Drosophila Metamorphosis: Life Cycle of a Fruit Fly

Fruit flies (Drosophila melanogaster) are often used as research specimens because they are small, easily obtained, and easy to maintain. Their fast reproductive cycle makes them favorable specimens for studying the process of metamorphosis.

In this project, you will observe the general life cycle of the fruit fly. Each stage of the metamorphosis will be timed, and effects of temperature on these times will be determined. You will examine the desired environment for development as well as the percentage of males and females produced in each generation.

Flies can carry diseases. When the experiments are completed, put the jar of flies in a freezer for at least one hour. Dead flies have their wings sticking out to the sides. Discard the dead flies.

Getting Started

Purpose: To observe the life cycle of fruit flies (Drosophila melanogaster).

Materials

• Overripe banana
• l-quart (1-liter) jar
• Paper towel
• Rubber band
• Magnifying lens

Procedure

1. Peel the banana and place it into the open jar.
2. Set the jar outside.
3. Observe the jar. After several flies are seen in the jar, wait 30 minutes and wave the jar back and forth to swish the flies out.
4. Bring the jar inside.
5. Cover the mouth of the jar with the paper towel and secure the paper with the rubber band.
6. Allow the jar to stand undisturbed for 14 days.
7. Use the magnifying lens to observe the contents of the jar. Make note of all changes.
8. Keep the flies in the jar for further experimentation.

Results

Small, white, moist-looking specks on the surface of the banana change into white wormlike creatures within two days (see Figure 22.1). At the end of four or five days, objects the color and shape of wheat grain can be seen stuck to the inside wall of the jar at various heights. Flies appear in the jar in another ten days.

Why?

The metamorphosis (development of an insect by passing through several stages) of the fly involves four separate stages (see Figure 22.2). In the first stage, there is the egg, which is elliptical and has two small projections on one end. These projections enable it to float in liquid mediums. In the second stage, which occurs within 24 to 48 hours after the first, the egg develops into a larva. The larva is the white wormlike organism commonly called a maggot. By eating continually, the larva forms channels in the fruit. After about four days, the larva crawls onto the side of the jar, contracts, and becomes more elliptical in shape. At this point, it enters the third, or pupa, stage. The larva is still white; however, within hours it darkens. No motion is observed during the pupa stage, but it is during this period that legs, wings, and a head develop. About ten days later, the fourth and final stage is reached and a pale-colored, fragile fly emerges. Shortly thereafter, the fly darkens in color.

Male and female flies can be identified. Male flies are noticeably smaller and have a pointed posterior that ends in a sort of "button." The rear end of the male's abdomen has intense black coloring that extends around the sides to meet ventrally (in the front, near the bottom). Females have small, black-colored lines across the abdomen that are confined to the upper part and never meet underneath. Note: Sexes are difficult to distinguish by color until the flies are a few hours old because newly emerged flies are very pale.

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