Trout Week Bug Day: Chitin and Color Choice

“What color though?”-  A question that often arises at the fly shop or in your box once you’ve settled on a particular pattern, color choice can be one of the most complicated and uncomplicated parts of fly fishing. To understand the why’s of color choice, you must first determine 2 things: Am I attractor fishing or matching the hatch, AND is a hatch occurring or about to occur? From there, you can at least make a very educated guess about what to use. 

When you are attractor fishing, you are doing just that, trying to get the fish’s attention through some flashy colorful attraction. Often times colors like purple, reds and oranges show up in these flies. Fish that tend to feed in less-selective environments are usually the target with attractor fishing. When you are matching the hatch, not only are you taking in to account the size, shape, profile and action of the fly, but you are also deciding on color. The fish you are trying to catch are much more picky and tend to reside in mores still, or spring-like water. I like to make the analogy that these trout are looking through glass all day. Fish in faster flowing waterways tend to be less selective, and just happy to grab a meal as it quickly passes by. 

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Making a quick study of bugs on the top water is always a good start for deciding what fly pattern to use. Skimming the edges of the banks, inspecting rocks near shore for shucks, and taking your fishing net to the nearest bush for a good whack, are all great ways to determine what is going on streamside. A small seine net, or even little green aquarium net is helpful for sampling nymphs as well. If a hatch is occurring, or is about to occur, that can make a difference in your fly color selection. 

Let’s start with a little metamorphosis lesson for some background on how hatch time influences color. Because insects don’t have spines (macroinvertebrates-remember?), they have to grow in a different way than we do. They are also extremely soft-bodied, prone to desiccation, and have the ability to respire through their skin. That is where the exoskeleton comes in. The exoskeleton provides the right protection and the ability to breathe cutaneously for the insect. Think of it as a Gore-tex hardshell. It keeps in 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 spurt, it must shed and regrow its exoskeleton. The phases of the nymph growth are called instars and the act of shedding the exoskeleton is called molting. Depending on the species, an insect can have just a few instars, or it can have upwards of 40. A molt to a new instar is triggered by an increase in the hormone ecdysone, which increases due to a variety of environmental factors. Insect color is conditional on what exact part of the life cycle they are in, and specifically, how progressed they are in to each instar. 

Chitin (rhymes with “titan”) is a polysaccharide that naturally occurs in arthropods and forms their exoskeleton along with a mixture of proteins. Arthropods are members of the invertebrate phylum of Kingdom Animalia that have a “jointed-foot”. Chitin also occurs in fungi, nematodes, and is the second largest macromolecule found on earth (behind cellulose). After an insect molts, their chitin has to harden up and go through a type of tanning process, called sclerotization, for it to become rigid and back to its normal coloration again. Immediately after the insect molts, it becomes almost white in color before that tanning process has begun. This process can take several hours, so as it is going on, the insect changes in color.  When the insect molts into that translucent-white version of itself, it is at its most vulnerable for predation due to the contrasting color, and their soft-bodied-ness that makes them extra easy to digest. This is called the “teneral” phase. If you ever flip over a rock and see a bug that looks like the ghost version of itself, you’ve likely found an insect or other arthropod in its teneral form. The see-through, teneral, insect is constantly in a state of darkening. 

When the aquatic insect reaches its final instar and final molt, they can do so on a structure outside of the water, in the surface tension of the water, and some even do it underwater before swimming to the surface as a winged adult. During that final sclerotization period where the exoskeleton is hardening, the adult insect’s color becomes darker, and they are changing rapidly as they pump their wings full of hemolymph (insect blood) and prepare to take flight as an adult. A special blend of chitin and protein forms the more flexible wing material. This is sometimes a prime opportunity to fish a bug that’s hatching- in one of the lighter colors. 

Sexually mature, winged-adult insects, don’t molt as an adult. EXCEPT that one funny exception, which is kind of a big deal in fly fishing. That is the Mayfly. Because of their prehistoric roots and lack of evolution, Mayflies actually DO have a second molt as an adult going from the subimago (dun) to the imago (spinner) phase. The biggest color change in that molt is the wings going from an opaque color as a dun to clear as a spinner. Your fly and color choice with an adult mayfly is going to be a little different than determining your nymph choice, mainly because the fish will be looking up at it, so wing shape and silhouette may matter more than color. In the underwater game, the variable color can make or break a day. 

When nymphing, we are imitating insects in the drift. These insects aren’t necessarily attached to anything under the water, they are instead tumbling, swimming, and working their way downstream or towards the shore. This is for a myriad of reasons that can be to either change their habitat or to emerge. If an insect is headed towards the shore ready for its last (or next-to-last, ahem Mr. Mayfly), molt, it will likely be in its darkest color form at that time. That is why, say when the salmonfly hatch is about to occur, you may choose that black version of a Pat’s Rubberlegs, as opposed to the lighter tan and brown versions. If you are fishing with a green drake nymph and you see some emerging on the shore, but fish still aren’t keyed in on dries, try something less green and more dark olive, almost black. 

One of the best days I have ever had fishing was because I had witnessed and documented a hatch in the previous year. I went out a few days earlier the following year and fished a darkened, swimming-close-to-the-surface nymph version of the bug and had a bang up time. Take pictures, keep a hatch log, and take time to notice the really, really little things. It will pay dividends.

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