Readers, Please Help! Problems I’m Having in Carrying Out Francisco Redi’s Experiment on Spontaneous Generation with my Third-Graders

Redi's Experimental Jars - Artist's Conception?

When I read about this experiment, I thought it would be really easy to do with my third-graders.  But some unexpected problems developed this week.  See a description in my previous post.

I had to think about a way to keep the experiment so that we could control the odor when putting it away in the classroom at night.  I also had to think about the safety issue of not having glass containers out where they could get broken by students.  Also, if we left the experiment outside at night, the uneducated (and illiterate) night guardians would surely throw it away as trash!

So, I came up with the idea of using individual plastic boxes, with tightly-closing lids, to solve both problems.  We set out the three completely covered boxes, the three completely exposed boxes, and the three gauze-covered boxes (each set of three containing one piece of meat, one piece of chicken, and one piece of fish).  We observed to see if flies were landing on them.  Generally, a few flies landed on the the exposed fish and gauze-covered fish the first day, and one or two landed on the exposed meat briefly.  But oddly, the flies didn’t seem very interested in the meat, and not at all interested in the chicken! The second day, the interest in the fish continued, with little or no interest in the meat and fish. Also on the second day, some small black ants came and crawled around inside the top of the gauze-covered chicken, but they were not going down into the chicken.  Oddly, they did not touch anything else, even though everything was set out together.

Every night we closed the boxes up tightly, put them in our classroom, and came in each next morning to find the classroom smelling to high heaven (exactly like a trash truck).  We opened all the windows, and set the experiment back out side.

The third afternoon, the exposed meat completely disappeared.  We didn’t see it disappear, but my assistant proposed that it had been stolen by a bird.  The fourth morning, the exposed fish disappeared, and a black bird with a bright orange beak was seen stealing it. The fourth afternoon, the gauze-covered meat disappeared (gauze ripped off the box, and meat gone), and I think it was the same bold bird!  We continued the experiment to the fifth day, as we were hoping to see some maggots (having never seen any except in pictures).

On the fifth morning (Friday) several dead flies were in the fish box (which my assistant had apparently closed in the box when he closed it the night before), but still no maggots.

So in five days, we got no maggots.  I haven’t thrown away the rest of the experiment yet, but I have a theory as to what might have happened.  If anyone else has any knowledge as to whether my theory could be correct, I would appreciate the feedback.

My conclusion:

When I saw the dead flies, it occurred to me that the bad smell (or perhaps poisonous gases given off by the decomposition of the contents) had killed the flies.  If the flies were killed, perhaps any maggots hatching out would have been killed by the same thing, which might be why we got no maggots.  A friend of mine also suggested that the germs which were decomposing the contents might have used up the oxygen in the box and produced a box full of carbon dioxide, which would have suffocated the flies, as opposed to having poisoned them.  Please, what does anyone else think about this?

I realized that Redi’s experiment did not say he closed his jars at night, which would have let any gases escape.  He also used a different-shaped flask, which birds would not have been able to penetrate (if the illustrations I’ve seen in science books are not just some artist’s conception of the flask shape).

I’m undecided as to whether or not I should try this experiment again.  I think we’d need a cage that birds could not penetrate, but which flies could, and containers which could be left open outside for one week.

Eileen

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