Like dry leaves or shapes of animals. How do they know what to mimic??
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Gonna have to Stretch this answer out because the SUB wont allow it when it is as short as it should be.
By dying If your Camouflage sucks. The better it is, the more likely they live on to reproduce. Which means that over thousands or millions of years, the Well camouflaged ones will outnumber the other so far that it will be the norm. And that it will still become better and better.
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By being the offspring of animals that didn’t get eaten or killed because they didn’t look enough like the thing they happens to”imitate”. Evolution is a passive process.
Accidentally and very slowly. The insects that look the most like an inedible leaf are the most likely to survive and reproduce.
Simultaneously, predators with the best eyesight who are therefore able to tell the difference between an insect and a leaf are more likely to find enough food to eat, survive and reproduce.
The insects must then look even more like a leaf in order to survive, and so on and so on
Every generation of a certain bug has a slightly different genetic makeup with some random mistakes or mutations. Sometimes the mutation makes them blend into the background better. Those more well hidden reproduce and pass on their good camouflage. Sometimes the good camo gets another mutation that’s even more beneficial or makes them even harder to spot. They reproduce successfully again and pass on the genes.
Times that over millions of years and then you have bugs that look like sticks and leaves and flowers.
They don’t know, and the ones that don’t do that also existed at some point, but they all got eaten. What you see is just the stuff that is currently working.
And to provide info that no one asked for: evolution and natural selection work on geological time scale, and the driving force are just random mutations that naturally happen. Tiny and random changes that happen across thousands or millions of generations of living creatures, and those with a favorable change survive to make babies that carry those genetic mutations, and those babies make more babies with more mutations over thousands of years. The accumulation of these tiny generational genetic mutations eventually give rise to all the
vastly different types of plants and animals you see today. And the common factor to all of these different species is that they’re “good enough” for their environment. You see, natural selection doesn’t mean nature “picks” the best traits and make more individuals carrying those traits, but rather the species randomly manifest a bunch of traits and nature just kills off all the ones that aren’t “good enough”.
Sometimes many different traits of a same species are all good enough, so they all live on and branch out – divergent (think of the Darwin’s finches), or certain traits are so good, that many different species that start manifesting that trait are all good enough and you see the opposite – a convergent (think of the species that all evolved into crabs).
To come back to the question, a leaf bug doesn’t just wake up and decide to look like a leaf, it’s the result of a long lineage of bugs that very slowly became more leaf-like over millions of generations, starting from a bug that mutated and became some % more leaf-like than it’s siblings, let’s say 1%, then it survived and made 100 buglets, and 1 of those mutated again to be 2% more leaf-like than the original bug, which made it 2% more likely to survive predation, and to make more leafy bug babies. Repeat this process for not just the appearance, but the anatomy and behavior, over millions of years, and you get a leafy ass bug that looks like a leaf, behave like a leaf, all the while having no say in the matter, unaware of it’s own leafiness, and if it’s good enough to survive to sexual maturity, it too will make leafy bug babies with teeny tiny mutations and the cycle continues
It is evolutionary, those with the best camouflage are the least likely to be eaten and thus the most likely to breed as others have said. I just want to add another thing to that: The smaller the creature, the shorter its lifespan and the faster it must breed as a result. Which means that the smaller the creature the faster evolution can change it. Most insects mature within the year and some reproduce several times a year (or within weeks) which makes it easier for them to find niche roles in a given environment.
This is also why single-cell organisms tend to have hyper-specific environments they evolve for despite them reproducing asexually.
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