Look up how evolution works it's not just one day the beetles said "hey guys lets try something different how about we shoot this stuff out of our ass." Instead it works on mutations in the DNA strand when it gets copied. (It gets copied in the reproductive organs to pass on DNA) Occasional a strand of the DNA gets mutated and can cause a visual change in the species, or it can change internal parts such as the beetle. Kind of like how certain family lines are more prone to dieses than others.