What Do Flies Eat?

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Author: Janice VanCleave


How do flies eat?


NOTE: This experiment requires a refrigerator.

  • eyedropper
  • jar of sweet-potato baby food
  • craft stick
  • masking tape
  • pen




  1. Place the tip of the eyedropper just below the surface of the potatoes in the jar. Try to fill the eyedropper with the sweet potatoes. Observe the amount of sweet potatoes that enter the eyedropper, if any.
  2. Wash the eyedropper and allow it to dry.
  3. Collect as much saliva in your mouth as possible, put the saliva on the craft stick, and transfer the saliva to the surface of the potatoes in the jar. Close the jar.
  4. Place a piece of tape across the lid and down the sides of the jar. label the tape DO NOT EAT.
  5. Place the jar in the refrigerator and leave it undisturbed for 1 day.
  6. After 24 hours, remove the jar from the refrigerator and repeat step 1.


On your first try, you can draw little or no potatoes into the eyedropper. After the saliva has been in the jar for 24 hours, the potatoes at the surface are liquid. You can then easily draw them into the eyedropper.


Human saliva, like the saliva of flies and many other insects, contains a chemical called amylase. Amylase breaks down starch, a complex chemical found in many foods, into less complex chemicals. When humans eat food containing starch, amylase in the saliva begins to digest (break down into useable forms) the starch in the food. In the experiment, the amylase in your saliva digested the potatoes, turning them to liquid. The digesting process started as soon as the saliva touched the food, but it took 24 hours for enough liquid to form for you to be able to draw it into the eyedropper.

As you did in the experiment, flies drop saliva on the food they plan to eat. The amylase in the fly saliva quickly begins to digest the starch in the food. The fly dabs at the liquefied food with the end of its tubelike mouthpart called a proboscis. The spongy end of the proboscis soaks up the liquid. The liquid food then moves through the proboscis into the insect's digestive system, where the food is further broken down and the nourishing parts are absorbed by the body.

Let's Explore!

Potatoes, rice, and pasta are all starchy foods. Can amylase in fly saliva dissolve nonstarchy foods? Repeat the experiment using jars of nonstarchy foods, such as chicken, meat, or spinach.

Show Time!

Demonstrate the coiling and uncoiling of a proboscis with a party blower. Place the party blower upside down in your mouth so that the end hangs down and coils toward your body. Blow into the tube, then suck the air out. Have someone take photos of you with the blower coiled and uncoiled. Prepare posters comparing photos of the coiled and uncoiled party blower with drawings of a butterfly's proboscis.


Find out more about the different mouthparts of insects. Prepare a display using materials to represent each mouthpart type, such as pliers for a cricket, a coiled party blower for a butterfly, and a sponge for a fly.

  1. Butterflies and moths use the proboscis to reach the sweet nectar (sugary liquid) of flowers. The proboscis stays coiled under the head of the butterfly or moth when not in use. Blood from the insect's body is forced into the proboscis, causing the proboscis to uncoil. When uncoiled, the proboscis is used by the insect to suck up nectar.
  2. Different insects have different types of mouthparts. Observe the plierlike mouth of a cricket. Catch a cricket and place it in a plastic resealable vegetable bag with a small piece of bread and a small piece of sponge moistened with water. Use a magnifying lens to observe how the cricket eats the food. Within 60 minutes, release the cricket where you found it.

Check it Out!

Insects take food into their body through their mouthparts. Through digestion, the food is chemically changed to supply the body with nutrients (substances used by the body for growth, repair, and energy) and get rid of any waste products. Find out more about how insects digest food. What is a crop? A gizzard? For information, see pages 48-51 in Janice VanCleave's Insects (New York: Wiley, 1998).

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