Laboratory Exercise 1: Scientific Method
Learning Objectives: After completing this exercise you should understand:
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Using the Scientific Method
Outline the steps of the scientific method
Distinguish among observations, hypothesis, conclusions, & theories
Observing a pillbug
Observe & describe the external anatomy of a pillbug, Armadillidium vulgare
Observe & describe how a pillbug moves
Formulating a Hypothesis
Formulate a hypothesis when supplied with appropriate observations
Performing an Experiment
Design an experiment that can be repeated by others
Reach a conclusion based on observation & experimentation
Introduction
This laboratory will provide you with an opportunity to use the scientific method in the same manner as scientists.
Scientists often begin by making observations about the subject of interest. Today our subject is the sowbug, or closely
related pillbug, a type of crustacean that lives on land.
Sowbugs and pillbugs are crustaceans, not insects, which means they are relatives of lobsters and crabs. They are
commonly found in damp leaf litter, under rocks, and in basements or crawl spaces under houses. These bugs breathe
by gills located on the underside of their bodies. The gills must be kept slightly moist, and that is why they are usually
found in damp places. They eat decaying plants and animals and some living plants. They can live up to three years.
Sowbugs and pillbugs have overlapping "armored" plates that make them look like little armadillos. Sowbugs have three
body parts: head, thorax, and abdomen. Sowbugs are adapted to living their entire life on land, although moisture is
required. Currently, it is believed that they do not transmit diseases, nor do they bite or sting. Because they eat dead
organic matter, such as leaves, they are easy to find and keep in a moist terrarium with leaf litter, rocks, and wood chips.
You will first become acquainted with your subject and how it normally behaves. Then, you will use your knowledge of the
sowbug to hypothesize whether it will be attracted to, repelled by, or indifferent to various substances of your choice.
After you have tested your hypotheses, you will conclude whether they are supported or not. Finally, your conclusions
may lead to other hypotheses, and if time permits, you may go ahead and test those also.
1.1 Using the Scientific Method
Scientists use the scientific method (Fig. 1) to come to a conclusion about the natural world. When a scientist begins a
study, he or she uses preliminary observations and previous data to formulate a hypothesis. A hypothesis is a tentative
explanation of observed phenomena.
After a hypothesis is formulated, it must be tested by doing new experiments and/or making new observations.
Experiments are done and observations made in such away that others can repeat them. Only repeatable observations
and experiments are accepted as valid contributions to the field of science.
Data are any factual information that can be observed either independently of, or as a result of, experimentation. On the
basis of the data, a scientist comes to a conclusion-whether the observations support the hypothesis or prove it false.
Scientists are always aware that further observations and experiments could lead to a change in prior conclusions.
Therefore, it is never said that the data prove a hypothesis true, but we could say that the data support the hypothesis.
The arrow in Figure 1 indicates that research often enters a cycle of:
hypothesisexperiments and observationsconclusionhypothesis.
After many years of testing and study, the scientific community may develop a scientific theory, a concept that ties
together many varied conclusions into a generalized statement. For example, after testing the cause of many individual
diseases, the germ theory of disease was formulated. It states that infectious diseases are caused by pathogens (e.g.,
bacteria and viruses) that can be passed from one person to another.
Figure 1. Flow diagram for the scientific method.
On the basis of observations, a scientist formulates a hypothesis. The hypothesis is tested by further experiments and/or
observations. The scientist then concludes whether the data support or do not support the hypothesis. The return arrow
shows that scientists often choose to retest the same hypothesis or test a related hypothesis.
1.2 Observing the Pillbug
Wash your hands before and after handling pillbugs? Please
handle them carefully so they are not crushed. When touched
they roll up in a ball or “pill” shape as a defense mechanism.
They will soon recover if left alone.
What You Need Per Group:
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pillbug in a Petri dish
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graduated cylinder
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metric ruler
What to do:
1.2 Observing the Pillbug:
1. obtain a pillbug that has been numbered with white
correction fluid or tape tags. First examine the shell and body
with the unaided eye & then with a magnifying lens or
dissecting microscope. Put the pillbug in a small glass or
plastic dish to keep it contained.
2. Examine the shell, shape, color & texture. Note the number
of legs and antennae and whether there are any posterior
appendages, such as uropods (paired appendages at end of
abdomen) or brood pouches. (Females have leaflike growths
at the base of some legs where developing eggs and embryos
are held in ouches.) Locate the eyes. Count the number of
overlapping plates.
1. Examine the diagram of the pillbug. Find the labeled parts on your own specimen.
Observation: Pillbug’s Motion
2. Describe the action of the feet & any other motion you see.
3. As you watch the pillbug, identify behaviors that might:
a. protect it from predators
b. help it acquire food
c. protect it from the elements
d. allow interaction with the environment
4. Allow a pillbug to crawl on your hand. Describe how it feels & acts
5. Gently touch the pillbug to demonstrate how it rolls into a ball.
6. Place the pillbug on a graduated cylinder. Experiment with the angle of the cylinder and the position of the pillbug to
determine the preferred direction of motion. For example, place the cylinder on end and position the pillbug so that it can
move up or down. Try other arrangements also. Compare this data with another group in and record this information in
Table 1.1 as well.
Table 1.1 Preferred Direction of Motion
Pillbug
Direction Moved
your pillbug
Comments
another group’s pillbug
7. Measure the speed of the pillbug. Use what you learned about each pillbugs preferred direction of motion (see Table
1.1) to get maximum cooperation from the pillbug. Place the pillbug on a metric ruler, and use a stopwatch t omeasure the
time it takes for the pillbug to move a certain number of centimeters. Record your results in Table 1.2. Calculate your
pillbug’s speed in millimeters (mm) per second, and record you data in Table 1.2. Compare this data with another group
and record this information in Table 1,2 as well.
Table 1.2 Pillbug Speed
Pillbug
Millimeters Traveled
Time (sec)
Average Speed (mm/sec)
your pillbug
another group’s pillbug
1.3 Formulating Hypothesis
Hypotheses are often stated as” if-then” statements. For example, if the pillbug is exposed to ____, then it will be ____.
You will be testing whether pillugs are attracted to, repelled by, or unresponsive to a particular substance. Pillbugs move
away from a substance when they are repelled by it, and move toward and eat a substance that they are attracted to. If a
pillbug simply rolls into a ball, nothing can be concluded, and you may wish to choose another pillbug or wait a minute or
two to check for further response.
1. Choose:
a. a powder (or two or three) such as four, cornstarch, coffee creamer, baking soda, fine sand
b. a liquid (or two or three) such as milk, orange juice, ketchup, applesauce, a carbonated beverage, water
2. Hypothesize in Table 1.3 how you expect the pillbug to respond, and offer an explanation for your reasoning.
Table 1.3 Hypothesis About Pillbug’s Reaction to Common Powders & Liquids
Substance Tested
Hypothesis About How Pillbug Will Respond to Substance
Reasoning for Hypothesis
1.4 Performing an Experiment
Design an experiment to test the pillbug’s reaction to the chosen substances. The pillbug must be treated humanely. No
substances must be put directly on the pillbug, nor can the pillbug be placed directly onto the substance. Since pillbugs
tend to walk around the edge of a Petri dish, you could put the wet or dry substance around the edge of the dish. Or for
wet substances, you could put liquid-soaked cotton in the pillbug’s path.
A good experiment design contains a control. A control group goes through all the steps of an experiment but lacks, or is
not exposed to, the factor being tested. If you are testing the pillbug’s reaction to a liquid, water can be the control
substance substituted for the test liquid. If you are testing the pillbug’s reaction to a powder substitute fine sand for the
test powder.
Experimental Procedure: Pillbug’s Reaction to Common Substance
1. What substances are you testing? Include in your list any controls, and complete the first column in table 1.4.
2. Obtain a small beaker and fill it with water. Rinse your pillbug between procedures by spritzing with distilled water from
a spray bottle. Then put it on a paper towel to dry off.
3. Test the pillbug’s reaction using the method described previously.
4. Watch the pillbug’s reaction to each substance, and record it in Table 1.4.
5. Do your results support your hypothesis? Answer yes or no in the last column.
Table 1.4 Pillbug’s Reaction to Common Substances
Substance Tested
Pillbug’s Reaction
liquid? ________________________
liquid? ________________________
control liquid: WATER
powder? ______________________
powder? ______________________
control powder: SAND
Hypothesis Supported?
Laboratory Review 1
1. What are the essential elements of the scientific method?
2. What is a hypothesis?
3. Is it sufficient to do a single experiment to test a hypothesis, why or why not?
4. What do you call a sample that goes through all the steps of an experiment but does not contain the factor being
tested?
5. What part of a pillbug is for protection, & what does it do to protect itself?
6. Name one criterion that you used to formulate your hypothesis regarding pillbug reactions toward various substances?
7. Why is it important to test one substance at a time when doing an experiment?
Indicate whether 8 or 9 are hypotheses, conclusions, or theories.
8. The data show that vaccines protect people from disease.
9. All living things are made of cells.
A Couple More Questions:
1. Based on your observations, and from that data collected by other groups, what type of environment do sowbugs
prefer?
2. What types of behaviors did you observe that support your conclusions in question 1?
3. In what types of natural habitats would you expect to find sowbugs, or other isopods?
4. The independent variable in a scientific experiment is the condition that the experimenter deliberately varies.
What condition in this experiment did you allow to vary to determine if it had an effect on the experiment’s results?
Independent variable __________________________
5. The dependent variable in a scientific experiment is what the experimenter designed the experiment to measure.
What variable did you measure at the completion of this experiment?
Dependent variable ___________________________