Flys/Flies

So, why don't I know the appropriate plural for fly? I should know. I have a freakin English degree. I know the plural of cul de sac (btw its culs de sac). Anyway, flys (or flies) ARE DRIVING ME CRAZY. In the past two weeks Ryan has learned to open every door in this house. We have put the child safety locks on all the major doors, but nothing has worked for the sliding door to the backyard. Now, I don't mind he wants to go outside and play. I like hanging out there with a book and watching him play in the sandbox or play with water. HE LOVES WATER PLAY. But, he wants the door open at all times. Even if he isn't out there. If I close it he freaks out. There is going to come a day here very soon where the AC will be on 24/7. It gets over 100 degrees here. Now, I've never one to care about an electricity bill. If its hot you turn on the AC. Thats what its for. I'm responsible, I set it to 78. Just enough to be comfortable. But, I know leaving the door open with the AC on is not a good idea. I don't even need to ask the engineer hubby about that! Plus, flies land everywhere and poop. YUCK.