Some will fly up and down off the water depositing eggs or even dive underwater to lay eggs which settle to the bottom adhering along the stream bed. Fishing mayfly duns is a tremendous and exciting experience. The last stage of a mayfly is the spinner. After it has mated and deposited eggs it become weak, and eventually dies. Some mayflies can be found spent during the morning hours, Tricos being a good example.
Spinners are an easy meal for trout.
They are completely dead and flat on the water. Trout don't have to expend any energy to feed on this stage. You will often see trout gently sipping spinners off the top. Large trout especially understand what it is to use as little energy as possible to feed on these easy targets. Devoting a fly box to just spinners is not a bad idea.