Laboratory Materials

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Topic 6.  Proboscis response.

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(Name)    

I. MATERIALS.

            A dozen or so young housefly or blowfly adults.

            Sticks for holders such as wood applicators.

Starve young adult flies for one day prior to the class (but provide water).  The insects must be hungry for this reflex response to be demonstrated, and the flies are notoriously variable in propensity to respond (so perhaps a dozen flies should be prepared).

            A few hours before the demonstration (for instance on the morning of the afternoon class), mount the adults by gluing or waxing the holder to the top of their thorax.  Be careful not to interfere with the movement of the wings.  The stick should be mounted in such a manner that the adult fly is held over a substrate of your choice, so the adults can maintain a posture or hold something with their feet. This can be a bit of cork that turns easily as the fly “walks,” or a bit of Styrofoam the flies can grasp.

            Sugar solutions in water, either sucrose, glucose or fructose.  A series of molar concentrations should be prepared:

10^-6 M, 10^-4 M, 10^-2  M, 1.0 M, and 10 M.

            This can be done easily by making the most concentrated first, then diluting one part in 9 or 99 parts of distilled water in succession to obtain the more dilute concentrations.  Exact accuracy is not that important.

            Saline solution.

II. LEARNING OBJECTIVES.

            Each student should be able to

1. describe the reflex behaviors triggered by given sensory inputs.

How:   

            Observe and describe the housefly proboscis response to sugar solutions.

III. INTRODUCTION.

            One of the best known reflex responses in insects is that of the extension of the proboscis of insects in response to feeding stimuli, popularized by Vincent Dethier. 

IV. DIRECTIONS.

            TA: mount the starved fly and a moving wheel in plain view of the class, or use a video camera focused on the fly and project the image onto a television screen for better visibility.  The head will have to be positioned so that when the proboscis is lowered, it will be able to reach the area of the substrate in contact with the front feet. This can be down with the flies upside-down if necessary.

            Present a small piece of filter paper soaked in pure water to the front tarsi. The fly may or may not drink from the filter paper.  Repeat until the fly stops responding by lowering its proboscis. Remove the wet filter paper.

            Repeat the procedure with the lowest concentration of sugar water.

            Repeat the procedure with increasing concentrations of sugar water until you obtain a proboscis extension response.   Record the concentration which elicits the response: _________.  Now allow the fly to feed on the sugar solution until it retracts the proboscis, and begins to move around.

            After 2-5 minutes, present the same concentration of sugar water to the same fly again.  What happens?

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            Now present the same fly the next higher concentration of sugar water.  Repeat with increasing concentrations until a proboscis response is obtained and again record the concentration of the sugar solution: ____________.

            Try presenting the sugar solutions to the back tarsi.  What happens?

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            For variations, these experiments may be conducted with starved virgin adult females, 5 days after adult emergence, versus gravid (mated) adult females 5 days after emergence.  It is important to starve the adult flies properly before the experiment starts.  Never-the-less, some adults may not respond at all, no matter what the experimenter does.  In trying to explain these reluctant insects, or false negatives, Dethier suggested that insects had to have a propensity to respond to stimuli under experimental conditions, and left it go at that.

            When finished with the procedures described above, and after the fly has had a chance to feed to satisfaction from the sugar solutions, before discarding the fly, present a salt solution in the form of the saline used in other lab sessions.  Describe the reaction of the fly here:

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Where are the sensory receptors that detect the sugar solutions?             

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Which receptors are sensitive to sugar (check the references)?             

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            Additional materials that may be prepared by the TA for this topic include feeding monitors in the fly rearing containers.  besides the solid materials provided for adult flies to feed on, also provide fluids in two U-tubes with graduation marks that enable amounts taken to be documented.  Provide water and sugar solution choices.  Have the students document how much water or sugar are taken and include a series of sugar solutions.  If the sugar solutions are high enough, the adults in the containers will drink only from the sugar water and leave the pure water untouched.  It should be possible to document this over a few days.

            (This suggestion is described in more detail in the Dethier reference.)

V. SUMMARY ANALYSIS.

            Describe what you learned from this exercise.

VI. REFERENCES.

Evans, H. E. 1984. Insect Biology, Addison-Wesley, Reading, MA.  See the section on Innate Behavior, pp. 174-179, in Ch. 7, Fundamentals of Behavior.

Dethier, V. G. 1962.  To Know a Fly. McGraw-Hill, New York, NY.

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(Name)                       

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