INSECTS ARE NOT AS STUPID as people might think. Studies have shown that several species can learn through association. This ability is crucial for many insects to survive in an environment filled with various chemical scents and visual cues. Parasitic wasps, for instance, locate their caterpillar hosts by learning odors that suggest the presence of the hosts. Such odors come from their hosts' feces and from the plants on which the caterpillars feed [see "How Parasitic Wasps Find Their Hosts'" by James H. Tumlinson, W. Joe Lewis and Louise E. M. Vet; page 100].

You can see how insects can be taught to respond to an odor by raising some fruit flies on various food sources and testing them in a wind tunnel. The tunnel isolates the insects in a given area and provides the odor plume that the flies will use to locate food. The setup described here is similar to those used by researchers to investigate associative learning in wasps. Most of the material can be purchased at a hardware store.

Rearing the insects is the easiest part Of these experiments. Anyone who has purchased fruit and let it sit on the kitchen counter has essentially "raised" the common fruit fly (Drosophila). To establish a number of Drosophila colonies, place a piece of fruit in the bottom of a clear jar. I used peeled bananas, but peaches, apples or oranges would also be good choices (use only one type of fruit). The jar should not be too small; otherwise you may end up with Drosophila that have developed poorly and cannot fly well.

Place the jar outside and watch the fruit for visiting adult insects. Many different insect species will come to the fruit, but they will not affect the project. If you have the patience, you can catch sight of the female Drosophila as she lays her eggs on the fruit. After two to three days, bring the jar inside. Place a double layer of cheesecloth over the jar opening and secure it with a rubber band. To keep the atmosphere inside the jar moist, it may be necessary to put a wet paper towel over the cheesecloth. Never leave standing water in the jar.

Depending on the room temperature, the eggs will hatch in about 21 hours. The larvae will not be visible for at least three to four days. On about the fifth day, they will crawl out of the fruit and up the walls of the jar. There they will pupate. The adults will emerge in about five days and will mate and lay their eggs in the fruit. In the experiments, I achieved the best results if the adult flies were at least four to five days old.

You will need to construct an aspirator to suck the insects out of the jar, because Drosophila are too small and fragile simply to pick up. To build the aspirator, you will need two copper pipes, about 1.5 inches long and about one-quarter inch in diameter, and 30 to 40 inches of flexible, plastic tubing, such as Tygon tubing. The pipes should plug tightly into the tubing; you may have to spend a few minutes at the hardware store mixing and matching. You will also need a clear plastic 3 5-millimeterfilm canister or something similar.

Puncture two holes in the lid of the canister and insert a copper pipe through each hole. Stuff some cotton in one of the pipes; do not pack it in tightly. Cut the Tygon tubing in half and attach one onto each piece of copper pipe. Snap the cover back on the canister. To check for leaks, place the tube connected to the cotton-plugged pipe in your mouth and hold your finger over the end of the other piece of tubing. You should be able to feel a vacuum when you suck. If you do not, there must be a leak. Make sure that the cap is on tight and that no air is getting in around the copper-tubing connections. Leaks can be plugged with modeling clay.

Now you can use the aspirator to collect the fruit flies. Insert the tube without the cotton plug into the fruit fly jar and suck on the other tube. The flies will end up in the film canister. The cotton plug will keep the flies from going up the tube into your mouth. There is no harm, however, in swallowing them. Most of us have eaten a bug or two before anyway. If your aspirator does not work properly, check to see if the cotton is packed too snugly or if there are leaks. You may need to reduce the length of the tubing or to adjust the diameter. Sucking harder is not likely to be the answer.

To test the flies, I constructed a wind tunnel 49 inches long, 24 inches high and 30 inches wide. These dimensions were chosen because I used a pre-cut material called Melamine. It is a kind of veneered particleboard often used in shelving. Melamine comes in white, which makes the insects more visible. Its smooth, polished surface also minimizes wind turbulence.

Secure three pieces of Melamine to form the sides and bottom. Two one-inch by two-inch pieces of wood were used for the support across the ends. I purchased a piece of Plexiglas of approximately the same dimensions for the top of the wind tunnel. At one end I pulled two layers of outdoor screening tightly across the frame to straighten the wind path and reduce its speed. At the other end I secured cheesecloth, leaving the bottom free so that I could easily reach into the tunnel to add the insects. (Make sure the cheesecloth is long enough to tuck under the bottom.)

A large standard box fan created the wind. I suggest setting it on its lowest speed and positioning it approximately four feet from the screened end. There should be just a slight breeze inside the tunnel. Remember, you are flying fruit flies, not jumbo jets.

For accurate results, an even and bright luminescence over the entire length of the tunnel is necessary. One long fluorescent light strip or two rather large and bright floor lamps placed on opposite sides and on opposite ends of the tunnel will do. Background odors may also present complications. I noticed that if I tried to fly the insects after cooking an odorous dinner, the flies did not seem to be as efficient at finding the fruit odors as they did when I went out to eat. Either they did not like my cooking or they were distracted by the extraneous odors. (I hope it was the latter.)

The fruit needs to be sitting on some kind of platform about 10 inches above the tunnel floor and about three to six inches from the screen. I found that inverted narrow drinking glasses worked nicely. Place two of them in the tunnel, each about two inches from the sides.

To see if your wind tunnel is working properly, you will need to "test fly" a few of your insects. After capturing the flies in the film canister, give them a few minutes to settle down. Detach both pieces of tubing from the canister. Set the canister upright in the tunnel on a platform elevated about six inches above the tunnel floor. Touch your finger to the fruit and then dab your finger on the copper pipe that does not have the cotton in it. The insects will crawl up this pipe. This process helps them orient to the odors. Observe how the flies "taste" the material you have dabbed on the pipe. If the insects cannot crawl up the pipe, the pipe is probably inserted too deeply into the canister. Pull it out so that the bottom of the pipe is almost even with the canister cover.

Once the insects have crawled out of the canister, they will usually walk around the top of the pipe and maybe down the sides. When an individual does "decide" to fly to the fruit odor, you will see that its flight pattern is initially a casting from side to side. As it moves closer to the source of the odor, it narrows in on the fruit and finally lands. Difficulties in landing may be a consequence of turbulences on the downwind side of the fruit. Try using a narrower drinking glass.

Once you are satisfied that your flies can function in the wind tunnel properly, you can begin the experiments. Place a piece of the fruit on which the flies had been raised (I used banana) on one of the drinking glasses and a piece of foam or some other inert material on the other. The foam should be about the same size and color as the fruit. To which do the insects fly? Then reverse the positions of the foam and fruit. Is there any difference? Flies might prefer one side of the tunnel over the other because of factors such as lighting, wind speed or turbulence; you may need to adjust the lighting, the placement of the fan or the size of the fruit platform.

Next, repeat the experiment but replace the foam with a different piece of fruit-say, peach. In some instances, I observed that if a banana-raised insect began to fly into the banana-odor plume but somehow "lost" the odor and then detected the peach plume, it did not land immediately on the peach but continued to search, casting back and forth in a larger arch. Often it was able to relocate the banana odor and then landed on the correct fruit. I also observed that when a banana-raised fly "accidentally" landed on the peach it usually did not stay long. It began searching again.

For a third set of choices, I offered a peach and an apple to a banana-raised fly. In this case, the number of flies that landed on each fruit were all about equal. The searching process seemed to take longer than when one of the choices was a familiar one. You could also see how the flies respond to "neutral" odors such as vanilla or almond.

Successful results will depend on the care taken during construction of the wind tunnel, the operating conditions such as wind speed and luminescence as well as the health of the insects. Perhaps the most frustrating problem you might encounter is when the insects do not fly at all. There are several possible reasons. The size of the tunnel plays a role; I tested larger and smaller sizes, and neither extreme works well. The flies might be too young; try waiting a few days. Like most scientific experiments, patience and a little imagination will pay off.

Bibliography

THE INSTINCT TO LEARN. J. L. Gould and C. Grant-Gould in Science 81, Vol. 12, No. 4, pages 44-50; May 1981.

INDUCTION OF HOST PREFERENCE IN DROSOPHILA MELANOGASTER. J. Jaenike in Oecologia, Vol. 58, No. 3, pages 320-325; June 29,1983.

THE BIOLOGY OF LEARNING. P. MarIer and H. S. Terrace. Springer-Verlag, 1984.

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