they don’t! there’s no actual brains behind the operation per se, simple patterns gradually evolve into more complex ones over time, like you say, due to what we call selection pressure! 

so say that you have a field of plain yellow butterflies. 

ordinary, completely boring, nothing special about them. 

we’re going to expect that all of these yellow butterflies will have a roughly equal chance of being captured and devoured alive by birds, with some corrective leeway for avian mishaps and dumb luck!

and lets say this continues for 100 butterfly generations, spanning a century of realtime. in that span, a random mutation occurs that gives one of the butterflies patches of additional melanin on its wings, resulting in small spots!

a very slight difference, sure, but birds are sensitive to environmental changes and this is a new and strange thing in our meadow, so we’ll say that these spots give our special butterfly a 20% survival boost by startling birds when they come in for the kill. if our initial spotted butterfly makes it, its offspring, if they have this mutation, also have the increased survival boost! 

and if we fast-forward our meadow another 100 butterfly generations, the spotted insects will have outcompeted and replaced the original plain-winged butterflies due to being slightly better at not getting murdered by birds all the time.

but UH-oh! what’s this?

at this point, the birds are starting to catch wise to this trick! a bird that can see through the butterfly spot defense has a better chance of getting a meal, which is now putting pressure on the bird population to develop this behavior!

so what’s a poor butterfly to do?

ESCALATE, OF COURSE.

it’s this ongoing arms race between animal populations that drives natural selection, pushing prey species to develop more elaborate and effective defenses while also driving predators to develop counteroffensives! this permanent natural war continues until something upsets the system entirely, like if a meteor slammed down onto our example meadow and set the whole continent on fire.

but before that inevitably happens at some point in the far future, our plain yellow butterfly species will have developed into a whole array of species, each with their own signature alarming, obnoxious, or just plain weird set of patterns and defensive behaviors! including, yes, some terrifyingly lifelike eyespots.

and it’s all down to the magic of random chance! 

Comet C/2014 Q2 Lovejoy and the Pleiades