• **Homing/trail pheromone**

An agent possesses chemical substances termed homing and trail pheromones. The agent secretes the homing pheromone while exploring for food and the trail pheromone while carrying food to the nest. Both pheromones are volatile substances that diffuse and evaporate quickly.

### • **Worker/non-worker**

An agent can be either a worker or a non-worker. While exploring food sources, an agent can perceive trail pheromones if it is a worker but cannot if it is a non-worker. While carrying food, the agent can perceive homing pheromones whether it is a worker or a non-worker.

**Table 1** shows the relationship among exploring/foraging behaviour, worker/nonworker and homing/trail pheromone.

Next, we describe the modelling of perception and action. As shown in **Figure 1**, an agent can perceive the difference in pheromone level between three front cells and the current cell and can select one of three action rules before moving to the next cell. The three action rules are as follows:


#### **Table 1.**

*Relationship among behaviour, pheromones and worker/non-worker.*

**Figure 1.** *Pheromone-following action rules.*


**Table 2.**

*Relationship between worker/non-worker and behaviour rules.*

### 1.**Random walk**

When the difference in pheromone level is negative, an agent randomly selects one cell from three cells and moves to that cell.

#### 2.**Pheromone trail**

When the difference in pheromone level is positive and an agent detects the pheromone in the front cell (**Figure 1a**), the agent moves forward. On the other hand, when the agent detects pheromones in the right and left front cells, it moves to the cell with the highest pheromone level (**Figure 1b**).

#### 3.**Turn around**

When an agent discovers food or exhausts its homing pheromone, the agent turns to the nest. However, the agent also turns to the nest if the angle between the moving direction and the direction of the nest is greater than 90° [17].

An agent can exhibit exploring or foraging behaviour by changing the combination of rules (**Table 2**). In exploring behaviour, an agent moves according to Rule 2 if it is a worker and can detect a trail pheromone, but an agent moves randomly according to Rule 1 if it is a non-worker. In foraging behaviour, both worker and non-worker agents follow Rule 3. If an agent detects a homing pheromone, it moves according to Rule 2. If it detects no homing pheromone, it moves according to Rule 1.
