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Break Out: What Happens to the Exoskeleton of a Growing Insect?

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Author: Janice VanCleave

Problem

What happens to the exoskeleton of a growing insect?

Materials

  • 12-inch (30-cm) round balloon
  • Spring-type clothespin
  • 1 tablespoon (15 ml) school glue
  • 1 tablespoon (15 ml) tap water
  • Small bowl
  • Spoon
  • 35 to 40 newspaper strips, each about 2 × 4 inches (5 × 10 cm)

Procedure

  1. Blow up the balloon to about the size of a grapefruit.
  2. Twist the end closed and secure it with the clothespin.
  3. Put the glue and the water in the bowl and mix them by stirring with the spoon.
  4. Dip a newspaper strip in the glue mixture and stick the strip on the balloon.
  5. Repeat this with a second paper strip, placing it on the balloon so that it overlaps half of the first paper strip.
  6. Continue adding strips, overlapping each one until most of the balloon is covered with paper. Leave a narrow strip around the balloon uncovered.
  7. Wait an hour or so until the paper dries.
  8. Once the paper is dry, remove the clothespin without letting the air out. Gently blow into the balloon, making it slightly larger.

Break Out

Results

A firm paper shell covers the balloon. When you blow up the balloon even more, the uncovered section of the paper shell separates.

Why?

The balloon and hard paper shell represent an insect during a growing stage. The body of the growing insect is covered by an exoskeleton (external skeleton). This exoskeleton does not grow with the insect. As the rest of the insect's body grows, the exoskeleton starts to become too small. A new exoskeleton begins to form under the old one.

When the old exoskeleton becomes too small for the growing insect, blood and sometimes air or water inside the insect is forced into the thorax by the contraction (squeezing together) of muscles in the abdomen. This splits the exoskeleton, usually along the middle of the back side, and the insect crawls out. This process of shedding the exoskeleton is called molting. Most insects molt four to eight times during their lives, but they typically do not molt or continue to grow in size once they reach the adult stage.

Let's Explore

When the insect first crawls out of its old exoskeleton, its new exoskeleton is still moist and flexible like the wet paper strips on the balloon. The insect gulps in air or water to expand the flexible exoskeleton before it dries and hardens like the paper strips. The dried, stretched exoskeleton provides growing space until the next molt.

Demonstrate the stretching of an insect's exoskeleton by repeating the experiment. While the glue is still wet, stand in front of a mirror and observe the paper covering as you slightly blow up the balloon. While the exoskeleton model is enlarged, twist the open end of the balloon and secure it with a clothespin. Allow the paper to dry, then let a little air out of the balloon. Science Fair Hint: Display diagrams showing the changes in the paper model of an exoskeleton.

Break Out

Show Time!

Mealworms are darkling beetles in a growing stage. Purchase a container of mealworms from a pet store. NOTE: You will need to plan what to do with your mealworms when you are through experimenting. See page iv (dedication page), "Handling Insects and Spiders. " Spread the contents of the container on a paper towel and look for exoskeletons that have been discarded by molting mealworms.

Sort the mealworms by length, placing them in three piles: short, medium, and long. Use three plastic containers with lids, such as empty, clean, 4-cup (1.36-kg) margarine containers, to house the three groups of mealworms. Fill each container about half full with cornmeal. Place a potato slice and mealworms on the surface of the cornmeal. Fold a paper towel in half twice and place it over the potato slice, mealworms, and cornmeal. Secure the lids, then make 10 to 15 airholes in each lid with the point of a pencil. Label the containers Small, Medium, and Large.

Place the containers on a tray. Set the tray in an area at room temperature and out of direct light. Observe the contents of each container every 2 to 3 days for 3 to 4 weeks. Measure and record the average length of the mealworms in each container. Record changes in the length of the mealworms and the number of exoskeletons you find. Observe the width of the mealworms and make note of any changes. NOTE: Remove the old potato slice and add a new one each week.

Break Out

Check it Out!

Insects produce hormones, which are chemicals that control molting and other processes. The hormone that controls molting in immature insects is called ecdysone. For more information about molting hormones, see pages 29-33 of Entomology by H. Steven Dashefsky (New York: TAB Books, 1994). What does the juvenile hormone control?

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