Caterpillar Development
Have you ever wondered how a caterpillar develops into a butterfly? Let’s look at a species of
caterpillars native to Colorado. When the caterpillar first hatches, it is in its first instar. An
instar is a stage of development. This species of caterpillar will then shed its exoskeleton (outer
skin) four times, making for a total of five instars. Each time it reaches the next instar the
caterpillar becomes larger and sheds a head capsule (Keep in mind that other species of
caterpillars may have more or less instars). After the 5th instar the caterpillar will then start to
make its cocoon. It is now called a pupa in this stage. The pupa will then develop into a
butterfly!
pupa
There are multiple factors to consider when we look at caterpillar development The time it
takes for a caterpillar to develop into a pupa depends on genetic factors, temperature, as well as
the quality of their food. Some caterpillars will develop more or less quickly depending on
genetics. Temperature can affect their metabolism, which is responsible for breaking down
matter as well as constructing cell components. Temperature also works to provide thermal
energy. The food source is important in terms of protein, water content, and other nutrients that a
caterpillar needs to grow and survive. Caterpillar development is also very compartmentalized,
so if they experience stress in one stage, it is possible for them to ‘compensate’ in following
stages (instars). In the following experiment, you will observe what happens to the time it takes
a caterpillar to turn into a pupa as well as the pupa mass when manipulating a key factor
involved in development: temperature.
Caterpillar Experiment
Name___________________
Date____________________
Read Caterpillar Development Handout
Introduction:
You have collected 30 butterfly eggs and you want to create an experiment to see if exposing
caterpillars to different temperatures early on will have an effect on how long it will take them to
pupate (make a cocoon) as well as if it will affect their pupa mass. You decide to expose the
caterpillars to different temperatures for two days after they’ve reached the 2nd instar. Then you
place them back into the control temperature until they pupate. You record the mass for each
caterpillar at the 3rd, 4th, and 5th instars as well as the pupa mass.
You decide to make the control temperature 25°C and have two experimental groups where you
will expose the caterpillars to a ‘heat shock’ for two days after reaching the 2nd instar. One
experimental group will be exposed to 33°C and the other experimental group will be exposed to
38°C.
Below is a brief description of the protocol for your experiment:
1. Give each egg an id name and place them in a separate petri dish
with plant and a wet sponge (to keep plant moist).
2. Place all 30 petri dishes in a control chamber that gets to an
average of 25°C. The chamber will also mimic night and day by
having light turn on at the time of sunrise and turn off at the time of
sunset.
3. Check each egg daily. Record which day each egg hatches and place back
in control chamber.
4. Record which day each caterpillar turns into a 2nd instar. The day they
turn into 2nd instars, you will split them up so that 10 go back into the
control chamber, 10 go into a 33°C chamber and 10 go into a 38°C chamber. They will
stay in these chambers for 2 days, though you will continue to check on them once a day
to ensure that they have enough plant and water on the sponge as well as if any have
changed into a 3rd instar.
5. You continue checking the caterpillars daily. Once the 2 days after the 2nd instar is up,
ALL caterpillars go back into the control chamber for the remainder of the experiment.
6. Each time a caterpillar reaches a new instar (3rd, 4th, 5th) you record the date as well as the
mass of the caterpillar. You also record the date each caterpillar turns into a pupa as well
as its mass.
1. Experimental Design Questions to Consider:
A. What is a control group? What is an experimental group?
B. What are you assuming about this population if you set the control temperature at
25°C?
C. In looking at development time overall, which is the dependent variable? Which is the
independent variable?
D. If you are testing to see if temperature has an effect on development time and/or pupa
mass, what other factors must you control for? How might you control for these factors
in your experiment? (HINT: refer to Caterpillar Development Handout)
2. Formulate a hypothesis for the following:
A. How will exposure to the ‘heat shock’ during the 2nd instar affect the time it takes a
caterpillar to change from a 2nd instar to a 3rd instar?
B. How will exposure to the ‘heat shock’ during the 2nd instar affect the overall time it
takes for a caterpillar to go from a 2nd instar to a pupa?
C. How will the final pupa mass be affected by this ‘heat shock’ early in development?
3. You have run your experiment and now you need to calculate the total time it took for each
caterpillar to pupate. Fill out the last column for ‘Total Time’ between the 2nd instar and Pupa in
the following tables:
Control--------Time Between Instars (Days)
Caterpillar
2nd &
3rd
Instar
3rd &
4th
Instar
4th &
5th
Instar
Control 1
Control 2
Control 3
Control 4
Control 5
Control 6
Control 7
Control 8
Control 9
Control 10
3
2
4
3
2
3
2
2
3
4
2
4
2
2
3
3
2
5
2
3
4
3
2
2
4
2
3
2
4
3
5th
Instar
&
Pupa
4
5
4
6
7
5
6
4
5
5
Ex. 33°C------Time Between Instars (Days)
Total
Time
(Days)
Caterpillar
2nd &
3rd
Instar
3rd &
4th
Instar
4th &
5th
Instar
Ex. 33C 1
Ex. 33C 2
Ex. 33C 3
Ex. 33C 4
Ex. 33C 5
Ex. 33C 6
Ex. 33C 7
Ex. 33C 8
Ex. 33C 9
Ex. 33C 10
2
1
3
1
2
2
3
2
2
2
3
3
4
3
3
2
4
3
3
2
2
5
3
2
3
3
2
3
4
3
5th
Instar
&
Pupa
5
4
6
5
5
6
5
6
5
5
Total
Time
(Days)
Ex. 38°C ------Time Between Instars (Days)
Caterpillar
2nd &
3rd
Instar
3rd &
4th
Instar
4th &
5th
Instar
Ex. 38C 1
Ex. 38C 2
Ex. 38C 3
Ex. 38C 4
Ex. 38C 5
Ex. 38C 6
Ex. 38C 7
Ex. 38C 8
Ex. 38C 9
Ex. 38C 10
3
1
1
2
2
1
3
2
2
2
4
3
4
2
3
2
2
2
4
3
2
4
4
3
2
4
3
4
4
3
5th
Instar
&
Pupa
5
4
7
4
4
6
5
6
3
4
Total
Time
(Days)
4.
A. Calculate the mean for time between 2nd and 3rd instars for each of the groups:
Control 25°C Group Mean:
Experimental 33°C Group Mean:
Experimental 38°C Group Mean:
B. Create a bar graph of the group means for time between 2nd and 3rd instars:
5. What do you notice about the mean time it takes a caterpillar to change from a 2nd instar to a
3rd instar in regard to temperature from the graph you just made?
6. Use an unpaired t-test to determine if the Experimental 33°C group developed into the 3rd
instar significantly more quickly than the Control group (p<0.05). Below are steps to help guide
you through a t-test:
A. Why would you use an unpaired t-test for this data? What are the null and alternative
hypotheses?
B. Fill out the following table. The first one has been done for you for each group.
Control (x)
3
2
4
3
2
3
2
2
3
4
Total sum=
(control x-control
mean)2
(Control x-control
mean)2
(3-2.8)2=0.04
Exp. 33°C (y)
2
1
3
1
2
2
3
2
2
2
Total sum=
(Exp. 33°C yExp. 33°C mean)2
(exp. 33°C y – exp. 33°C
mean)2
(2-2.0)2=0
C. Now calculate the variance:
So you can set it up like below (*note use your calculations from above to fill in the equation):
S2= (control x-control mean)2 + (Exp. 33°C y-Exp. 33°C mean)2
n1+ n2 – 2
D. Now, perform an unpaired t-test to find the t value:
E. Now look at the table of critical values of t-distributions and determine the
range the p-value must fall in (degree of freedom= n1+n2 – 2).
F. Did the caterpillars from the Experimental 33°C group develop into 3rd instars
significantly more quickly than the Control group? (p<0.05) Be sure to state
whether or not you can reject the null hypothesis. Why do you think that this
happened?
7.
A. Use an unpaired t-test to determine if the Experimental 38°C group developed into the
3rd instar significantly more quickly than the Control group (p<0.05). Be sure to write
out the null and alternative hypotheses. Reference the steps you did in the previous test
above to guide you.
B. Did the caterpillars develop significantly more quickly into 3rd instars in the 38°C
group than in the Control group? Be sure to state whether or not you can reject the null
hypothesis. Why do you think that this happened?
8.
A. Calculate the mean for total time between 2nd instar and pupa for each of the
groups:
Control 25°C Group Mean:
Experimental 33°C Group Mean:
Experimental 38°C Group Mean:
B. Create a bar graph of the group means for total time between 2nd instar and pupa:
9. What do you notice about the mean overall time it takes a caterpillar to develop from a 2nd
instar to a pupa in regard to temperature from the graph you just made?
10. Use an unpaired t-test to determine if the Experimental 33°C group developed into a pupa
significantly more quickly than the Control group (p<0.05). Be sure to write out the null and
alternative hypotheses and state whether or not you can reject the null hypothesis.
11. Use an unpaired t-test to determine if the Experimental 38°C group developed into a pupa
significantly more quickly than the Control group (p<0.05). Be sure to write out the null and
alternative hypotheses and state whether or not you can reject the null hypothesis.
Below you have recorded the final pupa masses for each group:
Final Pupa Mass (mg)
Control
Exp. 33°C
Exp. 38°C
211.7
156.7
148.6
158.6
190.3
216.5
240.9
180.7
178.3
173.5
206.5
162.1
140.2
178.4
158.2
175.5
143.4
185.5
169.6
151.3
191.7
182.4
194.2
172.7
194.7
184.1
200.3
157.2
201.8
157.2
12. Calculate the mean pupa mass for each group:
Control 25°C Group Mean:
Experimental 33°C Group Mean:
Experimental 38°C Group Mean:
13. Use an unpaired t test to determine if the mean pupa mass from the Experimental 33°C group
was significantly lower than the Control Group. (p<0.05). Be sure to write out the null and
alternative hypothesis and state whether or not you can reject the null hypothesis.
14. Use an unpaired t test to determine if the mean pupa mass from the Experimental 38°C group
was significantly lower than the Control Group. (p<0.05). Be sure to write out the null and
alternative hypothesis and state whether or not you can reject the null hypothesis.
15. Did the caterpillars in either experimental group develop into a pupa more quickly? Based
on what you have read, why do you think this happened? How does the data from the final pupa
mass support your claim?
16. What do you think may have happened if you kept the experimental groups in the ‘heat
shock’ temperature for the entirety of the experiment (instead of just 2 days after they reached
the 2nd instar)?
17. What do you think would happen in terms of how long it takes caterpillars to develop if the
average temperature for their environment increased due to climate change? Do you think this
will have any other ecological affects?