Identifying the Gender of Flies

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, 3 males, 2 Females,' he replied.
Intrigued, she asked. 'How can you tell them apart?'
He responded, '3 were on a beer can, 2 were on the phone'!!!