Trout Week Bug Day: Chitin and Color Selection


“What color though?”- A question that often comes up at the fly shop or in your box after you’ve decided on a particular pattern, choosing a color can be one of the most complicated and uncomplicated parts of fly fishing. . To understand the reason for the color choice, you first need to determine 2 things: Am I fishing lures or matching cover, AND is a hatch happening or about to happen? From there, you can at least make a very educated guess about what to use.

When you’re lure fishing, you’re doing just that, trying to attract the fish’s attention with a colorful, eye-catching lure. Often times colors like purple, red and orange appear on these flies. Fish that tend to feed in less selective environments are usually the target of lure fishing. When fitting a cover, not only do you consider the size, shape, profile and action of the fly, but you also decide on the color. The fish you are trying to catch are much pickier and tend to inhabit quiet habits or spring-like water. I like to make the analogy that these trout look through glass all day long. Fish in faster flowing waterways tend to be less selective and just happy to grab a meal as it passes quickly.

Doing a quick survey of topwater insects is always a good start to deciding what fly pattern to use. Skimming the edges of the banks, inspecting the rocks near the shore for debris, and taking your fishing net to the nearest bush for a good strike are all great ways to determine what’s going on along the creek. A small seine net, or even a small green aquarium net is also useful for sampling nymphs. If a hatch is happening, or will happen, it can make a difference in your choice of fly color.

Let’s start with a little metamorphosis lesson for some background on how hatching time affects color. Because insects don’t have backbones (macroinvertebrates—remember?), they have to grow in a different way than we do. They are also extremely soft, prone to drying out and have the ability to breathe through their skin. This is where the exoskeleton comes in. The exoskeleton provides adequate protection and the ability to breathe in the form of skin for the insect. Think of it as a tough Gore-tex shell. It keeps the right amount of moisture, allows you to be flexible and protects you from the bad stuff. After the insect hatches from its egg, it grows, but through each growth, it must shed and reproduce its exoskeleton. The growth stages of the nymph are called instars and the act of shedding the exoskeleton is called molting. Depending on the species, an insect may have only a few instars, or it may have over 40 instars. A molt at a young age is caused by an increase in the hormone ecdysone, which increases due to a variety of environmental factors. The color of the insects is conditioned by the exact part of the life cycle they are in, and specifically by how advanced they are in each star.

Chitin (rhymes with “titan”) is a polysaccharide that occurs naturally in arthropods and forms their exoskeleton along with a mixture of proteins. Arthropods are members of the invertebrate group of Kingdom Animalia that have a “jointed foot”. Chitin is also found in fungi, nematodes and is the second largest macromolecule found on earth (after cellulose). After an insect molts, their chitin must harden and undergo a type of tanning process called sclerotization for it to harden and return to its normal color. Immediately after the insect molts, it becomes almost white in color before the tanning process begins. This process can take several hours, so as it goes on, the insect changes color. When the insect molts into that translucent white version of itself, it is more vulnerable to predators because of its contrasting color and soft body which makes them very easy to digest. This is called the “internal” phase. If you ever turn over a rock and see an insect that looks like a ghost version of itself, you’ve likely found an insect or other arthropod in its teneral form. The translucent, strong insect is constantly in a state of darkness.

When the aquatic insect reaches its final instar and final molt, they may do so in an out-of-water structure, at the surface tension of the water, and some even do so underwater before swimming to the surface as a winged adult. . During that final period of sclerotization where the exoskeleton is hardening, the adult insect’s color darkens and they are rapidly changing as they pump their wings full of hemolymph (insect blood) and prepare to fly as adults. A special mixture of chitin and proteins forms the most flexible wing material. This is sometimes a good opportunity to fish a bug that is hatching – in one of the lighter colors.

Sexually mature, winged insects do not molt as adults. EXCEPT for that one funny exception, which is kind of big in fly fishing. This is Mayfly. Because of their prehistoric roots and lack of evolution, Fat Flies actually have a second molt as an adult that goes from the subimago (dun) stage to the imago (spinning) stage. The biggest color change in that fusion is the wings going from a dark color like a pimple to clear like a spinner. Your choice of fly and color with an adult fly will be a little different than determining your choice of nymph, mainly because the fish will be looking at it, so wing shape and silhouette may matter more than color. In underwater play, changing color can make or break a day.

When we nymph, we are mimicking insects in motion. These insects are not necessarily attached to anything underwater, they instead fall, swim and work their way downstream or towards the shore. This is for a multitude of reasons that could be either to change their habitat or to get out. If an insect heads for shore ready for its last (or last, ahem Mr. Mayfly) molt, it’s likely to be in its darkest color form at the time. That’s why, say when the salmon fly hatch is about to happen, you might choose that black version of Pat’s Rubberlegs, as opposed to the lighter, brown versions. If you are fishing with a green drake nymph and you see a few coming up the bank, but the fish are not yet locked in the dry, try something less green and darker olive, almost black.

One of the best days I ever had fishing was because I had seen and documented an opening last year. I went out a few days earlier the following year and fished a darkened nymph version, swimming close to the surface of the bug and had a fast time. Take pictures, keep a log and take the time to notice the really small things. It will pay dividends.

Article by Trout Unlimited, Maggie Heumann. Stay tuned for more content from this year’s Flylords x Trout Unlimited Trout Week!





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