The order of insects Diptera, Two-winged Flies, is made up of several thousand species on every continent with great variance of form and habit, occupying diverse niches. They can be broadly divided into aquatic and land-bred or terrestrial species. Only a small percentage of the two-winged flies are available to predation by fish, but those often produce significant hatches or falls and are therefore of particular interest to the fly fisher. These species include crane flies, marsh flies, hawthorn flies, reed smut, and midges to name a few. The two-winged flies have a full metamorphosis with stages as egg, larva, pupa, and adult. The duration of their cycle is influenced by local climate; warmer conditions result in shorter lifecycles. One species of midge for instance might have five generations in one year, while another in an arctic region might complete one cycle in two years.
The egg typically takes 3 or 4 days to hatch. The emerging larva then feeds and grows, sometimes through several moults or instars, over a four to six month period. The larva’s body often does not have obviously defined head, thorax or abdominal regions, but is segmented and often grub or worm-like in appearance. Diptera larvae do not have jointed legs but instead some species have small fleshy appendages known as prolegs found on their thorax and abdomen. In aquatic species, the larvae will be either free swimming; attach themselves to weed, stones and other structure (see Reed-smut larva illustrated); build tubes in the mud-water interface or attached to weed and stones. Fish will forage for these larvae at certain times and the wise angler will carry suitable artificials for just such occasions. The larvae of terrestrial flies are of little interest to the fly fisher.
When mature the larvae will change into one of two pupal forms. Some, whether land bred or aquatic, will become pupae that are free swimming (e.g. Large olive buzzerChironomus plumosus) and others will take the form of a cocoon (e.g. Reed SmutSimulium sp. and Hawthorn Fly Bibio marci). It is only the free swimming pupae of aquatic species that are of significance to fish and thereby fly-fishers. The pupal stage of midges is of great importance to fly-fishing and is outlined on another page.
With a few exceptions, when the pupae have matured, in the case of land-bred species they wriggle through the ground to the surface; or in water bred species float and swim to the surface. There they hatch out adult flies. In the case of the Reed Smut, the adult emerges from the cocoon below the water surface and floats to the surface in a bubble of air. The emergence phase in aquatic species presents an important opportunity to the fly fisher. The study of individual groups or species and their habits should lead to effective fishing of a particular hatch or fall.
The adults of aquatic species usually alight very quickly in search of food or a mate so dry flies are not necessarily successful in the hatch. Often it is when adults fall on the water during mating or return to the water to lay their that a matching dry fly is most effective. Though species like some of the crane flies are solely terrestrial insects, they often breed in the habitat around waterways where their clumsy piloting skills often leave them trapped on the water. If numbers are large enough the ‘fall’ can stimulate strong responses from the fish.