Ahead of us a mini twister moves across the road, leaving tumbleweeds spinning in its wake. On the back seat, Taco stirs. As I turn round to pinch his ear, he fixes me with his myopic squint, sucks in his tongue then drops it from his mouth again, where it hangs unnaturally long and thin, like a slice of carpaccio. The heat is intense, cloying. In the distance a lazy mist hangs over the Patagonia Mountains. I look for some orientation in the landscape; something to mark this mile from the last. We pass an iron bed, the remains of a fire, a charred shack.

‘It’s spooky here. Where are we?’

‘Parallel to the border.’ Duval turns the next corner then hits the brakes. The truck judders to a stop but it’s only when the dust begins to clear that I see why. The road ahead is blocked by a crude length of tree trunk acting as a barrier. Next to it, a military truck is parked.

Half a dozen soldiers in green uniform slouch around in lackadaisical ennui. A couple are smoking cigarettes, leaning their shoulders against a tree. Only mildly interested at our approach, they toss their stubs carelessly, stretch an arm for their guns, then swiftly turn and point them straight at us.

‘My God,’ I say, alarmed. ‘Who are they?’

‘Mexican Army,’ Duval replies briefly. Up close, the soldiers are startlingly young, their hair close cropped. ‘Military service,’ he adds, as though reading my thoughts. The soldiers keep their rifles trained on the dusty, smeared windscreen of the truck.

‘What do they want?’

‘Oh, I expect they just want to tell us a good way of getting that squished insect paste off the glass.’

‘Duval . . .’

‘Just follow my lead,’ Duval says and winds down his window.

The first soldier steps round, his gun still raised. ‘¿Tiene marijuana?’

Duval says something rapidly to them in Spanish.

The soldier’s eyes flick to me, then back to Duval, uncertainly. The second boy starts probing under the truck with his rifle, then pokes it through the back window. Suddenly, Taco, who up until this point has seemed content to sleepily follow the action from a seated position, lunges towards him, barking ferociously. The boy jumps back in alarm.

‘Oh, man! Does he bite?’ he asks in good English.

‘Sure,’ Duval says, ‘but usually just babies and old people.’

Involuntarily, the boy laughs.

‘¿Qué dijo?’ the other demands suspiciously.

‘Dijo que sólo muerde a bebés y a ancianos.’

The first soldier lowers his gun and now regards Duval with rank curiosity.

‘Y usted habla inglés,’ Duval says.

‘Sí,’ the boy acknowledges without explanation. ‘So where you going? Nogales?’

‘Eventually, but we were about to stop and have something to drink on the way. Here . . .’ Duval hauls a six-pack of Coke from the back seat and casually hands it through the window. ‘Go ahead, take it, we have plenty.’