system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do tiny organisms like insects, etc. think for themselves?

Like, how does a mosquito for example, function intelligently? It cannot possibly have a brain or even a similar structure. Yet it knows well enough to avoid my hand and fly above my reach.
gpt
The way mosquitos avoid your hand is known as a taxis. A taxis is a simple response which either moves towards or away from a stimulus. In this example, the stimulus is the hand moving towards the mosquito. The mosquito responds with a negative taxis, as it moves away from your hand.  
  
Both simple and complex organisms react with taxis, some more than others. In humans, taxis are a form of reflex. If you touch a hot stove, you move away your hand away (negative taxis).  
  
Some animals rely on this almost entirely. Mosquitos will use taxis to find food. The stimulus would be chemicals in the air coming from the food, and it's response would be to move towards it (positive taxis). Venus flytraps use this to their advantage, as they lure flies in with smells (a bit simplified) to eat them.  
  
Other animals don't even use taxis that often, and instead use kinesis. When you put a woodlouse into a dry and light place, it doesn't move in a certain direction. It runs around frantically in hopes that eventually it will find a dark and damp place. The woodlouse's behaviour is known as kinesis.  
  
More complex animals can consciously process actions rather than always acting on reflex. However, even in humans all our decisions are based on stimulus - we are simply able to interpret more stimuli in more detail.  
  
Feel free to ask any follow-up questions!