A Historical Context

The earliest reference to fishing with an artificial fly is to be found in the work of AElian writing in about 200AD. Though he did not travel outside Italy he was able to draw upon a vast range of reference work including Greek sources and in his “Natural History” he refers to a Macedonian way of catching fish with “speckled skins” which feed upon a fly that hovers on the river. He states that to handle the fly was to destroy the natural colour and therefore “They fasten red (crimson red) wool around a hook, and fix onto the wool two feathers which grow under a cock’s wattles”.
Presumably similar tactics to catch fish were used elsewhere and though little is known about fly fishing in medieval Europe it is mentioned in German  literature  that trout and grayling could be caught using a “feathered hook”. Other texts from 1360 onwards describe fly fishing as being popular amongst ”commoners”  in an area stretching from the Swiss plain across Styria.
A fifteenth century document kept in the Bavarian abbey of Tegernsee lists fifty different un-named fly patterns. There are at least three English treatises from this period which mention fly fishing. One can be found in the British Library describing how to take “trowte”. The second is Medicina Piscium in the Bodleian Library describing flies for both trout and salmon. The third is the “Treatyse of Fishing with an Angle” the earliest known printed reference work on fly fishing. It includes descriptions of how to make a rod, line, hooks as well as twelve fly patterns. It also promoted fly fishing as akin to hunting- at that time the sport of kings and nobles. It proved to be immensely popular and was reprinted many times over the century.
In the later seventeenth century writers on fly fishing include Colonel Robert Venables, Isaac Walton and Charles Cotton. Fly lines were home-made, usually from horse hair, tapering from seven down to three hairs at the point. Sometimes silk or silk/horse hair combinations would be used. The line would be fastened to the top of the rod, or some anglers would allow the line to run through a loop at the tip of the rod and either hand hold the line or fasten it to a reel.
Walton listed twelve flies from the Treatyse, but Cotton mentions sixty five trout fly patterns in the fifth edition of the Complete Angler.
Between 1850 and 1900 the false cast was discovered, dry fly technique emerged, split-cane rods were developed and what we know as the “modern reel” appeared. In 1857 a young Scotsman, Stewart, was advocating up-stream wet fly fishing with a shorter, light stiff rod. This along with the false cast and the emergence of dry fly fishing resulted in a trend towards rods made from lancewood, bamboo and whalebone of about nine to ten feet in length. By 1900 specialist fly rods were being manufactured, often made from split cane, which were much shorter than the typical eighteen foot rod used by Cotton and his contemporaries. The evolution of silk lines at this time also resulted in casting distances increasing greatly.
It was at this time that the concept of the up-stream dry fly was developed. The dry fly technique was to have a huge impact on rod design. Traditionally F M Halford is given the greatest credit for the evolution of dry fly technique however much is owed to others such G S Maryatt,  H S Hall, E J Power, and Dr T Sanctuary. It is through his writing as an amateur entomologist and fisherman that Halford came to prominence. He strived for an exact imitation in his fly patterns and was to refine his selection in the definitive thirty three flies described in “The Modern Development of the Dry Fly”. One can still hear echoes of his voice today on a river in his definition of dry fly fishing as “…presenting to the rising fish the best possible imitation of the insect on which he is feeding in its natural position.”
This he broke down into four conditions:
Finding a feeding fish
Presenting to him a good imitation of the natural insect both as to size and colour
Presenting it to him in its natural position, floating and “cocked”.
Putting it lightly on the water so that it floats accurately over him without drag
That the four previous points should have been fulfilled before the fish has caught sight of the angler or his rod
G E M Skues  observing  in his book The Way of a Trout with a Fly that  “..trout do not rise without something to rise at”  and that “ trout feed sub-surface and when not rising it is because they are busy with some preferable diet  under water“ . The concept of fishing sub-surface with nymphs  was to have an enormous impact on the fly fishing community and provoke the great debate between the advocates of Halford and the followers of Skues.
The above notes are compiled from:
For those interested in the  history of the subject Dr Andrew N Herd’s fascinating book “The History of Fly Fishing” is highly detailed and published by Medlar Press
F. M. Halford,  Dry Fly Entomology,  Second Series 1902, Vinton and Co.
G.E.M. Skues,  The Way of a Trout with a Fly, Fouth Edition, 1949