Why do horseflies need geometry to bite?

Question

formatting link

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!

hubops · Accepted Answer

On Sat, 25 Jul 2026 01:52:33 -0700, Maria Sophia   snipped-for-privacy@comprehensi  Just wear the wingman  all summer long  -  horse flies anddeer flies  will leave you alone  -  as will your friendsand family   :    John T.

rbowman · Answer

Could be worse... Nessmuk (George Washington Sears) was a 19th century author who wrote 'Woodcraft' and magazine articles. He had a recipe for an insect repellent that included pennyroyal oil, castor oil, and pine tar. He advised you to apply in frequently until you built up a good patina. It probably would have repelled bears at that point. Nessmuk was big on solo canoe trips through the Adirondacks.

I read an article yesterday about a new repellent derived from catnip. It advised waiting until you left to apply it if you happened to have cats lurking about.

Why do horseflies need geometry to bite?

Join the Discussion

Didn't find your answer?