Insects and their sex

The wife enters the kitchen and sees her husband holding a fly whisk
Wife : "What are you doing?"
Husband : "I'm Killing flies"
Wife : "Any success so far?"
Husband : "Yeap, three males and two females"
Wife : "? How did you tell their sex ??"
Husband : "Three flies were standing over my open beer can and two were standing on the phone."