What makes me tick…tock?
Lesson 2: Why do scientists study fruit flies to understand what makes us tick?
June 2012
Drosophila 4-1-1
When males and females are combined in a single vial, they will mate
within 1-10 minutes. The females will lay their eggs within 24 hours after
the original mating takes place and will be done laying eggs at around 48
hours after mating.
Egg Stage:
This is the first stage in the lifecycle of Drosophila. The eggs are small,
white and oval-shaped. They can be seen with the naked eye on the
surface of the blue food, but visualizing them does take practice. About a
day after the eggs are laid, the larvae will hatch.
Larval Stage:
As Figure 2 shows, the larvae look like white
worms. They eat almost continuously and
make tunnels throughout the food medium as they so. Their black mouths
can be seen moving on the side of the vials as they eat the medium (mixture
of water, yeast, and a formula for instant media that includes blue dye). The
larva sheds its skin three times in order to accommodate its increase in size.
Each shed marks the transition to the next instar (as shown in Figure 1).
Together the egg and larval stages take about eight days.
Figure 1: The stages of the fruit fly
lifecycle are shown.
Pupal Stage:
When it is time for the larva to become a pupa, it typically
Figure 2: Shown are larvae with clearly
travels up the side of the vial and away from the food. Then
visible black mouths.
the outer larva “skin” begins to darken and get harder. This
forms what is called the pupal case (Figure 3). In this casing,
the last stages of metamorphosis occur until it is time for the adult fly to emerge. The
entire pupal stage lasts about six days.
Figure 3: A young
adult, newly
emerged from the
pupal case.
Adult Stage:
Drosophila emerge from the pupal case and initially lack pigmentation in their bodies
(Figure 3). Their wings are a shade of purple and are not fully expanded so they appear
crinkled up. After a few hours, their wings will fully expand and their bodies will begin to
darken as their exoskeleton hardens (Figure 4). Adult flies should easily live for two
weeks, and many can live a month or more before
they die.
The time of development for Drosophila melanogaster is
dependent on many factors, such as the lifecycle may not
continue exactly as described above. One factor affecting the
length of time in each stage is temperature. Typically fruit flies
that are kept at room temperature will take 10-12 days to
complete their life cycle. If classroom temperatures are above
room temperature, the stages will progress more quickly.
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Figure 4: Adult fruit fly with developed
wings and darkened body
What makes me tick…tock?
Lesson 2: Why do scientists study fruit flies to understand what makes us tick?
June 2012
After the fruit fly has hatched, it is possible to distinguish between males and females by using a
microscope to examine the genital organs. Males have dark bristles that surround their genitals,
whereas females do not. In Figure 5 below, the fruit fly on the left is a female and the one on the right is
a male. By looking at this image, you will notice that the posterior end of the male fruit fly is much
darker than the posterior end of the female. It is important to know the sex of the flies in crossing
experiment.
Figure 5: Comparison of female fruit fly (left) and
male fruit fly (right)
Images taken from:
http://www.greenharvest.com.au/pestcontrol/fruit_fly_info.html
http://farm4.static.flickr.com/3235/2993342324_3066810afb.jpg
http://www.scq.ubc.ca/wp-content/white_drosophila.gif
http://www.backyardecosystem.com/tag/urban-composting/
http://berkeley.edu/news/media/releases/2002/07/03_paras.html
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What makes me tick…tock?
Lesson 2: Why do scientists study fruit flies to understand what makes us tick?
June 2012
Model Organism Reading Discussion
1. Why do scientists prefer to use one type of model organism over another? Give an example from the
readings.
2. What does the term “unnecessarily burdened” mean in the context of animal research? Why is this
important for how humans conduct scientific research?
3. Do you believe flies are a good model organism to study circadian rhythms? Why or why not? What
other organisms could be used to study circadian rhythms?
4. What have you learned about model organisms that you did not know before the class discussion and
reading? Have your ideas on scientific research changed?
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