The most likely are fruit flies, although perhaps you are faced with something different and smaller. No flies are likely to infest a refrigerator due to the low temperature, they will become unconscious and inactive even if one or two should go there. However, the fruit will not taste as good from the fridge unless allowed to reach room temperature for some hours..... Under no circumstances should bananas be stored in a refrigerator as this will ruin them, unless you like bland and dying/dead bananas.