METABOLISM IS BASIC TO life. Everything that breathes combines chemical energy stored in its tissues with oxygen contained in the atmosphere to liberate energy to grow, move and reproduce. Despite long-running efforts, biologists have so far surveyed only the basics of metabolism. Life is so diverse that discoveries await anyone who ventures carefully into these deep waters. Of course, few amateurs can grapple with the many technical and ethical complexities involved in experimenting on large or warmblooded animals. Fortunately, most of the earth's 10 million or so species are quite small and cold-blooded. Indeed insects offer myriad opportunities for amateur exploration.

It is actually quite simple to measure the metabolism of an insect. When an organism is enclosed in an air-tight container, its respiration removes oxygen molecules from the air and releases carbon dioxide. Often fewer molecules are added to the air than are removed. The resulting loss causes the pressure inside the container to fall.

That pressure drop is key to measuring metabolism, and it can easily be observed using a device called a Warburg apparatus [see illustration below]. The instrument consists of two stoppered test tubes connected by a capillary tube. A tiny droplet of oil (or soap) in the capillary tube moves in response to pressure differences between the test tubes. Therefore, as the respiration of an insect in one of the test tubes causes a decrease in pressure in that tube, the oil drop will slip toward it. You can witness this movement if you warm one test tube with your hand. That makes the air inside expand and push the droplet to the cooler test tube. To quantify the movement of the droplet, photocopy a ruler with millimeter gradations and tape the copy to the capillary tube.

As the warming with your hand demonstrates, the device is sensitive to small temperature differences between the test tubes. The best way to ensure equal temperatures is to submerge the tubes in a large basin of water. To keep them below the surface, attach them to the sides of the plastic cup. Weigh the cup down with sand, pebbles or a fistful of spare change. The cup also permits you to view the oil in the capillary tube in dry air. To reduce further the effects of temperature gradients, set the water moving slowly with a handheld body massager.

If you know the air pressure, the water temperature and the distance the oil drop moves, you can determine the number of molecules the insect respires. The box on the opposite page lists the exact relations.

The next step is to find the ratio of carbon dioxide produced to oxygen consumed. Called the respiratory quotient, it represents a fundamental measure of metabolism. It tells you what biological fuel the organism is burning. If it is converting sugar, the ratio is 1; for fat, about 0.70; for protein, about 0.80; for alcohol, about 0.67. For most creatures, the quotient ranges from 0.72 to 0.97, because organisms metabolize several kinds of energy sources simultaneously.

To measure the respiratory quotient, you will need some sodium hydroxide (NaOH), which absorbs carbon dioxide from the air. Purchase this compound m solid form from any chemical supply house; check your Yellow Pages. But watch out-sodium hydroxide is caustic and will burn skin and eyes if not handled properly. Rubber gloves and safety goggles should be worn.

Before conducting trials, you will need to clear all the carbon dioxide out of the test tubes. Place several grams of NaOH m just one test tube. The toe of a nylon stocking makes an excellent pouch to hold the chemical; ball the nylon up on the open side to prevent the insect from touching the NaOH. Measure how long it takes for the droplet to stop moving, that is, for the NaOH to remove the carbon dioxide from the air. Make sure to wait at least that long before you start each trial. You will want the system to come to equilibrium quickly, so use a lot of NaOH. (Alert readers may wonder about water vapor, which NaOH also absorbs; for technical reasons, it will not affect the measurements.)

Now you are ready to begin the experiments. First, place both the wrapped NaOH and the creature inside one test tube and just NaOH m the other. Measure how long it takes for the droplet to move at least five times the smallest distance marked on your scale. Then run a second trial for exactly the same length of time, but with only the organism. The box lists the equations needed to obtain the respiratory quotient. Note that your results will be valid only if the organism is in the same physical state in both trials (not calm in one and agitated in the other, for example), if the NaOH removes all the carbon dioxide from the air before that trial begins and if both trials are run for exactly the same length of time.

With an investment of about $100, you can collect professional-quality data suitable for publication in a research journal. You will need to buy an electronic differential pressure transducer, a device that converts pressure differences into voltages that can be measured with a voltmeter. I used a Honeywell model (No. 163PCOlD36) that registers pressure differences as small as 0.0003 percent of one atmosphere. (For more information about this sensor, call the Honeywell Corporation at 1-800537-6945.)

The power-supply circuit for the device could not be simpler. It consists of an AC-to-DC adapter wired to a type 7812 integrated-circuit chip. Its voltage drifts a bit, causing the transducer's output to wander about 10 millivolts, but that should not pose much of a problem. You will need to calibrate the transducer with a manometer-a clear, U-shaped plastic tube with some water inside [see illustration above]. I made mine out of two thin, stiff tubes I found m the garden department of a hardware store. Aquarium pet stores also stock similar kinds of tubes. I joined the pieces by inserting each into opposite ends of a six-inch-long flexible acrylic tube. The difference in height in the water column on either side of the U is a direct measure of the pressure. You can plot the output voltage against that difference, in inches (to match the standard units that pressure transducers made in the U.S. use).

The transducer has two inlets that permit easy connection to the rubber stoppers of each test tube in the Warburg apparatus. I used this setup to measure the respiratory quotient of a beetle, which I found to be 0.701, averaged over several breaths. Of course, you can also measure the metabolism of other living things: mushrooms, seeds, bread mold, to name a few.

For more information and suggestions for additional experiments, send $2 to the Society for Amateur Scientists, 4951 D Clairemont Square, Suite 179, San Diego, CA 92117, or download the information free from http://www.sas.org/ or on Scientific American's area on America Online. Special thanks are due John Lighton, professor of biology at the University of Nevada, for his assistance in preparing this column.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.