Insects and microbes multiply rapidly, and often. IIRC doesn't the lowly cockroach lay egg capsules every other day, with each capsule containing hundreds of eggs?
This kind of pace hastens evolution, and allows biochemical changes to occur with greater probability.
The rate measured by generations is not that much different from other organisms. My point is that the changes I describe are demonstrated vividly, insects change proteins relatively quickly to deal with change but they do not change phenotype.
In fact while insects can adapt biochemically rather quickly, their basic layout and structure are very stable over millions of years thus making my point.