Mayfly nymphs can and do emerge in almost any manner imaginable. However, the angler is mainly concerned with water emergence and most mayflies are surface emergers. The physical means by which the nymphs get from the bottom of the stream to the top is vital knowledge both for the angler and the fly tyer. Mayfly nymphs crawl, swim, or float to reach the surface. The swimming is accomplished by undulating the abdomen up and down in a vertical plane. This movement flips the fringed tails up and down, and both actions provide propulsion in a dolphinlike movement....
The float portion of the swim /float equation is interesting but more complex. Many mayfly nymphs often make several attempts to reach the surface before they succeed. During this time they propel themselves by the up-and-down motion of the abdomen and tails. They swim up toward the surface but then will cease their swimming motion and slowly fall back toward the bottom with abdomen and tails curled up. At this point they have negative buoyancy and may make many attempts to swim to the surface before succeeding.... After one of their tries to break the surface they do not sink, but hover just under the surface. The mayfly nymph is now in a perfect position to break the surface film and sometimes does so rather quickly.
During the hatch, some individual insects do not get through the film quickly. Some of the nymphs will hover one or two millimeters under the surface for long periods of time.... They lie very still, without even their gills working much of the time, giving the appearance of death. After a period of time, they will work their gills and legs rapidly and make one or more attempts to break through the surface tension. (Swisher and Richards, 1991)
Once the caddis worm is mature, it builds a cas or closes off its existing case, and, over a two- or three-week period, transforms into a pupa.
After cutting their way out of the cocoon on thebottom, some pupa will struggle out of the case, swim rapidly to the surface, and emerge quickly. Some individuals will lay on the stream substrate, legs curled back under their undulating godies in what we call the "tucked" position. They will lie in this position for various periods of time, which can be as short as a few seconds to as long as ten or more minutes.... Sooner or later they will extend their legs, swim up a little, and drift back down, repeating this action several times. Finally, they will dart into the current, swim rapidly to the surface, and emerge.
Many pupa swim on the surface to the shore and emerge on land, and some emerge just under the surface. Species emerging from water break the surface tension and pop outof the pupal shuck much like the mayfly does. There the similarity ends. Caddisflies quickly flip their wings two or three times and, almost immediately, fly off; they do not float for long periods of time as do the mayfly.
...cadddis pupae swim in almost exactly the opposite manner of mayfly nymphs. Rather than resembling the dolphin's form, their legs mimic a breast stroke or the oars of a scull. They swim almost entirely with their legs. The middle and forelegs pump back and forth rapidly. The hind legs usually trail back, under the abdomen. This is a striking visual characteristic. While swimming, the body becomes almost perfectly straight and rigid. The long antennae slope back over the top, or can be tucked back under the middle legs along the side of the body. (Swisher and Richards, 1991)
The big difference between stoneflies, mayflies, and caddisflies as far as the fly fisher is concerned, is that stoneflies are crawlers, unlike caddis puape and mayfly nymphs, which swim. Stonefly nymphs possess little swimming ability. When dislodged from the stream substrate, they curl up legs ready to grab any object they are carried to by the current. they will sometimes move their legs back and forth rapidly, producing some forward motion. Since they are primarily crawlers, they traverse the bottom from the riffles and pools to the stream bank and up to dry land. Here they break the nymphal shuck; a dry (emerger) imitation is thus of no value during emergence. (Swisher and Richards, 1991)
The larva of most species live in cylindrical or conical cocoons but some are free-swimming and burrow in the silt or in stone or bark cases, much like caddis pupae.