If you think your reaction time is fast, compare yourself to the bugs. Our insectoid friends are much quicker.
The fly is twelve times faster.
Swatting a fly is difficult because it has an early warning system. In less than a 10th of a second, the fly has to perceive the threat using its eyes, determine what direction it's coming from, and then make the appropriate movement with its legs so it jumps in the right direction. And all this is accomplished by a brain that's the size of a poppy seed.
They have compound eyes – instead of one lens they see through a sphere with many hundreds or thousands of eyes, called ommatidiums. Household flies, for instance, have a very well-developed 3D visual system with the capacity of seeing motion, color and pattern of objects in their environment due to their advanced compound eyes. It is now serving as a model for advanced electro-optical design.
Flies appeared in the middle Triassic. They have a symbiotic relationship with other life on earth. More than 2/3 of a fly's genes are involved in guiding cognitive function, which, along with its short lifespan and plastic adaptability, makes it ideal for structural neurocognitive research into changes from Alzheimer's and similar conditions.
The University of Arizona has published a fly brain atlas (c. 2000) using Java applets and VRML, or virtual reality mark-up language, which as of present is the number one search result for 'fly brain.'