How to tell the sex of a fly

How to Tell the Sex of a Fly A woman walked into the kitchen to find herhusband 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.