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I live in the South of England. It is currently winter.

Whenever I buy bananas, even one, if I leave the discarded banana peel in my kitchen, within hours or even minutes, there is a cloud of little flies to be seen in the vicinity.

Given that this doesn't happen with other fruit such as apples, I hypothesise that these flies spontaneously generate in the presence of opened bananas and/or their peel.

Notes

Bananas are not native to the British Isles and are imported from hot countries.

I can go months or even years without purchasing a banana and yet, when I do, these flies appear almost immediately. They also disappear almost immediately when I dispose of the fruit and its peel.

The flies do not appear until I unpeel the banana.

Question

How can I disprove my hypothesis of spontaneous generation?

Note: A secondary hypothesis is that fruit fly eggs are distributed all around the world (including in my kitchen) and simply waiting for a banana to appear. This seems less likely but more difficult to disprove. If this is so, how do the flies have time to mate and lay the next generation of eggs in my kitchen when the banana detritus is disposed of almost immediately.

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    $\begingroup$ Francesco Redi performed this experiment 350 years ago: courses.lumenlearning.com/microbiology/chapter/… . Further work by Louis Pasteur and John Tyndall ~150 years ago definitively convinced the science community that spontaneous generation does not occur -- a belief which is in no way controversial to this day. $\endgroup$ Dec 7 '20 at 17:41
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    $\begingroup$ @theforestecologist: But that rather begs the question: where DO the fruit flies come from? $\endgroup$
    – jamesqf
    Dec 7 '20 at 17:46
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Regarding spontaneous generation:

Francesco Redi performed an experiment 350 years ago disproving spontaneous generation of insects (see here).

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Source: Lumen Learning

Further work by Louis Pasteur and John Tyndall ~150 years ago definitively convinced the science community that spontaneous generation does not occur -- a belief which is in no way controversial to this day.

Regarding fruit flies:

Fruit flies lay their eggs in fruit. According Orkin, the eggs will hatch into larvae in ~30 hours when in an optimal temperature (75-80 oF).

Produce is typically refrigerated during shipment and storage to prolong shelf-life. This means that the hatching process is halted until the fruit nears your home (or on the grocer's shelf if they are not managing their temperatures or unboxing procedures effectively). Assuming your grocer is taking good care of their fruit (i.e., keeping it cool), then you are likely only starting (or restarting) the egg-hatching timeline after you bring the fruit home. Result: you find fruit flies in your home that you didn't see in the store.

  • Ripening bananas, by the way, produce their own heat. So even if ambient temperatures are cooler than optimal for fruit flies, the heat from the ripening bananas might push the temp within optimal conditions. This is especially true if bananas are in any form of storage container (e.g., their shipping box). This is why a good grocer opens their banana boxes upon arrival -- it extends shelf-life and prolongs/prohibits fly development. (This also means that if the grocer doesn't unbox their bananas or leaves them in dense piles in poorly refrigerated display shelves, the 30-hour timer started before you brought them home).
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    $\begingroup$ So if Redi had performed his experiment with bananas instead of meat, he would have found that spontaneous generation occurred :-) $\endgroup$
    – jamesqf
    Dec 8 '20 at 2:28
  • $\begingroup$ @jamesqf actually, if he'd just let it sit long enough, bananas would've grown out of the meat. $\endgroup$ Dec 8 '20 at 2:36

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