What determines an organism's brain size and at what point is a brain necessary?

[Adyssa]

At what point, or scale, do organisms/lifeforms/whatever have brains?

 

[Greg Bernhardt]

At what point, or scale, do organisms/lifeforms/whatever have brains?

I'm going to guess it's the Fruit Fly
http://www.nytimes.com/2010/12/14/science/14neuron.html

Any smaller and you just have a collection of neurons such as in worm and starfish etc...

 

[Antiphon]

Nah. Much smaller. I believe the parasite that lives in your eyelashes has a brain and it's practically invisible.

Edit: reference http://en.m.wikipedia.org/wiki/Demodex

(If this doesn't have you washing your face then you have nerves of steel.)

 

[Pythagorean]

First, define what you mean by brain. Some people call the nerve ring in c. elegans the brain, defining "brain" functionally.

With something like a starfish or a jellyfish, nearly everyone agrees that there is no central nervous system; they have a distributed "neural net".

On the other side of the nerve bundles in worms, we have insects with appendages that generally require a clear CNS of some sort to integrate sensory signals and motor skills (ants and flies).

Second, define what you mean by small. The general scientific consensus is that we should compare brain size to body size and the ratio is what's significant (so a small brain is defined relative to the body it's in) whereas you may be thinking in absolute terms.