Under the surface of the water is an entirely different universe than above. Down here is where much of the lives of important flies to fisherman begin. The nymphs, or larva are considered to be an imitation of any aquatic insect in a stream or lake, or on its way to the surface to hatch (emerging). Nymphs form a massive part of a trout’s diet, they are available year round, and that is the reason that so much focus is put on various techniques for nymphing for trout. High Stick Nymphing is an example of this.
From what I’ve learned, there are two basic stages in which trout will actively be feeding on nymphs. Nymphs spend quite a long time below the surface before emerging, and they are active during the entire phase, except in the case of some species which pupate, such as caddis flies. In this stage of development, the nymphs are available to trout as they move around in the water, or when they loose their grasp and are swept away by the current. Then there is the stage where the nymph begins is rapid transition into an airborne fly. This is the emerger stage, and it is one phase of insects that can drive a fly fishing nuts! It is important also to note that when a particular species of nymph is emerging, it is key to have a pattern on hand that resembles the insect in size and colour. Most fish will key in on emerging insects, and in the process ignore other items drifting downstream in the current.
I find the emerging process to be one of the best opportunities to fish with nymph imitations. During the last few days of the insects life as a sub surface creature, they become quite restless. In the process they begin to rise to the surface and then dropping back down to the bottom of the stream or lake, and again rising to the top. Naturally, this sort of thing attracts the attention of trout, who promptly move in to gorge themselves as thousands of nymphs begin the process of transforming to an airborne insect. Activity of trout during this time can be seen as surface boils or flashes below the surface as the fish dart around feasting.
There are a vast number of insect species that occupy the water below its surface, and the species that in exist in each waterbody will differ from one to the next. Your best bet is to investigate the streams or areas you will be fishing in order to find the right patterns for you.