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Entomology 101

Updated: Sep 17, 2022


Often as a guide I am asked what flies to use and how do I determine that. There are many factors that come into play such as time of year, temperature, and weather. You might get a hatch in the middle of the day, but if it's rainy, those fish might not be feeding around the surface. Flies that have been hatching for a few days might not be as prolific if a cold front is coming in. There is a lot to consider for sure, but having a basic knowledge of the life cycles of the bugs is the foundation of fly fishing for trout.


There are all kinds of aquatic critters that trout will eat, but they mainly subsist on two types of macroinvertebrates called caddisflies and mayflies. Stoneflies are also present in larger streams, but the basic life cycles are the same for all of these "flies". First of all, these bugs are always present in the stream and live under the rocks for roughly 364 days out of the year. They cling to the bottom of rocks eating very small things (think plankton) all year long. Mayflies and stoneflies hold onto the rocks with their legs. Some caddis swim through the water from rock to rock, while others actually build small burrows attached to rocks. They are like little worms in a casing.


These critters hang out all year until the right conditions, water temperature, and sunlight finally tell them its their special day and they start the transformation to adults. Mayflies will go from their nymph stage to an adult. They float to the top of the water, where they get stuck in the surface film until their wings split out of their thorax and they can fly off. They mate shortly thereafter and return to the water where they lay their eggs and then die. 364 days of waiting for 1 day of partying. Caddisflies and stoneflies do the exact same except that they swim or float to the banks or dry rocks above the stream. They molt and have wings as they emerge from their aquatic casing.


So what does this all mean to us? If it is the right day for these particular bugs then they will be out in mass. Its not an individual survival thing, but a numbers thing. The concentration of this bug means that the trout will eat nothing else. This is what we call a hatch and usually when you see fish rising everywhere eating bugs off the surface. Your best bet is to "match the hatch". Try to skim one of these bugs off the surface, or check in some spiderwebs. Getting the right fly can mean an amazing day. However, we can prepare in advance with a little research. A local fly shop can tell you what's going on and sell you the hot fly, but what if you don't have a shop nearby?


Hatch charts! Yes, hatch charts. Some biology student in the area has likely put together a hatch chart for the more known streams. A quick google search for "waterway you want to fish" + hatch chart and you will have a pretty good guide to what is likely hatching that time of year. For example "Shenandoah National Park Hatch Chart" and there is an example here: https://www.totalflyfishing.com/virginia/shenandoah-hatch-chart/

This is a pretty basic chart, however it gives you some ideas of flies to carry in a particular time of year.


Be patient and persistent. You might think you have the perfect fly and its not quite it. Maybe go smaller, more tippet, a slightly different color. Once you have the right idea of how it works together then it is only a matter of time and experience, and messing up plenty. When it all comes together you will have all kinds of stories to tell.


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