In the case of flies, of course, we provide food resources that include not only our own foods (things put out for picnics for example) but breeding resources such as pet droppings and piles of yard debris. A restaurant omits odors that are highly attractive to house flies, as does a dairy full of cattle and the resulting mountains of manure. While the house fly is "capable" of traveling that 20 miles, the study went on to say that nearly 90% of the flies will not travel more than 2 miles, and this will be most often when they detect serious sanitation problems. One study showed that up to about 80% of flies stay within 1 mile of their origin, another 8-10% might go that 2 miles, and the last few might end up much further away. These studies were done by marking flies and then releasing them, and then capturing them again to determine the distances flown. 

Another study used the good old House Fly once again to test how long it can continue to fly before total exhaustion brings it down, and the number 4.5 hours was used several times. The house fly, it seems, utilizes an energy chemical in its body called trehalose, and during flight this compound may be a kind of sugar molecule. Flies that were forced to fly until total exhaustion was accomplished were then fed a soup of sugars, including trehalose, and they were able to resume flight immediately. Flies flown to exhaustion and then fed sugars withOUT trehalose were unable to resume flight, so the trehalose compound is a critical component for their flight longevity. This sugar is a natural substance that is found in plants and many animals, so the flies apparently find a way to secure it when needed.