Experts always tell us that crayfish are one of the major baits in the diets of all species of bass everywhere. In the beginning of their life, bugs are nutritious, small morsels for a growing bass. However, a bass is generally weaned off insects as it quickly outgrows its juvenile phase. Sure, they will still take bulky-sized bugs whenever they can, and in some remote ponds and streams that I go to, the adult bass continue to feed like trout on insects and their hatches because there really isn't anything much more than bugs, flies and nymphs for bass to eat. In most places however, adult bass tend to become piscavorous - fish eaters.
Some bodies of water have LOTS of pelagics.
In such waters, bass focus lots of their time and activities around these pelagics. In some waters, for instance, it may become unproductive to fish shoreline cover at night because bass are focusing themselves on eating mid-lake pelagics by day. Other waters have no pelagics, but plenty of juvenile panfish, and bass will orient their diets and feeding habits to intercept these young panfish. Unbeknown to most anglers, many bodies of water are infested with crayfish. But unless you start overturning underwater rocks or pulling up water weeds, you may never see many crayfish.