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Aquatic insects make up the largest part of a freshwater fish's diet. Their abundance provides the fish with a never-ending food supply. Fish can recognize the different stages of the insects' life, larval and adult, and a fly fisherman can use this to their advantage.


Mayflies make up the greatest part of many fishes' feeding pool. The insects are generally between 3 and 25 millimeters in length and there are literally hundreds of different species, calling for the use of different size flies if you fish in many different regions. Mayflies have a lifespan of about a year and spend most of that time in the water. The millions of nymphs floating in the water after hatching present an irresistible target to the fish.

The first adult, or dun, stage is also important, because when the nymphs emerge from their casings as adults they must sit on the water until their wings dry. During that time fish come to the surface to feed, presenting the opportunity for dry fly fishing. The duns that survive fly to the bank to change into the next stage. This stage is called the spinner stage and is when mating takes place. Mayflies gather in great clouds over the water and mate in midair. Those who are successful fall into the water and the female releases her egg mass. The flies die soon after.


The caddisfly is also populous, but its entire metamorphic process takes place under water. Larva take three forms, case-building, net-making, or free-swimming. The free-swimming is the most frequently imitated by fly fishermen. The freeswimmers build small cocoons around themselves in the pupa stage and when they begin to hatch their pupa moves quickly up through the water. When the pupa reaches the surface and cracks open, they must quickly fly away or risk being eaten. After they mate and return to the water to lay their eggs, they again become a target for hungry fish. Caddisflies are most active in late afternoon and early evening.


Stoneflies reside in fast-water streams and are between 7 and 50 milimeters in length, making them a target for small and large fish. Nymphs can remain underwater for three years and, because they crawl instead of swimming, can be easily preyed on if they slip from the bank. Stoneflies crawl up onto land to make their metamorphosis into adults. Once fully mature, they go further up the land to mate and they fly back to deposit their eggs in the stream, again presenting fish with an opportunity for feeding.