Stigmergy in Insects
Entomology is the scientific study of insects. Insects are widely spread across the whole globe. Insect behaviour has given insight into new coherences and leads to new fields of research within social sciences and biology. In the study of insect behaviour, the activities performed in the surrounding environment is central. These activities include locomotion, feeding and communication. Their behaviour can be described as an action carried out by an individual as a response to a stimulus from the environment. Insects perceive the environment through their sensory system, which allows them to act accordingly (Hoy, 2013). From their behavioural patterns, scientists can learn how social insects have adapted to the surroundings over time or how they respond to stimuli. The knowledge has been applied to a wide range of areas (Bonabeau et al., 2000).
The Origins
Termites are tiny blind insects that, despite their size, can join their work efforts into building large complex structures. A recent study illustrates how complex and massive termite mounds might be built. According to the findings, connected termite mounds cover an area of approximately 230 000 square kilometres in northeast Brazil. In that area, some individual mounds reach up to 4 meters above ground level, with a diameter up to 9 meters. The estimated mound density of the area was 1800 mounds per square kilometre. The oldest mounds are dated back almost 4000 years (Martin et al., 2018). A massive network of underground tunnels is interconnecting the mounds. Straight or helix-shaped ramps are connecting the internal chambers and floors of the mounds, as seen in figure 1 (Garnier et al., 2007). Despite the large structures above ground, the chambers lay underneath the mounds. Mounds are mainly a result of the extracted materials from the underground chambers (Heylighen, 2016a).
The term stigmergy originates back in 1959 when the French zoologist Pierre-Paul Grassé first described the phenomenon in his work. Grassé had been studying how termites coordinated their work efforts through indirect collaboration. He looked at how termites could build something as complex as the system of mounds described above. Grassé discovered that traces left in the environment triggered subsequent behaviour from either the termite itself or other individuals. In this way, the environment serves as a way of indirect communication. He named the phenomenon stigmergy from the two ancient Greek words stigma and ergon. The first word, stigma, means a sign or mark. Ergon means work or action. The combination of these words captures the notion of stigmergy, that is environmental marks which trigger actions (Dipple et al., 2014).
Figure 1 Helix ramps and straight ramps connecting chambers
The nest-building of termites
The Stigmergic Feedback Loop
One of the most central parts in stigmergy is to understand how one action leads to subsequent actions. The environment plays a significant role as the carrier of stigmergic marks. An action performed by an individual result in a change in the environment that acts as a mark. The mark is likely to trigger a new action. New actions might be carried out by the same individual or a different one; it is insignificant who does it. These following actions are the elements of the stigmergic feedback loop, as seen in figure 2. When the steps are repeated, a loop pattern emerges, and stigmergy is recognised (Heylighen, 2016a).
In social insects, collaborative activities commonly result in physical structures. Such structures are seen in nests of the termites, ants and paper wasps. For termites, the nest under construction stimulates new and more specific actions. The result is collaborative activities that do not depend on the ability of the workers to collaborate, but on the marks found in the structure. It is the structure itself that triggers the actions (Theraulaz & Bonabeu, 1999). The stigmergic feedback loop provides an understanding of how small independent actions contributes to the big picture.
ACTION
MARK
stimulates
produces
Agent
Environment
Figure 2 The stigmergic feedback loop (Heylighen, 2016a)
Core Concepts
Three core components of stigmergy are the agent, the environment, and the mark (Dipple et al., 2014). An agent is an individual that executes an action. Social insects, like the termites Grassé studied, are common stigmergic agents. The marks are a result of an action and in the next place what triggers new actions. The environment carries the marks, and this is often where the action itself takes place.
The state of the environment drives agents without the need for internal memory. With no awareness of the overall state, the agents keep repeating the steps of the stigmergic feedback loop. As a result of the stigmergic actions, the state of the environment is continuously changing (Dipple et al., 2014). Both agents and the environment can be associated with a state. These states can be distinguished in their accessibility. The agent has a hidden internal state, but the environment is openly accessible to all agents with appropriate sensors (Parunak, 2005).
In stigmergy, there is no central control. All agents are interacting with the environment locally. Individuals are not concerned with the overall system or the environment they are a part of. No matter how large the environment is, it does not influence the work capacity of an individual. Agents are only involved in local interactions that create a self-organised system. A shared output at the system-level is achieved from the local interactions (Parunak, 2005).
Further, an insect colony looks very well organised and coordinated. At the same time, each insect is pursuing only its agenda. An individual does not have much focus on their nestmates and the activities they perform (Theraulaz & Bonabeau, 1999). On an individual level, termites work as if they are not involved in collective behaviour (Susi & Ziemke, 2001).
Swarm Behaviour
A concept that is closely related to stigmergy is swarm behaviour. These two concepts share some of the same elements. A swarm is a large group of insects from the same species. Social insects are most common in swarming, but the behavioural patterns are recognised in many other living organisms. The behaviour is seen in colonies of bacteria, fish schools, bird flocks, sheep herds, and even crowds of human beings (Garnier et al., 2007).
When a group behaves like a swarm, it collectively changes how it interacts with the environment (Rubenstein et al., 2014). The individuals are neither aware of the global environment nor the state of the whole colony. No upper leader is controlling the motion of the group, or its behaviour. The environment provides local information to the individuals, who execute their actions based on these cues. Swarm behaviour commonly occurs when a single individual cannot find an efficient solution to a problem on its own. In this case, the colony collaborates as a whole, to find a solution. To achieve swarm behaviour, social insects have a certain amount of behavioural rules they follow.
Ants
Ants are among the most successful species on earth and are one of the most commonly studied social insects in terms of stigmergy. They have several behavioural patterns of stigmergic character (Dorigo et al., 2000). In a similar fashion to termites, ants build relatively big nest structures through means of stigmergic collaboration. When ants communicate, they only do so indirectly through chemicals in which they leave behind in the environment. These chemicals are more commonly known as pheromones and act like stigmergic marks. When an ant is foraging, it deposits a pheromone trail. The pheromones work like a feedback mechanism and are left behind for other ants to follow. Pheromone trails are commonly connecting a food source back to the nest. When other ants come across the trail, they sense the pheromone chemicals and will likely follow it. Since the trail attracts ants, more ants are likely to follow it, which results in more ants discovering the food source (Sumpter & Beekman, 2013).
Reinforcement
Ants are good at finding the shortest or most efficient route between, for instance, a food source and the nest. The shortest path is not necessarily found at the first attempt, but through indirect coordination they collectively find it. Each ant that makes its way between the food source and the nest will deposit their pheromones. Wherever paths are crossing or following each other for some time, the pheromone trail is reinforced. An ant that comes across a pheromone trail is likely to follow it for some distance (Sumpter & Beekman, 2013). The stronger the scent of the pheromones is, the more likely it is that other ants will follow it for longer distances. The trail experiences a positive reinforcement from the pheromones deposited by the ants that follow it. Pheromones are not everlasting, with time they will fade. This fading is better known as negative reinforcement from the environment on the marks.
Utilising the two reinforcement mechanisms, ants can find the shortest path between two points. The ants that take the shorter paths back to the nest are likely to arrive there first (Dorigo et al., 2000). Other ants are likely to follow these paths back to the food source. Shorter paths are traversed faster and will experience more positive reinforcement. The negative reinforcement on the less travelled paths will make sure they disappear with time. The accumulation of pheromones in specific paths might indicate how good of an option the path is. After some time, this path traversal will result in the ants finding the shortest path, without direct communication.