I think that insect variations are due to very short lifespans with high reproductive rates and a more simple design.

I just made the following calculations: fruit flies for example live 50 days, this accounts for more than 500 generations during a typical human lifespan. They lay up to 500 eggs. Of course survival rate must be very low but just for the fun of it let assume everyone survives (maybe a natural disaster has eliminated predators). This means that during the time evolution has tried 10 new human designs it can try also on the order of 10^1300 new fly designs (500^500).