如何分辨蒼蠅的性別？

A woman walked into the kitchen to find her husband stalking with a fly swat.
太太在廚房中看到先生拿著蒼蠅拍。
"What are you doing?" She asked.
“你在幹嗎？”
"Hunting Flies" he replied.
“打蒼蠅”

"Oh!    Kill any?" She asked.
“打到了嗎？”
"Yep, 3 males, 2 Females," he replied.
“打到三隻公的，兩隻母的”

Intrigued, she asked.
"How can you tell them apart?" 
“你如何區分公的母的？”

He responded...
 "3 were on a beer can…
“三隻在啤酒罐上
the other 2 were on the phone”.
其他兩隻在電話上”