How does a line of ants find its way to food?

The next morning, Na spilled a little sugar on the kitchen floor at Grandma's. At first a single ant wandered up, zigzagging for ages before it found the sugar. It ate a little, then headed home, round the chair leg and into a crack in the wall.

Cú Bông guessed the ants could see the sugar from far away and would all come over. Or a leader ant would walk in front and show the way.

Half an hour later, a long line of ants ran from the crack to the sugar. Oddly, they didn't go straight: they went round the chair leg, just where the first ant had gone home. It was as if an invisible path had been drawn on the floor.

In many kinds of ant, an ant that finds food leaves a scent trail on its way home. The scent is called a trail pheromone. Other ants follow it from the nest to the food.

Some ants that reach the food may leave more pheromone on the way back, strengthening the trail. A stronger trail usually draws more ants. Choosing where to feed needs no single ant in charge: the trails let the colony decide together.

Step through to watch a scent trail being made. At which step does the scent trail first appear?

In many kinds of ant, one that finds food leaves a pheromone trail on its way home. Other ants follow that scent to the food. Ants that reach the food may add pheromone, strengthening the trail and drawing more ants.