Pests in dog food

Question
I have been feeding my 5-year-old Cocker Spaniel Purina One Healthy Weight Management for over a year now.  Quite some time ago, I bought a bag from the supermarket, brought it home, and when I opened it a bug flew out. I tossed the bag out the door and returned it to the store.  A few months later, after purchasing the same brand of food from a pet superstore, my mother found the same type of insects in my dog's air-tight dog food container.  We disposed of it and purchased another container.  We've had this container since just before Christmas and things have been fine.  Today, we went to another pet superstore and noticed the same type of insects flying around the dog food.  Of course we did not purchase it there.  Instead, we drove to yet another store and bought his food.  When we got home, to our dismay, there was one of those bugs sitting in the empty container.  What are these things?  They kinda look like large fruit flies.  I don't think they are something that is in our home because we have never seen them anywhere else besides on the dog food or inside the container.  Are they something that originates from the food?  Is there anything I can do to keep these pests away from my dog's food?  Are they in any way harmful to my dog?  I know this is a long story with a ton of questions, but I am really concerned and perplexed.  I've done just about everything I can do to keep his food sanitary.  Any suggestions or information you can provide would be GREATLY appreciated.  Thanks!

Answer
Although these bugs are unsightly, they are probably harmless.  What you can do is to empty the food into a pre-cleaned plastic garbage can with a good, tight fitting lid.

If it is a problem with the bags of food, notify your grocer or change places where you buy it.