A common type of taxis is called phototaxis, which is movement of organisms in response to light. Moths show a positive phototaxis when they flutter around lights at night. Cockroaches, on the other hand, show a negative phototaxis—they scurry into dark corners when a flashlight is shined on them. Geotaxis is the movement of living things in response to the Earth’s gravity, and chemotaxis is the movement of organisms in response to chemicals. In this experiment, you investigate phototaxis, geotaxis, and chemotaxis in fruit flies. Observing chemotaxis in this experiment should be particularly fun, because you can test whatever foods you’d like.
Some Suggested substances for chemotaxis investigation, but feel free to try your own!
- Baking soda
- Fresh fruit (some under-ripe and some over-ripe)
- Rubbing alcohol
- First, you will need to make two “choice chambers.” Using the scissors, cut the bottoms off the empty water bottles.
- Slide one bottle into the empty bottom of the other bottle.
- Secure the edges with clear packing tape.
- Place cotton balls at each end.
- Make a second choice chamber identical to the first.
- For the phototaxis experiment, tape a piece of black construction paper around one side of one of the choice chambers.
- Because you’ll want to load a small number of fruit flies into the chambers, distribute some of the flies into the extra vials first. You may also want to put your vials of fruit flies in the refrigerator for 15-30 minutes to slow them down before doing so. Here’s how:
- Uncap an empty vial and set it on the table.
- Take the vial your fruit flies shipped in and tap it on the table so that many of the adult flies fall to the bottom of the vial.
- Uncap and place the vial with fruit fly culture on top of your empty vial.
- Transfer 10-20 adult flies into the new vial. The flies might move into the new vial, but if they are very sleepy, you might have to tap them in.
- Now, it is time to put some flies into your choice chamber.
- Remove the cotton ball from one side of your chamber, hold the chamber upright, and insert the narrow end of the funnel into the opening.
- Gently tap the capped vial so the flies fall to the bottom of the vial before you open the vial’s cap.
- Carefully pour 6-20 flies into the funnel, counting them as they enter the chamber.
- Place the cotton ball back in the opening of the chamber. Set the chamber back on its side.
- Wait about twenty minutes for the flies to warm up again and start moving around.
- Count how many flies are in the light side of the chamber. Subtract this number from the total number of flies to determine how many were in the dark side of the chamber.
- Record your results.
- Remove the black paper and allow your flies to rest for a few minutes.
- Next, you will test geotaxis in the second chamber you made. Load 6-20 flies into this chamber.
- Hold this chamber vertically for ten minutes.
- After ten minutes have elapsed, count the flies at the bottom half of the chamber.
- Compare this number to the number of flies present in the top half of the chamber.
- Record your results.
- Let the flies in both of your chambers rest before you set up your chemotaxis experiment.
- If you are interested in testing dry substances like baking soda or sugar, dissolve a spoonful of the dry substance in water first.
- Place 5-10 drops of the substances you are interested in on separated cotton balls.
- Place a cotton ball soaked with one of the two substances you’re testing in one end of each choice chamber. Each of the other ends should be plugged with a cotton ball soaked in water only. Why is it a good idea to test a cotton ball soaked in water in one of your choice chambers?
- After 10 minutes, count how many fruit flies are on each side of your choice chambers.
- Record your data and think about your results.
Fruit flies show a strong negative geotaxis, which means they like to move up. Adult fruit flies generally show a positive phototaxis, meaning they move towards light, but they show some variability in this preference. During your chemotaxis experiment, you should have observed that fruit flies generally move towards the chemicals in their preferred food: rotting fruit. Sweet substances, wine, and vinegar are often popular fruit fly choices.
A lot of fruit fly behavior can be understood by thinking about how the action helps the fruit fly to better survive in its natural environment. Moving away from gravity makes sense for a flying insect. Preferring light over dark also helps the flies find their food. Many of the chemotaxes you observed can be explained by investigating what fruit flies regularly eat. Since alcohol and vinegar are common byproducts of rotting fruit, you might expect fruit flies to show a strong positive chemotaxis towards them. For your chemotaxes experiment, we asked you to use water as a control to see whether it was the water or the substances dissolved in it that the flies were attracted to.
There is so much you can do with fruit flies! Next, you should learn how to tell males from females. You could repeat your experiment and determine whether one gender likes certain substances more than the other gender does. You also could experiment with fruit fly larvae instead of adults—and if you continue to enjoy experimenting with fruit flies, you might consider doing a genetics experiment.