Bruschelli, one of the most successful riders in a century, as their jockey.

Weeks earlier, when I'd been offered a ticket to il Bruco's pre-race dinner, I accepted without much thought. One contrada seemed as good as any other. I wasn't even sure if the Caterpillar would race (10 of the 17 contrade ride in each contest). But when I arrived in Siena, the local newspapers were practically guaranteeing a victory for my adopted clan. Drums were beating all around me, and a never-ending stream of revelers, wrapped in green-and-yellow il Bruco scarves, flooded the streets. I fell under the spell of the Caterpillar.

Looking to share my enthusiasm, I met up with Dario Castagno, author of A Day in Tuscany and a passionate Caterpillar. When I asked him about the race, though, he looked anxious. "I don't think we're going to win," he said. "We can't afford it." Perhaps he was just being superstitious, but I sensed there was something more at hand. The Palio isn't just a sport.

"You have the best horse. What else do you need?" I asked.

"We've won three Palios in 10 years," he said. "We don't have enough favors left." He rolled his eyes conspiratorially. "There are too many people who don't want us to win."

The Palio is a violent, unruly race. The start is chaos. Then the horses must circle the steeply angled Piazza del Campo three times. The track is narrow. The curves are tight. The jockeys carry stiff whips to spur on their horses and lash out at their opponents. Riders are often thrown and horses stumble. If the horses make it to the starting line in full health (allegations of doping and other pre-race tampering are common), there are plenty of chances for one rider to sabotage another or for a group of riders to work together. The Palio is famous for bribery and backroom deals.

"If you don't race, but your rival is racing," Cristina Amberti, another Caterpillar told me, "you can make agreements with other contrade to help stop your enemy." The night after the horses are chosen, the wheeling and dealing begins. "The three elements of the Palio horse race," said Amberti, "are luck, money, and political strategy."

Il Bruco did have a few advantages. The Caterpillar is only one of three contrade without a sworn enemy. "That has helped us," Castagno said. "The Owl and the Unicorn, the SHE-Wolf and the Porcupine — there are many others — they are always rivals." Plus, il Bruco had a master of brute force and Machiavellian cunning on their side. Known universally as Trecciolino ("the Ponytail") but affectionately called "Gigi" by his Caterpillar fans, the jockey had built a remarkable résumé — 10 wins in 10 years — and had carved a larger-than-life persona. He was a short, thin man, but he carried himself with intense confidence. He almost never spoke and never smiled. Members of the Caterpillar treated him more like a mafia boss than a sports star. "There goes Gigi," they would say quietly and nod slowly, as if they were on intimate terms with him, but didn't want to disturb the great man. "He is a very powerful character," one Caterpillar told me. "He can make many agreements."