The Concept of Emergent Behaviour - An Ant Colony

Ant colonies are one of the most popular examples of emergent behaviour. Below I explain some of the amazing things a colony of 500000 ants can achieve without a central controller...
Please note the information presented here is taken from Nigel R Franks article 'Army Ants: A Collective Intelligence' in American Scientist Vol 77 (1989) p139-145.

• Regulating Temperature within the Nest
  The ant colony maintains the temperature at the centre of the nest where the brood is cradled at 28.5 degrees, even though the temperature in a tropical rain forest can vary by 8 degrees a day!
  This is most probably done not as might be expected by some form of central thermostat, but rather through the ants following a simple set of rules. What probably happens is that colder individuals on the outside press towards the centre. This makes the nest more compact, and so closes ventilation channels, and hence raises the temperature in the centre of the nest. When individuals are too hot, the process is reversed.
  No central thermostat, but lots of individual ones!
  If the idea of just following simple rules to achieve something quite complex seems odd, then consider the act of tying a knot in a shoe lace, something small children notoriously have difficulty with. It looks quite complicated, but can be broken down into these three steps:
  1. Place left thread over right thread, making a cross.
  2. Place left thread under right thread.
  3. Pull both threads.
  Are we aware that these are the steps we take to tie a knot, probably not, it's just instinct. Not that different from ants obeying simple rules to govern temperature...

• Carrying Large Items of Prey
  Disproportionatly large items of prey can be carried by large groups of ants. For instance the E.burchelli colony will retrieve 20000 or more items of prey in a single day. Some of these will be carried 100m or more. The equivalent for a human would be to carry the items 30km.
  How this is achieved: A carrying team is initiated by a large porter ant, who begins to move the item. The ants themselves then follow the simple rules:
  IF prey item in trail is moving below the standard retrieval speed AND you are not carrying an item THEN help out OTHERWISE continue.
  No individual ant either chooses the team or selects themselves, they are just following a simple set of rules.

• Communication
  Does the ant know anything of the complex architecture, foraging systems or economic stategies of the colony? Probably not. All of this is beyond the scope of the individual, rather this is achieved through communication.
  Ants communicate by passing information from individual to individual through the 'writing' and 'reading' of symbols, often in the form of chemical messages or trails of pheromenes, which act as stimuli for changing behaviour patterns.
  A human parallel to this is that the individual human brain cell could not process even a simple thought. The processing power of the human brain is the result of the connection between 100 000 000 000 brain cells. These neurons then communicate with each other through chemical messages sent between each other.