Project No. 2: Measure the Metabolism of Insects.

Metabolism is essential to life. It’s also easy to measure. When an organism breathes, it takes in oxygen and expires carbon dioxide. All living creatures consume more gas than they breathe out. This means that when a living thing is sealed inside an airtight jar, more and more of the gas gets used up over time and the pressure inside the jar drops. By monitoring the pressure as it falls, you can also monitor the life processes of whatever is inside the jar.

What You Need

Empty baby-food jar

Silicone aquarium sealant (You can find this at pet stores and petco.com)

Narrow bending drinking straw (no more than 1/2 centimeter wide)

Metal nut, through which the drinking straw can just barely fit

Small piece of cotton

Several insects of the same species (Make sure they’re small enough to fit inside the jar.)

Glass of dark-colored water (Use food coloring to stain it.)

Petroleum jelly

Ruler

What You Do
Step 1: Drill or punch a hole through the center of the lid of the baby-food jar. The hole should be just large enough to hold the drinking straw snugly.

Step 2: Install the straw.
Using ample aquarium sealant, attach the nut over the hole in the top of the lid. Poke the bendy end of the straw down through the nut and hole so that the end of the straw penetrates the lid by about an inch. Run a thick bead of aquarium sealant around the joint between the straw and the nut and gently push the straw in a little deeper (so that the cement is drawn into the nut and makes an airtight seal between the nut and the straw). Apply a dollop of sealant on the underside of the lid, around the straw. Let the sealant set overnight.

Step 3: Plug the end of the straw that will be inside the jar very loosely with a small twist of cotton. (You want to prevent the insects from escaping, yet still allow air to easily pass through the straw.)

Step 4: Collect several insects for the experiment. Any species will work — ants, beatles, sow bugs — as long as you can find more than one of them. Place them in the jar. Note: This experiment also works with plants, fungi, and invertebrates — as long as they’re small enough to fit inside the jar.

Step 5: Create a “liquid plug.” Dip the long end of the straw in the glass of colored water. If you stop up the other end with your finger as you withdraw the straw, a droplet of colored water (a “liquid plug”) will remain inside.

Step 6: Seal the bottle. Bend the straw at a right angle so that the long part of the straw is parallel with the lid. Tightly screw the lid onto the jar, making sure the liquid plug remains in the straw. Apply a thick layer of petroleum jelly to the area where the lid meets the jar to ensure the lid is airtight.

Step 7: Measure the rate at which the liquid plug moves along the straw. As the creatures breathe, the pressure inside the jar will drop and the liquid plug will slowly creep inward. The faster the liquid plug moves, the greater the metabolism of what’s inside. (Need to say how metabolism is measured? Is there a rate? The answer: millimeters along the straw per minute.)