Spontaneous generation of fruit flies

Spontaneous generation of fruit flies

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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.


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.


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.

Regarding spontaneous generation:

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

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).

Spontaneous generation

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Spontaneous generation, the hypothetical process by which living organisms develop from nonliving matter also, the archaic theory that utilized this process to explain the origin of life. According to that theory, pieces of cheese and bread wrapped in rags and left in a dark corner, for example, were thus thought to produce mice, because after several weeks there were mice in the rags. Many believed in spontaneous generation because it explained such occurrences as the appearance of maggots on decaying meat.

By the 18th century it had become obvious that higher organisms could not be produced by nonliving material. The origin of microorganisms such as bacteria, however, was not fully determined until Louis Pasteur proved in the 19th century that microorganisms reproduce. See also biopoiesis.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen, Senior Editor.

Origin of Life: Spontaneous Generation

It was once believed that life could come from nonliving things, such as mice from corn, flies from bovine manure, maggots from rotting meat, and fish from the mud of previously dry lakes. Spontaneous generation is the incorrect hypothesis that nonliving things are capable of producing life. Several experiments have been conducted to disprove spontaneous generation a few of them are covered in the sections that follow.

Redi's Experiment and Needham's Rebuttal

In 1668, Francesco Redi, an Italian scientist, designed a scientific experiment to test the spontaneous creation of maggots by placing fresh meat in each of two different jars. One jar was left open the other was covered with a cloth. Days later, the open jar contained maggots, whereas the covered jar contained no maggots. He did note that maggots were found on the exterior surface of the cloth that covered the jar. Redi successfully demonstrated that the maggots came from fly eggs and thereby helped to disprove spontaneous generation. Or so he thought.

In England, John Needham challenged Redi's findings by conducting an experiment in which he placed a broth, or gravy, into a bottle, heated the bottle to kill anything inside, then sealed it. Days later, he reported the presence of life in the broth and announced that life had been created from nonlife. In actuality, he did not heat it long enough to kill all the microbes.

Spallanzani's Experiment

Lazzaro Spallanzani, also an Italian scientist, reviewed both Redi's and Needham's data and experimental design and concluded that perhaps Needham's heating of the bottle did not kill everything inside. He constructed his own experiment by placing broth in each of two separate bottles, boiling the broth in both bottles, then sealing one bottle and leaving the other open. Days later, the unsealed bottle was teeming with small living things that he could observe more clearly with the newly invented microscope. The sealed bottle showed no signs of life. This certainly excluded spontaneous generation as a viable theory. Except it was noted by scientists of the day that Spallanzani had deprived the closed bottle of air, and it was thought that air was necessary for spontaneous generation. So although his experiment was successful, a strong rebuttal blunted his claims.


Pasteurization originally was the process of heating foodstuffs to kill harmful microorganisms before human consumption now ultraviolet light, steam, pressure, and other methods are available to purify foodsin the name of Pasteur.

Pasteur's Experiment

Louis Pasteur, the notable French scientist, accepted the challenge to re-create the experiment and leave the system open to air. He subsequently designed several bottles with S-curved necks that were oriented downward so gravity would prevent access by airborne foreign materials. He placed a nutrient-enriched broth in one of the goose-neck bottles, boiled the broth inside the bottle, and observed no life in the jar for one year. He then broke off the top of the bottle, exposing it more directly to the air, and noted life-forms in the broth within days. He noted that as long as dust and other airborne particles were trapped in the S-shaped neck of the bottle, no life was created until this obstacle was removed. He reasoned that the contamination came from life-forms in the air. Pasteur finally convinced the learned world that even if exposed to air, life did not arise from nonlife.

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384&ndash322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (&ldquovital heat&rdquo). As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water. 1

This theory persisted into the 17 th century, when scientists undertook additional experimentation to support or disprove it. By this time, the proponents of the theory cited how frogs simply seem to appear along the muddy banks of the Nile River in Egypt during the annual flooding. Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont, a 17 th century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish.

However, one of van Helmont&rsquos contemporaries, Italian physician Francesco Redi (1626&ndash1697), performed an experiment in 1668 that was one of the first to refute the idea that maggots (the larvae of flies) spontaneously generate on meat left out in the open air. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers (Figure (PageIndex<1>)). Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.

Figure (PageIndex<1>): Francesco Redi&rsquos experimental setup consisted of an open container, a container sealed with a cork top, and a container covered in mesh that let in air but not flies. Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container.

In 1745, John Needham (1713&ndash1781) published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. 2 He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.

Lazzaro Spallanzani (1729&ndash1799) did not agree with Needham&rsquos conclusions, however, and performed hundreds of carefully executed experiments using heated broth. 3 As in Needham&rsquos experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani&rsquos results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani&rsquos findings, Needham argued that life originates from a &ldquolife force&rdquo that was destroyed during Spallanzani&rsquos extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation (Figure (PageIndex<2>)).

Figure (PageIndex<2>): (a) Francesco Redi, who demonstrated that maggots were the offspring of flies, not products of spontaneous generation. (b) John Needham, who argued that microbes arose spontaneously in broth from a &ldquolife force.&rdquo (c) Lazzaro Spallanzani, whose experiments with broth aimed to disprove those of Needham.

  1. Describe the theory of spontaneous generation and some of the arguments used to support it.
  2. Explain how the experiments of Redi and Spallanzani challenged the theory of spontaneous generation.

Disproving Spontaneous Generation

The debate over spontaneous generation continued well into the 19 th century, with scientists serving as proponents of both sides. To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem. Louis Pasteur, a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. In 1858, Pasteur filtered air through a gun-cotton filter and, upon microscopic examination of the cotton, found it full of microorganisms, suggesting that the exposure of a broth to air was not introducing a &ldquolife force&rdquo to the broth but rather airborne microorganisms.

Later, Pasteur made a series of flasks with long, twisted necks (&ldquoswan-neck&rdquo flasks), in which he boiled broth to sterilize it (Figure (PageIndex<3>)). His design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks&rsquo necks. If a life force besides the airborne microorganisms were responsible for microbial growth within the sterilized flasks, it would have access to the broth, whereas the microorganisms would not. He correctly predicted that sterilized broth in his swan-neck flasks would remain sterile as long as the swan necks remained intact. However, should the necks be broken, microorganisms would be introduced, contaminating the flasks and allowing microbial growth within the broth.

Pasteur&rsquos set of experiments irrefutably disproved the theory of spontaneous generation and earned him the prestigious Alhumbert Prize from the Paris Academy of Sciences in 1862. In a subsequent lecture in 1864, Pasteur articulated &ldquoOmne vivum ex vivo&rdquo (&ldquoLife only comes from life&rdquo). In this lecture, Pasteur recounted his famous swan-neck flask experiment, stating that &ldquo&helliplife is a germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.&rdquo 4 To Pasteur&rsquos credit, it never has.

Figure (PageIndex<3>): (a) French scientist Louis Pasteur, who definitively refuted the long-disputed theory of spontaneous generation. (b) The unique swan-neck feature of the flasks used in Pasteur&rsquos experiment allowed air to enter the flask but prevented the entry of bacterial and fungal spores. (c) Pasteur&rsquos experiment consisted of two parts. In the first part, the broth in the flask was boiled to sterilize it. When this broth was cooled, it remained free of contamination. In the second part of the experiment, the flask was boiled and then the neck was broken off. The broth in this flask became contaminated. (credit b: modification of work by &ldquoWellcome Images&rdquo/Wikimedia Commons)

Theory of Spontaneous Generation (1668-1859)

From the time of the ancient Romans, through the middle Ages and until the late nineteenth century, it was generally accepted that some life forms arose spontaneously from non-living matter. Such “spontaneous generation” appeared to occur primarily in decaying matter.

For example, a seventeenth century recipe for the spontaneous production of mice required placing sweaty underwear and husks of wheat in an openmouthed jar, then waiting for about 21 days, during which time it was alleged that the sweat from the underwear would penetrate the husks of wheat, changing them into mice.

Although such a concept may seem laughable today, it is consistent with the other widely held cultural and religious beliefs of the time.

The first serious attack on the idea of spontaneous generation was made in 1668 by Francesco Redi, an Italian physician and poet. At that time, it was widely held that maggots arose spontaneously in rotting meat. Redi believed that maggots developed from eggs laid by flies.

To test his hypothesis, he set out meat in a variety of flasks, some open to the air, some sealed completely, and others covered with gauze. As he had expected, maggots appeared only in the open flasks in which the flies could reach the meat and lay their eggs.

This was one of the first examples of an experiment in the modern sense, in which controls were used. In spite of well-executed experiment, the belief in spontaneous generation remained strong, and even Redi continued to believe it occurred under some circumstances.

The invention of the microscope only served to enhance this belief. Microscopy revealed a whole new world of organisms that appeared to arise spontaneously. It was quickly learnt that to create “animalcules” as the organisms were called, you needed only to place hay in water and wait a few days before examining your new creations under the microscope.

The debate over spontaneous generation continued for centuries. In 1745, John Needham, an English clergyman, proposed what he considered the definitive experiment.

Everyone knew that boiling killed micro-organisms, so he proposed to test whether or not microorganisms appeared spontaneously after boiling. He boiled chicken broth, put it into a flask, sealed it, and waited-sure enough, microorganisms grew. Needham claimed victory for spontaneous generation.

An Italian priest, Lazzaro Spallanzani, was not convinced, and he suggested that perhaps the micro-organisms had entered the broth from the air after the broth was boiled, but before it was sealed. To test his theory, he modified Needham’s experiment – he placed the chicken broth in a flask, sealed the flask, drew off the air to create a partial vacuum, then boiled the broth.

No microorganisms grew. Proponents of spontaneous generation argued that Spallanzani had only proven that spontaneous generation could not occur without air.

Events in Spontaneous Generation:

The theory of spontaneous generation was finally laid to rest in 1859 by the young French chemist, Louis Pasteur. The French Academy of Sciences sponsored a contest for the best experiment either proving or disproving spontaneous generation.

Pasteur’s winning experiment was a variation of the methods of Needham and Spallanzani. He boiled meat broth in a flask, heated the neck of the flask in a flame until it became pliable, and bent it into the shape of an S. Air could enter the flask, but airborne microorganisms could not, they would settle by gravity in the neck. As Pasteur had expected, no microorganisms grew.

Spontaneous Generation

Spontaneous generation, also called abiogenesis, is the belief that some living things can arise suddenly, from inanimate matter , without the need for a living progenitor to give them life.

In the fourth century b.c., the Greek philosopher and scientist Aristotle argued that abiogenesis is one of four means of reproduction, the others being budding (asexual), sexual reproduction without copulation, and sexual reproduction with copulation. Indeed, the Greek goddess Gea was said to be able to create life from stones. Even Albertus Magnus (Albert the Great), the great German naturalist of the thirteenth century Middle Ages, believed in spontaneous generation, despite his extensive studies of the biology of plants and animals.

Through the centuries, the notion of spontaneous generation gave rise to a wide variety of exotic beliefs, such as that snakes could arise from horse hairs standing in stagnant water , mice from decomposing fodder, maggots from dead meat, and even mice from cheese and bread wrapped in rags and left in a corner. The appearance of maggots on decaying meat was especially strong evidence, for many people, that spontaneous generation did occur.

Spontaneous generation found further support from the observations of the Dutch merchant Anton van Leewenhoek, the inventor of the first, primitive microscopes. From 1674 to 1723 Leewenhoek corresponded to the Royal Society in London, describing the tiny, rapidly moving, "animacules" he found in rain water, in liquid in which he had soaked peppercorn, and in the scrapings from his teeth (which, to Leeuwenhoek's surprise, had no such animacules after he had drunk hot coffee).

In the seventeenth century, however, some scientists set out to determine whether living organisms could indeed arise through spontaneous generation, or if they arose only from other living organisms (biogenesis).

In 1668, even before Anton van Leeuwenhoek began his study of microscopic organisms with the microscope , the Italian physician Francisco Redi began a series of experiments that showed that dead meat does not give rise spontaneously to maggots.

Redi filled six jars with decaying meat, leaving three open and sealing the other three. The unsealed jars attracted flies , which laid their eggs on the decaying meat, while the meat in the sealed jars was unavailable to flies. When maggots developed on the meat in the open jars, Redi believed he had demonstrated that spontaneous generation did not occur. However, supporters of the notion of spontaneous generation claimed that the lack of fresh air-not the absence of egg-laying flies-had prevented maggots from appearing on the meat.

Therefore, Redi undertook a second experiment, in which he covered the tops of three of the jars with a fine net instead of sealing them. Once again, maggots failed to appear on the meat in the covered jars, but did appear on the meat in the open jars, where flies were able to lay their eggs.

Nevertheless, the tiny "animacules," described by Leeuwenhoek in his observations on microscopic life in drops of water, still held the imagination of many scientists, who continued to believe that such creatures were small and simple enough to be generated from nonliving material.

John Needham, an eighteenth century English naturalist and Roman Catholic theologian, began his study of natural science after reading about Leewenhoek's animacules. Needham became a strong advocate of spontaneous generation, and performed an experiment that he felt supported his belief in biogenesis. In 1745, he heated chicken and corn broths, poured them into covered flasks. Soon after the broths cooled, they teemed with microorganisms , prompting Needham to claim that the organisms arose through spontaneous generation.

Needham's work was contradicted by another religious investigator, the Italian physiologist Lazzaro Spallanzani. Spallanzani, who was educated in the classics and philosophy at a Jesuit college, went on to teach logic, metaphysics, Greek, and physics . About 20 years after Needham announced the results of his own investigation of spontaneous generation, Spallanzani showed that when broth was heated after being sealed in a flask, it did not generate life forms. He suggested that Needham's broths had probably supported growth after being heated because they had been contaminated before being sealed in their containers.

Undeterred, Needham counterclaimed that heat destroyed the "vital force" needed for spontaneous generation, and that, by sealing the flasks, Spallanzani had kept out this vital force .

The argument continued into the nineteenth century, when the German scientist Rudolf Virchow in 1858, introduced the concept of biogenesis living cells can arise only from preexisting living cells.

But the matter remained unresolved until two years later when the great French scientist Louis Pasteur, in a series of classic experiments demonstrated that (1) microorganisms are present in the air and can contaminate solutions and (2) the air itself does not create microbes.

Pasteur filled short-necked flasks with beef broth and boiled them, leaving some opened to the air to cool and sealing others. While the sealed flasks remained free of microorganisms, the open flasks were contaminated within a few days.

In a second set of experiments, Pasteur placed broth in flasks that had open-ended, long necks. After bending the necks of the flasks into S-shaped curves that dipped downward, then swept sharply upward, he boiled the contents. The contents of these uncapped flasks remained uncontaminated even months later. Pasteur explained that the S-shaped curve allowed air to pass into the flask however, the curved neck trapped airborne microorganisms at the bottom of the curve, preventing them from traveling into the broth.

Pasteur not only executed a brilliant set of experiments, he also used his zeal and skill as a promoter of his ideas to strike a decisive blow to spontaneous generation. For example, in a lecture at the Sorbonne in Paris in 1864, Pasteur said that he had water for his experimental liquids to generate life. But, he said, ". it is dumb, dumb since these experiments were begun several years ago it is dumb because I have kept it sheltered from the only thing man does not know how to produce, from the germs which float in the air, from Life, for Life is a germ and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment!" Pasteur's work not only disproved abiogenesis, but also offered support to other researchers attempting to show that some diseases were caused by microscopic life forms. Thus, in a simple, but elegant set of experiments, Pasteur not only struck the doctrine of spontaneous generation a "mortal blow," but also helped to establish the germ theory of disease .

How do fruit flies appear seemingly from nowhere?

One answer is that fruit flies have very good noses. O.o And can smell fruit/food from far away.

18 year member
4575 replies

15 year member
11752 replies

You can get these totally cool bug zappers that look like a child's tennis racket. and go around doing expert-like tennis strokes while you fry those little puppies. zap. zap. zap!

I got a blue one so it would match my black and blue spots from trying to hit them with my hand. and missing.

15 year member
2520 replies

Dj's link is superb--it even says,

"This very mystery -- vermin from nowhere -- gave rise to a charming and durable belief called 'spontaneous generation.'. But I think fruit flies still make a fair case for spontaneous generation."

Baloo would like Victor Hernandez Cruz.

"How would your family
feel if they had to tell
The generations that you
got killed by a flying

and our bugzapper is yellow like the one in Zb's link.
(Not really "ours"--I refuse to use it.)

My husband loves that poem, (There's more than that fragment.) but then, he loves to zap mosquitoes, too. I think I have some spontaneously generated bugs in my belfry. Or in my brain. Oh, bats!)

15 year member
11752 replies

Currently voted the best answer.

Those "fruit flies" could actually be gnats that hatch in plant soil. If you have live plants, check the soil when you water and see if the little critters are running around the pot and plant.

The adult fungus gnat lays tiny eggs, singly or in clusters, on the soil in houseplants. In about 4 days, the eggs hatch into tiny, silvery white larvae (resembling tiny earthworms), with shiny black heads. When mature the very slim, and transparent larvae can measure up to 6 mm in length. Within a 2-week period, the larvae begin to spin tiny silken cocoons in the soil. The pupal stage lasts only 1 week. Adult fungus gnats emerge and mate. The adult stage lasts only 1 week, but the female will lay around 100-150 eggs during that time. The entire life cycle lasts only 4 weeks -- from egg to larvae to pupae and back to egg.

So, it's in the third week that the bugzapper works great. However, to completely get rid of these annoying pests. one must interrupt the cycle. which Zbeckabee has been unable to do, at least so far. zap.

Spontaneous generation of fruit flies - Biology

Among these ideas, for centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms. It was common “knowledge” that simple organisms like worms, beetles, frogs, amd salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life. For example:

Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times. Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs.
Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs (like Shakespear’s house). As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around. Conclusion: It was obvious to them that the mice came from the moldy grain.
Observation: In the cities, there were no sewers nor garbage trucks. Sewage flowed in the gutters along the streets, and the sidewalks were raised above the streets to give people a place to walk. In the intersections, raised stepping stones were strategically placed to allow pedestrians to cross the intersection, yet were spaced such that carriage wheels could pass between them. In the morning, the contents of the chamber pots were tossed out the nearest window. When people were done eating a meal, the bones were tossed out the window, too. A chivalrous gentleman always walked closest to the street when escorting a woman, so if a horse and carriage came by and splashed up this filth, it would land on him, and not the lady’s expensive silk gown. Most of these cities also had major rat problems which contributed to the spread of Bubonic Plague (Black Death) — hence the story of the Pied Piper of Hamelin, Germany. Conclusion: Obviously, all the sewage and garbage turned into the rats.
Observation: Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in summer, meant battling the flies around the carcasses. Typically, carcasses were “hung by their heels,” and customers selected which chunk the butcher would carve off for them. Conclusion: Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies.

From this came a number of interesting recipes, such as:

Recipe for bees: Kill a young bull, and bury it in an upright position so that its horns protrude from the ground. After a month, a swarm of bees will fly out of the corpse.
Jan Baptista van Helmont’s recipe for mice: Place a dirty shirt or some rags in an open pot or barrel containing a few grains of wheat or some wheat bran, and in 21 days, mice will appear. There will be adult males and females present, and they will be capable of mating and reproducing more mice.

In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat. This was a true scientific experiment — many people say this was the first real experiment — containing the following elements:

  1. Observation: There are flies around meat carcasses at the butcher shop.
  2. Question: Where do the flies come from? Does rotting meat turn into or produce the flies?
  3. Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies.
  4. Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots.
  5. Testing: Wide-mouth jars each containing a piece of meat were subjected to several variations of “openness” while all other variables were kept the same.
    control group — These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop.
    experimental group(s) — One group of jars were sealed with lids, and another group of jars had gauze placed over them.
    replication — Several jars were included in each group.
  6. Data: Presence or absence of flies and maggots seen in each jar was recorded. In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat. In the sealed jars, no maggots or flies were ever seen on the meat.
  7. Conclusion(s): Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms.

In 1745 - 1748, John Needham, a Scottish clergyman and naturalist showed that microorganisms flourished in various soups that had been exposed to the air. He claimed that there was a “life force” present in the molecules of all inorganic matter, including air and the oxygen in it, that could cause spontaneous generation to occur, thus accounting for the presence of bacteria in his soups. He even briefly boiled some of his soup and poured it into “clean” flasks with cork lids, and microorganisms still grew there.

A few years later (1765 - 1767), Lazzaro Spallanzani, an Italian abbot and biologist, tried several variations on Needham’s soup experiments. First, he boiled soup for one hour, then sealed the glass flasks that contained it by melting the mouths of the flasks shut. Soup in those flasks stayed sterile. He then boiled another batch of soup for only a few minutes before sealing the flasks, and found that microorganisms grew in that soup. In a third batch, soup was boiled for an hour, but the flasks were sealed with real-cork corks (which, thus, were loose-fitting enough to let some air in), and microorganisms grew in that soup. Spallanzani concluded that while one hour of boiling would sterilize the soup, only a few minutes of boiling was not enough to kill any bacteria initially present, and the microorganisms in the flasks of spoiled soup had entered from the air.

This initiated a heated argument between Needham and Spallanzani over sterilization (boiled broth in closed vs. open containers) as a way of refuting spontaneous generation. Needham claimed that Spallanzani’s “over-extensive” boiling used to sterilize the containers had killed the “life force.” He felt that bacteria could not develop (by spontaneous generation) in the sealed containers because the life force could not get in, but in the open container, the broth rotted because it had access to fresh air, hence the life force inherent in its molecules, which contained and replenished the life force needed to trigger spontaneous generation. In the minimally-boiled flasks, he felt the boiling was not severe enough to destroy the life force, so bacteria were still able to develop.

By 1860, the debate had become so heated that the Paris Academy of Sciences offered a prize for any experiments that would help resolve this conflict. The prize was claimed in 1864 by Louis Pasteur, as he published the results of an experiment he did to disproved spontaneous generation in these microscopic organisms.

The Health Risks of Fruit Flies

Unlike their insect cousins wasps, spiders and mosquitoes, fruit flies don’t sting or bite. The main risk they pose to humans is exposure to the diseases and bacteria the flies spread by moving throughout the home. While fruit flies spend the majority of their time on fermenting food, they also make the occasional pit stop on plates, cutlery, drinking glasses, and even toothbrushes. While you may not consume the contaminated produce itself, when you eat or drink from these objects, you’re exposed to the same bacteria the fruit fly has transported from site to site.

Approximately 48 million people a year get sick as a result of foodborne illness.   When you find yourself facing an infestation, eradication isn’t optional—it’s necessary.


Due to the overwhelming increase of large-scale food recalls, research has determined that fruit flies are a significant vector of Escherichia coli (E. coli), a dangerous food-borne health hazard to humans in the developed and undeveloped world.  

Spontaneous generation of fruit flies - Biology

As late as the 17th century, some biologists thought that some simpler forms of life were generated by spontaneous generation from inanimate matter. Although this was rejected for more complex forms such as mice, which were observed to be born from mother mice after they copulated with father mice, there remained doubt for such things as insects whose reproductive cycle was unknown. [Maria Sibylla Merian (1647 - 1717) was the first person to document the stages of metamorphosis in butterflies].

To test the hypothesis, Francesco Redi placed fresh meat in open containers [left, above]. As expected, the rotting meat attracted flies, and the meat was soon swarming with maggots, which hatched into flies [left, below]. When the jars were tightly covered so that flies could not get in [middle, above], no maggots were produced [middle, below]. To answer the objection that the cover cut off fresh air necessary for spontaneous generation, Redi covered the jars with several layers of porous gauze [right, above] instead of an air-tight cover. Flies were attracted to the smell of the rotting meat, clustered on the gauze, which was soon swarming with maggots, but the meat itself remained free of maggots [right, below]. Thus flies are necessary to produce flies: they do not arise spontaneously from rotting meat.

Redi went on to demonstrate that dead maggots or flies would not generate new flies when placed on rotting meat in a sealed jar, whereas live maggots or flies would. This disproved both the existence of some essential component in once-living organisms, and the necessity of fresh air to generate life.

Note that is unnecessary to observe or even imagine that are such things as fly eggs, nor does the experiment prove that such exist. Redi's experiment simply but effectively demonstrates that life is necessary to produce life. Redi expressed this in his famous dictum as " Omne vivum ex vivo" ("All life comes from life").

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