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Physiology Activity
Perception and Behavior

Objective:

To study fundamental concepts such as perception and behavior using the easily obtainable house fly or the blowfly. To design an experiment to compare the fly's sensitivity to that of a human's.

Background:

Flies are easily obtainable (see appendix) and their feeding behavior is quite predictable. Sugar is attractive to flies, but just how sensitive are they to different concentrations? Flies are able to taste a substance simply by walking on it, provided the substance is moist. The fly exhibits a simple behavior - the feeding response, made obvious by the lowering of the proboscis (mouthpart). The labellae are paired, padlike structures at the end of the proboscis. They are grooved and act like a sponge to "sop" up liquids. When placed over a Petri dish of distilled water, the fly retracts its proboscis when it has had enough to drink. This activity is designed to determine the threshold concentration of sucrose - common table sugar -- to which 50% of the flies will respond by extending their mouthparts to paper discs containing known concentrations of sucrose. With its legs, the fly will hold the paper disk which has been saturated with a solution of sucrose until it is satisfied.

Materials: (for each group)

6 flies source of CO2
6 2 inch insect pins styrofoam block (to hold flies)
very sticky glue or wax forceps
distilled water Petri dish
scissors
6 triangle-shaped strips of paper cut from the corners of index cards
paper discs (made from filter paper by using a hole punch)
test solutions (different concentrations of sucrose, lemon juice, etc.)

Experimental Design:

This experiment is designed to determine the limits of the fly's ability to detect sugar, the behavioral response made, how past experience alters this response, whether the fly can be fooled, and how our own sensitivity compares to the fly's. Prepare solutions as follows:
  • tube # 3 = 10 % sucrose solution (10 g in 100 ml water)
  • tube # 2 = 5% sucrose solution (5 ml from tube # 3 + 5 ml distilled water)
  • tube #1 = 1 % sucrose solution (1 ml from tube # 3 + 9 ml distilled water)1
  • tube # 4 = a few drops of vinegar or acetic acid in 10 ml of tube #3
  • tube # 5 = a few drops of lemon juice in 10 ml of tube # 3
  • tube # 6 = a few drops of 5% NaCl in 10 ml of tube # 3

In order to test a fly's responses, it must first be "captured". The most effective way to do this is to anesthesize the flies with CO2 only long enough to keep them from escaping. After they have been exposed to the gas long enough to stop moving, carefully clip one of the wings in case they begin to wake-up too soon. Obtain 1 fly at a time from the CO2 source, place a drop of glue on the paper point, press the point onto the fly's back between its wings so that the paper point is parallel to the fly's body. Run each of the 6 pins through the broad end of a paper point and move the paper towards the blunt end of the pin until it is about 3/8 inch from the blunt end. (see diagram). Once the fly is immobilized, stick the pin into the styrofoam block so that the fly's feet are pointed downwartd. Repeat for all 6 flies.

Before beginning the actual testing procedure, clean the flies' tarsi (feet) by exposing them to a dish of distilled water. Begin the testing by using forceps to offer a distilled water-soaked disk to each fly's legs, letting them hold the disk and drink until they no longer extend their mouthparts. Repeat this procedure immediately after each solution used. Dip a paper disk into each solution for each of the 6 flies, beginning with the most dilute solution, which is contained in tube # 1. Offer a disk soaked with each test solution to all flies. Repeat the procedure using tubes in the following sequence: 2, 3, 4, 5, and finally 6. Observe the behavior of each fly based on the following responses:

  1. Mouthparts are lowered and touch the disk.
  2. Mouthparts are only partially lowered.
  3. Paper is retained, but mouthparts are not lowered.
  4. Paper is rejected.

Record the response of each fly for each solution tested. If the fly responds at level 1, remove the paper disk immediately before the fly has a chance to drink much of the fluid. Flies which respond at levels 2 or 3 should have the disks taken away after 10 sec. Always, after removal of the disk, you will want to "clean the flies' palates" by giving them a fresh disk saturated with distilled water. Rinse forceps as well. If the observer notes that the fly is no longer responsive to the solutions, it is advisable to test fresh flies. Then try to determine what will make a fly reject sugar. It is also possible to test other substances that have the same effects on the flies, such as artificial sweeteners, alcohol, caffeine, and insect repellent. Combine data collected by all groups in the class and determine the lowest concentration of sucrose that flies respond to.

Questions: (all levels)

  1. Why should a fly require a much more delicate chemical sensation than a human? How does each get its food?
  2. Does the fly taste with is feet, mouth parts, or both?
  3. Why is it important to "clean the flies' palates" and the forceps ?
  4. Did you find a concentration at which all 6 flies responded with a level-1 response? (level 2)
  5. Why might other groups arrive at different answers to these last two questions? Could you redesign the experiment to make this less likely?
  6. Was there a control in this experiment? What was the control and what variable was it supposed to remove? (level 3) Design and conduct an experiment to compare the sensitivity of flies to that of humans. Be careful to keep anything that may have contacted the flies, either directly or indirectly, from entering the mouths of humans. Any solutions to be tasted by humans should be sterilized in advance and kept sterile during use.
  7. How many human subjects did you include, and why did you choose this number?
  8. What was the mean threshold concentration of sucrose among your human subjects?
  9. Which are more sensitive to sucrose, flies or humans? Explain.
  10. How do your results compare to those of other groups?

Resources

Dini, M. and Harris, J. (1991). "Taste Reception in Flies." Animal Physiology Update Workshop Manual. LSU Baton Rouge.

Oakley, B. and Schafer, R. (1978). "Feeding Behavior: Taste Reception in Flies." Experimental Neurobiology: A Laboratory Manual. The University of Michigan Press.

Pentz, Lundy (1989). "Perception and Behavior." The Biolab Book (Second edition). The Johns Hopkins University Press. Baltimore.

Appendix:

Houseflies (Musca spp.) can be used, but blowflies (Phormia spp.) are suggested. Obtain about 40 pupae or larvae per group per laboratory period. They can be obtained from entomology departments, researchers, and biological supply houses. Order pupae which have just pupated. Place them on a thin layer of slightly moistened vermiculite in an enamel pan. Cover with more vermiculite and moisten slightly. Do not let the vermiculite dry out. The pan should be placed in a fly cage screened on four sides and one end. Tacked around the perimeter of the other should be a cloth (old piece of nylon panty hose works fine) which is long enough to twist and tie securely with a string. When you are ready to remove flies, insert your hand and trap the flies into a small bottle. Place a petri dish containing dry table sugar in the cage along with a jar of water inverted on two pieces of filter paper in another petri dish. Food and water sources are essential for the flies as soon as they emerge. It will take a period of approximately two weeks for all the flies to emerge. If the adults need to be kept longer, store them at 4 degrees C in a container covered with a plastic bag. Deprive the flies of food (but not water) for 48 hrs. at room temp. prior to testing. Testing two or more days after mounting on paper strips provides even better results. Feed the flies before placing them in cold storage (4-10 degrees C) until using them again.

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