Trout Flies in the Surface Skim

The cohesive forces between liquid molecules are responsible for the
phenomenon known as surface tension. A surface "film" is formed which makes it
more difficult to move an object through the surface than to move it when it is
completely submersed. If an insect is perched on the surface film its six legs
and/or other portions of its body may protrude through the film. The parts of
the insect or fly that extends below the surface can be seen by the trout even
when it is outside its window of vision. Seeing the legs of an insect or other parts
of its body may alert the trout that something is coming into its window of vision. A
midge may make such a sight indentation in the surface film that would be almost
impossible to see outside the window simply because the parts penetrating the
film are so tiny. A grasshopper’s legs and maybe even part of its body would be
visible outside of the window from much farther away. I could go on and on
explaining light refraction, Snell’s Law and just how it affects the trout’s vision of
the fly but I would be getting away from some of the main points I want to make.

When a trout sees an insect on the surface that has drifted into its window of
vision, it determines whether or not to take the insect. If the trout attempts to
take the insect, it moves its fins in such a way that allows the current to assist it
in propelling its upward motion. It takes the insect in its mouth and then moves
its fins in such a manner as to propel back down into its holding position.
Copyright 2013 James Marsh
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