Just imagine the first rove beetle was adapted to be a predator of social insects. It runs to the colony and tries to grab some eggs. It's often detected and killed. So maybe by chance one beetle has a mutation that makes one of its pherhormones (or whatever) smell slightly more like termite. It's a bit less likely to get killed by the termites, so it has an advantage and higher reproductive success. The new genes spreads through the population. The another one gets a mutation that makes it's antennae just a bit more termite-like... You get the idea.
Why rove beetles? No idea, but maybe their body plan was for some reason more adaptible to change morphology? Also maybe because they were closely associated to colonial insects from the beginning as predators, and so had high selective pressure to avoid detection.
Why rove beetles? No idea, but maybe their body plan was for some reason more adaptible to change morphology? Also maybe because they were closely associated to colonial insects from the beginning as predators, and so had high selective pressure to avoid detection.