Why do horseflies need geometry to bite?
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Horseflies cannot reliably identify a host without polarization cues. It's all about right triangles and the arc tangent of skin.FACT:
- I get bitten by horseflies when I leave the pool.
- They fly around me and then bite my shiny wet skin. Why?
- It's all about geometry. They bite only when the geometry is right!
a. Brewsters angle is an arctan of the normal vector. b. n=1.4 for skin. The arctan(1.4) is about 54° c. So Brewsters angle is 54° for skin.
A. Point your finger at the sun B. Move it 36° to one side C. Now the "normal" vector is 54° from the sun
If a horsefly is at the other side of that 54° normal, it bites you. It flies around, literally, until it "sees" that "laterally polarized" reflection at 54° to the normal vector. Then it bites.
What can you do about it? Dry your skin and rough it up and then paint it with zebra stripes. Oh, and sit in the shade.
Moral of the story? Horseflies don't find hosts by color or heat; they find hosts by geometry. Never give a horsefly the 54 degrees it wants!