Looking at the caddis and mayfly life cycle in detail.
After spending most of its life under water as a nymph (fig. 1), a mayfly swims to the surface to hatch. During the swim-up (emerger) stage (fig.2). The emerger hatches into a dun (fig.3) which, while on the surface is a primary food source for trout. The dun then flies off the water (fig.4) to nearby foliage where it undergoes another transformation. When it becomes a spinner (fig.5), it will join others and be seen swarming over the surface in mating flights. Some spinner will drop their fertilized eggs, others will touch down on the surface to deposit them (fig.6).
Finally the act of renewing the species complete, the spinners fall to the surface (fig.7) to be eaten in great numbers by the trout.
Like the mayfly, the caddis fly begins its life in an egg (fig.1).
After the egg stage the caddis spends most of its life as a larva, encased in a protective shell it manufactures either of small sticks or pieces of gravel (fig.2).
When its ready to hatch the caddis swims up as a pupa (fig.3) and rides on the flow where it may be taken by a trout if it doesn’t escape to a nearby shore as an adult (fig.4).
When adults have mated, some deposit their eggs on the surface of the river (fig.5), while others swim to the bottom (fig.6), deposit them and then swim up again (fig.7).
It is these diving ovipositors that anglers frequently confuse with hatching caddis, because both are usually spotted after swimming up from the bottom.
Note: Snipped from Art Lee Fishing Dry Flies for Trout On River and Streams