Appearances aside, you are ...

Recent studies have proven that trout possess vision in the ultraviolet range of the spectrum; those wavelengths which to humans are "non-visible" light. Ultraviolet wavelengths in the (UVA) range of 320-400 nm (nanometers - the measurement of the wavelength of light) penetrate water well, permitting trout to see their prey, even at depths, or in conditions, which filter out most visible light.

Fur-bearing animals, from whom we get much of our fly-tying material (let's not question their generosity), can, in many cases see in the UV; so it behooves them, from a survival perspective, to not reflect light in the UV to their predators or prey. Aquatic insects, on the other hand, lacking means of attracting their mates by vocalization (ah, the dulcet tones of the mating cry of the March Brown...), must use a pattern of visible markings instead. Since the mayfly, for example, has vision in the UV range - in order to discern moving water at night, which reflects UV - species specific markings will be present in the UV, easily seen by potential mates, though not discernible to the human eye.

The materials we commonly use in our mayfly imitations - furs and feathers - do not provide a UV key, the all-important "trigger", which tells the wary trout that the fly drifting above him is real. Given the choice of taking a fly which has no UVA signature (our artificial) or the natural insect that glows in a certain banding, the trout's instinct is to sip the natural drifting inches away rather than our artfully created, but unmarked, imitation. If we could only see the mayfly as the trout does, in the long-wave ultraviolet, then we could tie properly "colored" representations of the insects. The simple addition of a UV pattern - unique for each species of aquatic insect - to our fly tying, should greatly improve our fishing success.

[Tech talk: The two photos above represent the same insect. The top photo was taken with "normal" light, which has a certain percentage of UVA. UVA can penetrate the glass lenses of the camera, unfortunately, the shorter wavelengths of UV cannot. Nevertheless, we have a beginning point. I took the first image, divided it into its constituent Red, Green, and Blue images, used a histogram on each to permit me to isolate the lowest, non-visible values, and then recombined the result to create a mask. I applied the mask to the original and used bright green as a pseudo-color to represent the areas of Ultraviolet. The use of multiple masks would have permitted me to represent the gradations of UVA, but I thought a single mask sufficient for this representation. I used green in this instance to represent the unseen light, however, any false color would do.]

The world that trout and insects see has only fairly recently been exposed to our view through the work of skilled UV photographers such as Bjørn Rørslett. His work has shown us how flowers have adapted their coloration in the UV to attract pollinators. For example, below is a Potentilla anserina L." prostrate cinquefoil" in visible light --
and here is the same photo showing the UV bulls-eye in pseudo-colors --

Therefore, as astute anglers, if we wish to present a far better imitation of the natural aquatic insect than we have at present, we must alter our patterns through the controlled, subtle application of a UV reflective material. Through the following series of articles, I will demonstrate how simple this is to do, and how the result might appear to a trout's eye. I hope in the near future to complete a book of the major aquatic insects and freshwater baitfish as seen in UV.

BTW, you know the "urine-stained vixen fur" that is proper for the body of the Hendrickson? Urine is highly UV reflective, so there was an unintentional cleverness in that choice of dubbing.

© 2008 Reed F. Curry