No. 79: Jan-Feb 1992
We all learned about the Tropics of Capricorn and Cancer in high school, but the voyagers of old also recognized a "louse line."
"They wrote about arriving at a longitudinal point in the tropics -- the so-called 'louse line' -- where fleas and lice abandoned even healthy humans. Although a specific line is a myth, cultural entomologist Charles Hogue of the Natural History Museum of Los Angeles believes part of the story. 'Voyagers in the tropics often experience a rise in body temperature of as much as 4 degrees F.' That's enough, he says, to kill some species of fleas.
Also, the traumatic experience of being caught in a trap causes the body temperature of animals to rise. Thus, trappers often witness fleas jumping off a trapped animal by the dozens. (Johnson, Donna; "How to Tell Time by a Cat's Eye," National wildlife, 29:12, October/November 1991.)