Dancing Flies

A Guided Discovery Illustration of the Nature of Science

Moti Nissani

Source: American Biology Teacher 58 (#3): 168-171 (1996).

Note to Science Teachers:  Copy the Students Manual below for your students, not the background materials which follow it!

The Mystery of the Dancing Fruit Flies

To derive pleasure from the art of discovery, as from the other arts, the consumer+in this case the student+must be made to re-live, to some extent, the creative process. In other words, he must be induced, with proper aid and guidance, to make some of the fundamental discoveries of science by himself, to experience in his own mind some of those flashes of insight which have lightened its path. . . . The traditional method of confronting the student not with the problem but with the finished solution, means depriving him of all excitement, [shutting] off the creative impulse, [reducing] the adventure of mankind to a dusty heap of theorems.

Arthur Koestler, 1964

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Preliminary Remarks (Distributed on Week 1)

1. If you own a magnifying glass, please bring it to class during the weeks for which this experiment is scheduled.

2. The main point of this entire exercise is discovering things on your own. If you look up the answers beforehand, you would deny yourself the fun and excitement of rediscovery.

3. Your writing assignments for this exercise will not be graded on the basis of your ability to come up with "correct" answers; they will be graded only on the basis of effort, creativity, and reasonableness.

4. Unlike traditional lab reports, each writing assignment will only ask you to answer specific questions. So, while performing various tasks and observations, you need to bear in mind the specific questions that you will need to answer later. Next week, for instance, you will submit Writing Activity A+which will only consist of written answers to the five questions which appear at the end of Part II.

5. Parts IV-VII of this exercise will be handed out throughout the term. By next week, you only need to read Parts I, II, and III.

6. Please submit your writing assignments on time, before the answers are discussed in class!

Part I: Dancing Flies? (Distributed on Week 1 and performed on Week 2)

If you take time to sit and stare at flies, you might notice a few things you have never noticed before. You might see them extending their mouth part to imbibe food, or laying eggs on your tomato salad. With a little bit of luck, you might even stumble across a striking spectacle. Besides their ability to do such amazing things as walk on ceilings and taste with their feet, flies+when the mood strikes them+actually dance! Science professors have been known to pull a leg or two (other than those of flies), and so you have excellent reasons to disbelieve us. But you will not, we hope, doubt your own eyes.

Form groups of fours. Each group will receive two vials, each containing 10-20 flies of a species known as Drosophila melanogaster. Because in nature these insects feed on overripe or rotting fruits, they are commonly referred to as "fruit flies."

Place both vials horizontally on a desk at the center of the group. Make sure the vials remain stationary throughout the entire observation period (we are interested in the behavior of undisturbed flies). Please record the behavior of the flies in both vials. In particular, do you see any dancing flies? Are they interacting with each other or doing their own thing?

The flies in each vial have been together for days; familiarity in this case might have bred indifference. So let's try watching the behavior of strangers as they are brought together for the first time.

To begin with, relax! These flies don't bite, they are not noisy, and they carry no disease. Also, they are replaceable+if you failed in your first attempt to mix them, your instructor will give you another pair of vials. Now, when a vial is lightly tapped on an open hand with the food part down, the flies are forced to the bottom. One member of your group should tap one vial while another member taps the other. While still gently tapping, remove the plug from both vials. Quickly invert one vial and place it on top of the other, so that both vials form a single, closed-ended tube. Grasp both vials at the point of contact and tap them again so that all flies are knocked to the bottom of the lower vial. Remove the top vial and plug the lower vial+rapidly but cautiously, so that the flies don't escape or get injured. As before, record their behavior.

After about ten minutes (while still observing the flies in the first vial and without disturbing them), repeat the procedure you have just carried out with a second pair of vials. Continue to observe both for 25 minutes or so.

At the end of the observation period, your group will have two empty vials and two vials with flies. Transfer about half the flies from one of the occupied vials to an empty vial. Repeat this step for the second pair of vials. Each group member should take home one vial.

Besides the above, the class as whole will keep a few vials in which no mixing took place. These vials will serve as controls: taken home by a few students (in addition to a vial of mixed flies), and treated in the exact same manner as the mixed vials.

Part II: The Dissecting Microscope (distributed on Week 1 and performed on Week 2)

These flies are too tiny to be seen clearly with the naked eye. A more impressive view is afforded with a dissecting microscope. Take turns today examining a few flies under the microscope. After consulting the labels which identify microscope parts, make the following adjustments:

1. Make sure that the object you are about to observe is at the center of the stage and that the light switch is on.

2. Adjust the two eye pieces to fit the distance between your eyes. You know it's OK when you get a clear image while looking at the specimen with both eyes open.

3. Always start with the smallest magnification.

4. Slowly adjust the focus. If the fly does not become clearly visible while you steadily move the focus one way, try moving the focus the other way. Also, you need to slightly readjust the focus to see different body parts.

5. Play with the mirror knob at the bottom of the microscope (different specimens appear best with different degrees of illumination).

6. After making sure that the object as seen through the microscope is at the center of your field of vision, switch to the next higher magnification, remembering to readjust the focus and mirror knob again. 7. When you are done, switch back to the lowest magnification and bring the fly again into focus.

The other microscope is for individual exploration--of coins, fingers, hairs, or any other object which strikes your fancy.

Writing Activity A (due Week 3)

1. Describe the fly dance and its sequel.

2. What, do you think, is the meaning of this dance? Does it, in your opinion, serve any specific purpose?

3. Let's say you wanted to be sure that your explanation of the meaning of this dance is correct. Can you think of a way to test this explanation? Note: To answer this question, don't conduct observations or experiments; instead, describe in writing steps which might test your interpretation.

4. Does each fly have its own dance, or is the dance of all flies of this species essentially identical? (You can answer this question by recording and comparing the dances of different flies in the same vial, of flies in the two consecutive trials of your group, and of flies observed by other groups in this class).

5. Can you describe the appearance of these flies? Do you see their three main body parts? Can you describe their eyes, antennae, wings, legs? Can you draw a sketch of a single fly? What's your opinion now of flies: Are they ugly or beautiful?

Part III: Life Cycle of Fruit Flies (Distributed on Week 1; performed at home on Weeks 2-4)

This part of the exercise will be conducted at your home for the next 3 weeks. Your task is figuring out on your own the developmental stages of fruit flies.

At home temperatures and conditions, the food may become too dry, causing the flies to starve to death (they don't chew their food, but imbibe it). Hence, they must be placed in a humid environment. Pour a small amount of water (one inch high or so) in a glass jar, place the vial right-side-up inside the jar, and loosely cover the jar with a lid (if you close the lid tight, the flies may suffocate). Instead of a glass jar, you can use a milk carton or any other container.

Two to four days after you brought them home, release all the adults outdoors and plug the vial. Then, every two days, remove the vial from its container and observe what's going on inside it. Three weeks after you took the vial home, release all remaining flies, thoroughly wash the vial, and bring it back.

Writing Activity B (due on Week 5)

At the end of the three-week period, describe in writing the life cycle of fruit flies.

Part IV: An Experimental Proposal (distributed on Week 3)

If you haven't figured this out already, in Part I of this exercise you observed courtship behavior of fruit flies. One vial contained receptive females (3- to 10-days-old virgins); the other, sexually-starved, mature males (6- to 13-days-old virgins). As in many other animal species, in fruit flies foreplay often precedes copulation. When an amorous male encounters a female, he becomes visibly excited. In rapid succession, he intermittently faces her, taps her with his forelegs, follows her, or runs from one side of her body to the other. At the same time, he may stick out and vibrate the wing closest to her. Now and then, he touches the rearmost portion of her body with his head. He tries to mount her while bending the back part of his body. If she permits copulation, the two may remain linked for a few minutes, after which time they disengage. If she rejects him, he either keeps chasing her until he either succeeds, tires, loses contact, or loses out to another male.

Figure 1: Drosophila Courtship Behavior

Top Row:  Orientation, Following, Single-Wing Vibration

Bottom Row: Attempted Copulation, Copulation

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But how can we be sure that this is a mating dance? To begin with, we know from other species, including our own, that males often court females and that mating often involves physical union. You may have noticed, as well, some physical differences between dancing and non-dancing flies. We can also compare mixed and unmixed vials. In vials where the presumed mating dance took place, you detected at home the production of offspring. In contrast, when no dancing was observed (as was the case in the few vials in which no mixing of flies took place), no progeny were observed.

To the curious observer, all this raises myriad questions. Which muscles and nerves are involved in the courtship display? Why don't females court males? How does a male recognize a female? By sight? By smell? By her shape?

At this point, we would like to focus your attention on just one scientific puzzle: Where does courtship behavior in fruit flies spring from? A little reflection suggests that there are three possible guesses (or hypotheses) about the origins of this uniform behavioral pattern:

On the face of it, none of these hypotheses appears far-fetched. Children learn to write, they instinctively recoil from electrical shocks, and they combine innate linguistic abilities and personal experiences to master the art of speaking.

Writing Activity C (due on Week 4)

1. Propose in writing a test which will help decide if the dance you saw is learned, instinctive, or interactive. 2. Write down, as well, what the possible outcomes of this test might be. 3. Explain how each outcome supports one of the three possibilities and discredits the others.

Part V: A Scientific Experiment (Distributed and performed on Weeks 5-7)

Today we begin investigating whether courtship behavior in flies is wholly learned, instinctive, or the product of both. Note that, because the larvae you will encounter in class have never come in contact with adults, they could not learn the courtship ritual.

Following your instructor's example, place instant fly food and water in three small vials. Use a swab to gently pick one mature larva (a little white worm-like creature; plural: larvae) from the side of a bottle and place it on the food layer at the bottom of one vial. Pick two other larvae and similarly place them in the other two vials. Plug the three vials, take them home, and deposit them in a secure, shady, warm area. As before (consult Part III, paragraph 2) keep them in a humid environment. Keep the walls of each vial wrapped with paper or aluminum foil so that the insects cannot see each other.

For the next two weeks, occasionally observe what's going on. A short time after you take the larvae home, they would crawl up the side of the vial and become pupae. The pupae would gradually darken, undergo metamorphosis, and emerge, in about 6-8 days, as adult flies.

Two weeks after you had taken the three vials home, please bring them to class.

Part VI: The Crucial Test (distributed on Week 5 and performed on Week 7)

Team up in groups of fours. With your instructor's help, or with the help of a magnifying glass and the figure below, determine the sex of your flies. Note in particular that (1) the female is larger than the male, (2) her body curves to a point at the rear end while his is more rounded, and (3) she has separate black bands on her rear back side while his are fused together.

Figure 2: Male (left) and Female (right) Drosophila


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Wait until all groups have sexed their flies before proceeding to the next step. If one group does not have enough flies, or if one group has too many flies of one sex and too few of the other, we shall consolidate groups or exchange flies.

Now bring one male and one female together in a single vial (consult Part I, paragraph 6). Similarly combine other pairs. Then, while keeping the vials stationary, carefully record the flies' behavior.

Writing Activity D (due on Week 8)

The point of this lab exercise is to rely on your own personal observations and deductions, so please don't consult the pertinent literature. Instead, answer the two questions below only on the basis of class discussions, your observations, and the observations of fellow students.

1. What does your experiment disclose about courtship behavior in fruit flies? Is it instinctive, learned, or interactive? Please explain your answer.

2. Your investigations of the nature and origins of courtship behavior is typical of science as a whole. What do they tell you about the process of scientific discovery? What do they tell you about the role in scientific inquiries of observations, of imaginative guesses as to what the causes of these observations might be, and of experimental tests of these guesses?

Part VII: An Overview (distributed on Week 9)

The Nature/Nurture Controversy. Scientists today still wonder: Which part of our behavior is traceable to our genetic make-up and which to our environment? In humans the subject is controversial, in part because of its explosive social implications, in part because it is often difficult to separate hereditary and environmental influences. Scientists fall roughly into three camps: those who attribute most of our behavior to our genetic make-up; those who trace most of it to experiences and culture; and those who take an intermediate position, ascribing some of our behavior to heredity, some to environment, and most of it to the ongoing interaction between the two.

When faced with such complex problems, scientists often try to study them in simpler systems. Flies, as you probably suspect by now, are not educable. Virtually their entire behavioral repertoire is programmed into them from birth. Insects need not, therefore, spend valuable time learning how to cope with their environment; unlike robins, blowflies require no flying lessons.

Insects obviously are well-adapted to their environment, as humans are daily reminded when they try to eradicate cockroaches, fleas, or cucumber beetles. But built-in behavior has its shortcomings too, even its comical side. A famous Frenchman once studied the life cycle and behavior of a wasp species. After mating, this species digs a tunnel in sandy soils, flies away, catches a prey caterpillar and anesthetizes it, carries it back to the tunnel, puts it near the opening, goes into the tunnel to "check" that everything is OK, comes out, drags the caterpillar down to the end of the tunnel, injects an egg into the caterpillar, then flies away, never to return. From the egg a wasp larva hatches, gradually eats up the caterpillar, turns in due course into a cocoon, metamorphoses, and emerges as a full-fledged sand wasp. If it's a female, she now flies away and either hibernates over the winter or immediately repeats the previous cycle of mating, digging tunnels, catching caterpillars . . .

Common sense suggests that this behavior is not learned. These wasps mature alone in the tunnel; everything they do seems to be accomplished with no one's instructions. Like the shape of their wings, or like the color of your eyes, their behavior is genetically determined.

This conclusion receives further confirmation from experimental interventions. While the wasp was going in to "check" the tunnel, the naughty Frenchman removed the caterpillar a short distance away. Emerging from the tunnel, the wasp brought the caterpillar back to the same place, then proceeded to go down the tunnel once more. The Frenchman removed it again, and the wasp, upon re-emerging from the tunnel, brought it back, once more going to "check" the tunnel. This process continued many times, until the wasp finally gave up and took the caterpillar straight in, or until our naturalist gave up and let the caterpillar be (I forgot which). All this is reminiscent of a broken computer loop; you short-circuit the loop and get stereotypic, maladjusted behavior.

You have reached similar conclusions about your former housemate, Drosophila melanogaster. All flies were reared in isolation, with no opportunity whatsoever of learning anything from anyone, yet most males performed the same mating dance. Learning, therefore, seems to play no part in this behavior.

Other lines of evidence boost the belief that this behavior is programmed from birth. For instance, geneticists can create fruit flies which appear and behave like males, but which have a patch of female integument (=insect skin). Because it is the smell coming from the integument which triggers the mating dance, normal males dance when they encounter such laboratory creations. In this case, although coitus is physically impossible, males go on "courting" such patched-up creatures for hours, day in and day out, like a broken computer. This shows that this behavior can't be unlearned or modified by a process of trial and error, thereby confirming its rigid, probably innate, nature.

Reflections on the Scientific Method: Our fly escapades illustrate the nature of science. Scientists often begin with simple observations. They note, for example, that upon encountering a female fly, a sexually-starved male follows, vibrates one wing at a time, etc. They ask: Where does this behavioral sequence come from? Knowing that human courtship behavior is determined in part by culture (e.g., serenades in Mexico, dinner for two in Canada), they may begin by surmising that the mating dance is, in part, learned. Ingeniously and methodically, they subject this hypothesis to experimental tests. If their observations refute one hypothesis, they switch to another. If they are lucky, they come up with a tentative answer to their original question. But then they begin to wonder about a related puzzle, e.g., is spider courtship instinctive too? Why does a recently fertilized female reject courtship overtures? How can a male distinguish a female from a male? They publish their results. Other people all over the world may check these results, build upon them, teach them to others, and raise more questions. Gradually, researchers build up the biological sub-discipline of ethology (=animal behavior).

At times, scientific investigations enhance humanity's control of nature. For instance, we can apply our understanding of the sexual habits of insects to the eradication of gypsy moths and to the cultivation of silk moths.

It takes intellectual effort to understand scientific terms, observations, and theories. It takes years to gain expertise in a scientific discipline. But these hurdles should not blind us to the enjoyable, commonsense quality of science, the essence of which you have just experienced first hand.

End of Exercise

Instructors' Background Notes and References (Don't give this part to your students!)

The science education community is troubled by the post-instructional persistence of naive misconceptions. Students may take, for instance, a course in astronomy, do well on tests, and yet continue to believe that lunar phases are caused by Earth's shadow. Educators are concerned, as well, with widespread misconceptions about science itself, especially among high school students and nonscience majors in college. These students often believe that science is dull, irrelevant, incomprehensible, authoritarian, or purely descriptive.

On the face of it, the guided discovery approach seems capable of facilitating conceptual change, exciting students' interest, overcoming their negative attitudes towards science, and providing a meaningful, hands-on, experience with the process of scientific discovery. Surprisingly, although this approach has been championed for centuries (DeBoer 1991; Matthews 1991), it has never taken a firm hold in the science curriculum. Here we can set aside inertia (Nissani & Hoefler-Nissani 1992; Tobias 1991) and other possible reasons for this failure, addressing instead just one: the unavailability of ready-to-wear versions of guided-discovery texts and laboratory manuals (Schoenfeld 1991; see also, National Research Council 1982).

My colleagues and I have described elsewhere a few activities which satisfy the ready wearability requirement (Nissani 1989a, b, 1994; Nissani, Maier, & Shifrin 1994). Here I would like to sketch the longest and most complex, but also most engaging, activity in our program. Because the relevant, self-explanatory, student manual is reproduced below, only a few introductory remarks are called for.

This longitudinal activity can be readily implemented in introductory biology and nature of science classes, costs little, does not require a laboratory, requires no background knowledge on the student's part and only a modicum of preparation and technical know-how on the instructor's part, captivates students' interest, allows students to share their experiences and data with classmates, provides an excellent illustration of the process of scientific inquiry, and is remembered for years or lifetimes. It fleshes out abstract lectures about life cycles, insect morphology, patterns and causes of animal behavior, and, above all, the nature of science. Instead of moving from one experimental organism to another, students get comfortable with just one, observing it over a few generations, taking it home and sharing their experiences with friends and family. Some students, one is almost tempted to say, grow attached to the little bugs.

To lend further confirmation to these intuitive impressions, a more formal questionnaire was given to students in our two fall 1994 classes. Data for all 23 respondents are given in Table 1.

Table 1. Student Survey (N = 23)

  Yes No Undecided No Answer
As a result of performing this experiment, do you have a better . . .        
Attitude towards science than before? 22   1  
Understanding of the life cycle of insects (egg, larva, pupa, adult)? 22   1  
Understanding of insect morphology (how insects look like)? 22 1    
Understanding of the nature of science (how scientists conduct experiments; the kinds of questions they ask)?  




Understanding of insect behavior? 21 1   1
Understanding of instincts, learned behavior, and the difference between them?  


Chance of reading newspaper articles about such topics as insects, mating behavior, and instincts?  








Students were also invited to evaluate this activity in their own words. Again, with one exception, their views were positive. A few examples: "I really enjoyed this experiment (except for looking at the larvae, I have a phobia). It taught me about insects and their behavior. I feel everyone should partake in this experiment." "I shared the experiments with my teenaged daughters. They learned something and enjoyed them too." "I personally enjoyed the home experiments. I had opportunity to explain them to my family & friends who also became involved. This type of experiment should be utilized more in other classes." "I feel that the experiment . . . helped me understand science a little better than I did before." The one negative comment was: "I had no interest in it whatsoever."

Its charms notwithstanding, this activity has its fair share of drawbacks. Instructors must know how to handle and sex flies (cf. Hall 1994; Nissani 1975; Strickberger 1962). Fly cultures must be started a couple of months before the first session. For large classes, the isolation of virgins is tedious, so a lab assistant may be essential. Other technical problems stem from the guided discovery nature of this activity and from its multifaceted nature. Some parts of the manual must be handed in class throughout the term because they contain answers to earlier questions. Owing to the complexity of this activity, students and instructors must adhere to a written schedule (which can be most conveniently incorporated in the overall class schedule). On rare occasions, students may know the answers beforehand, or they find them by consulting friends or journals, instead of struggling with the material on their own. These problems notwithstanding, this activity has been used for six years by six different instructors to teach nonscience majors in our program, with uniformly excellent results. Moreover, our choice of living material and topic is traceable to nothing more than the appeal of this particular topic, the contents of the course which accompanies this activity, and our professional backgrounds. Because this approach worked so well, we have already extended it to atmospheric pressure (Nissani, Maier, & Shifrin 1994) and to a causal analysis of moon phases (Nissani 1995). Depending on course contents and on instructor's background and aims, countless other variations could be devised.


This activity could not have been developed without the help and encouragement of Jerry Bails, Beth Dover, Donna Nissani, Ethan Nissani, Marsha Richmond, Veronica Riha, Norma Shifrin, and, above all, hundreds of students in our program.


DeBoer, G. (1991). A History of Ideas in Science Education, New York: Teachers College.

Hall, J. C. (1994). The mating of a fly. Science, 264, 1702-1714.

Koestler, A. (1964). Act of Creation. London: Hutchinson, pp. 265-266.

Matthews, M. R. (Ed). (1991). History, Philosophy, and Science Teaching: Selected Readings. Toronto: Oise.

National Research Council. (1982). Science for Non-Specialists: The College Years. Washington: National Academy Press, p. 63.

Nissani, M. (1975). A new behavioral bioassay for an analysis of sexual attraction and pheromones in insects (GO TO ARTICLE). Journal of Experimental Zoology, 192, 271-275 .

Nissani, M. (1989a). A hands-on instructional approach to the conceptual shift aspect of scientific discovery. Journal of College Science Teaching 19, 105-107.

Nissani, M. (1989b). A class exercise for teaching the genetic code. The Science Teacher, 56 (3), 76-78 .

Nissani, M. (1995). Phases of the moon: a guided discovery activity for clarifying the nature of science. Science Activities, in press.

Nissani, M. & Hoefler-Nissani, D. M. (1992). Experimental studies of belief-dependence of observations and of resistance to conceptual change. Cognition and Instruction, 9: 97-111 .

Nissani, M., Maier, C. L. & Shifrin, N. (1994). A guided discovery exercise for introductory physics labs (GO TO ARTICLE). The Physics Teacher, 32, 104-107.

Schoenfeld, A. H. (1991). On mathematics as sense-making. In J. F. Voss et al. Informal Reasoning and Education, p. 341. Hillsdale, NJ: Erlbaum.

Strickberger, M. (1962). Experiments in Genetics with Drosophila. New York: Wiley.

Tobias, S. (1992). Revitalizing Undergraduate Science. Tucson: Research Corporation.

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