How to Tell the Gender of a Fly

[MBM888]

A woman walked into the kitchen to find her husband stalking around with
a fly swatter.
"What are you doing?" she asked.
"Hunting flies," he responded.
"Oh! Killing any?" she asked.
"Yep, three males, two females," he replied.
Intrigued, she asked, "How can you tell them apart?"
He responded, "Three were on a beer can, two were on the phone."

[_fly_]

I can confirm that I am male....