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E2.4 · Investigate an invertebrate response to a stimulus

Learn to investigate an invertebrate response to a stimulus through clear examples and targeted practice.

Ontario Grade 12 Biology

Homeostasis

Planning and interpreting a pill bug behaviour investigation

A pill bug moves from a dry area toward a damp area. The difference in moisture is a stimulus: a change in the surroundings that may affect an organism. The movement is a response: an observable change in activity or behaviour. Pill bugs are invertebrates, or animals without a backbone. This lesson uses pill bugs to explore how to investigate an invertebrate response. Any numbers identified as practice data are invented for learning, not observations from real animals.

What you will learn

Identify a stimulus and an observable response in an invertebrate.
Plan a fair investigation that tests how one stimulus relates to behaviour.
Record and summarize evidence while distinguishing observations from interpretations.
Draw a cautious conclusion that recognizes limits in the evidence.

1. Bridge from earlier biology: connect a stimulus to a response

In earlier biology, you studied how organisms interact with their environment. A condition such as light, moisture, or touch can act as a stimulus. A response is what an organism does when a condition changes. You can observe a response without knowing exactly how the animal detects the change.

Imagine a pill bug in a container with a damp area and a dry area. Saying that it “likes” the damp area suggests a feeling that the investigation does not measure. A clearer statement is that more pill bugs were counted in the damp area after a set time. This describes an observation rather than guessing what the animals feel.

A fair investigation changes one factor at a time while keeping other relevant conditions similar. The factor deliberately changed is the independent variable. The outcome measured is the dependent variable. Conditions kept similar are controlled variables. These terms help describe how an investigation is planned.

A stimulus is a change in the surroundings that may affect an organism.
A response is an observable change in activity or behaviour.
Define the response so it can be observed and recorded.
Separate what you observe from what you infer.

2. Build a simple investigation model

A pill bug is a small land-dwelling crustacean. For this investigation, focus on how its location changes under different moisture conditions. There is no need to identify a particular sensory organ or explain a detailed process inside the animal. The question is about a relationship between a condition and observable behaviour.

A simple model has three parts: a stimulus, detection of a change, and a response. Detection means sensing a change in the surroundings. In this example, the response being studied is movement into, out of, or within an area. This model organizes the question, but it does not prove how the animal detects moisture.

A choice chamber is a container that lets an organism move between areas with different conditions. Place damp paper in one half and dry paper in the other. Keep the halves similar in size. Keep the container, lighting, temperature, observation time, and handling as similar as possible. Here, moisture condition is the independent variable. The number of pill bugs in each area at a chosen time is one possible dependent variable.

The comparison is clearer if the areas differ mainly in the factor being tested. If the damp side is also darker, the counts might be related to light rather than moisture. Make other conditions as similar as possible. Let the animals move freely, and avoid repeatedly disturbing them during observation.

A simple model links stimulus, detection, and an observable response.
Change moisture while keeping other relevant conditions similar.
Choose a response measure before collecting data.
The model organizes the investigation; it does not establish an internal mechanism.

3. Collect evidence and interpret it carefully

Before starting, decide how many pill bugs to observe and how long to wait before counting. Record the number in the damp area, the dry area, and any central area. Recording the centre helps account for all animals. Decide in advance which locations will be included in a calculation, then use the same rule for every trial.

An observation is a recorded result, such as “six pill bugs were in the damp half after five minutes.” An interpretation is what the results suggest, such as “more pill bugs were found in the damp half in these trials.” Keep the two separate. A useful interpretation matches the evidence and avoids claims that the investigation cannot support.

You can summarize a count as a percentage. Divide the count in the selected area by the total count used, then multiply by 100. State what the total includes. If the calculation uses every animal counted, include animals in the centre. If it compares only the two halves, say so and apply that rule consistently.

Repeated trials help show whether a pattern occurs more than once. A repeated pattern does not show that every pill bug will respond in the same way. A small group, uneven dampness, handling, or another uncontrolled difference could affect the results. Describe a conclusion as applying to the animals and conditions tested, not to all invertebrates.

P=\frac{n_{\text{selected}}}{n_{\text{total}}}\times100\%

Record location counts, conditions, and observation time.
Distinguish observations from interpretations.
State what is included in a percentage total.
Use cautious conclusions that reflect the evidence and its limits.

4. Turn results into a conclusion

A focused question could be: “Does moisture condition affect where pill bugs are found after five minutes?” It names the condition being changed and the behaviour being measured. A prediction states an expected pattern, but it is not a result. For example, a student might predict that more pill bugs will be counted on the damp side.

Before collecting data, check that the method can answer the question. Use comparable areas, set the observation time, record the relevant counts, and keep other conditions similar. If the results do not match the prediction, report them honestly. A result that differs from a prediction can still provide useful evidence.

A conclusion should state the pattern, connect it to the tested conditions, and include a limitation. For example: “In these trials, more pill bugs were counted on the damp paper after five minutes. This pattern is consistent with a response to moisture under the tested conditions. The group was small, so the result may not describe all pill bugs.” The phrase “consistent with” connects the evidence to the question without claiming certainty about every animal or how it detects moisture.

A prediction is an expectation, not evidence.
Connect a conclusion to the measured results and tested conditions.
State at least one limitation when drawing a conclusion.

Plan and interpret a pill bug investigation

Investigation part	Pill bug example
Stimulus changed	Moisture: damp paper compared with dry paper
Response measured	Number of pill bugs in each area after a set time
Conditions to keep similar	Area size, observation time, lighting, temperature, and handling
Evidence-based conclusion	Describe the pattern, tested conditions, and at least one limitation

Worked example

Identify the parts of an investigation

A student places pill bugs in a chamber with one damp half and one dry half. After five minutes, the student counts how many are in each half. Identify the stimulus, response measure, independent variable, and one controlled variable.

Name the stimulus
The condition being tested is moisture. The damp and dry halves provide different moisture conditions.
Define the measured response
The student records where the pill bugs are after five minutes. The count in each half is an observable measure of their location at that time.
Identify the variables
Moisture condition is the independent variable because it is deliberately changed. The location counts are the dependent measure. Observation time is one controlled variable if it is kept the same.

Answer: Stimulus: moisture condition. Response measure: the number of pill bugs in each half after five minutes. Independent variable: damp versus dry condition. One controlled variable: observation time.

Check: The investigation measures where the animals are, not what they feel.

Worked example

Calculate a percentage from practice data

Practice data, invented for this example: in one trial, 8 pill bugs were counted in the damp half, 3 in the dry half, and 1 in the centre. What percentage of all counted pill bugs were in the damp half?

Find the total count
The question asks for the percentage of all counted pill bugs, so include the damp half, dry half, and centre.
$8+3+1=12$
Calculate the percentage
Divide the damp-half count by the total, then multiply by 100. Round to one decimal place.
$\frac{8}{12}\times100\%\approx66.7\%$
Interpret the result
About two-thirds of the counted animals were in the damp half in this invented single-trial dataset. One trial does not establish a reliable pattern.

Answer: Approximately 66.7% of the counted pill bugs were in the damp half.

Check: The denominator is 12 because the question includes all counted pill bugs.

Worked example

Choose a conclusion supported by the evidence

Practice data, invented for this example: three trials each began with 10 pill bugs. At five minutes, the damp-side counts were 7, 6, and 8. What conclusion is supported, and what is one limitation?

Compare the counts
Half of 10 is 5. Each damp-side count is greater than 5, so more than half of the animals were on the damp side in each practice trial.
$7>5,\quad6>5,\quad8>5$
State a cautious conclusion
The results are consistent with more pill bugs being found on the damp side after five minutes under the tested conditions. This describes the pattern without claiming that moisture is the only possible influence.
Name a limitation
Only three trials are shown, and the example gives limited information about how other conditions were controlled. The evidence supports a pattern in these trials, not a claim about every pill bug.

Answer: In these invented trials, more than half of the pill bugs were on the damp side each time. This pattern is consistent with a response to moisture in the tested setup. One limitation is the small number of trials and limited information about other conditions.

Check: The conclusion is limited to the example’s results and conditions.

Common mistakes and how to avoid them

Saying pill bugs “prefer” the damp side as if the investigation measured a feeling.

Correction: Report the observable evidence: where the animals were counted and when.

Changing moisture and light at the same time.

Correction: Keep other relevant conditions similar so the investigation tests moisture more clearly.

Leaving animals out of a total without stating a rule.

Correction: Define which locations count in the total and apply that rule consistently.

Treating one trial as proof of a general rule.

Correction: Repeat trials where practical and limit the conclusion to the evidence and conditions tested.

Lesson summary

A stimulus is an environmental change that may affect an organism; a response is an observable change in activity or behaviour.
A choice chamber can compare pill bug behaviour under damp and dry conditions.
Change one factor, define the response measure, and keep other relevant conditions similar.
Record observations clearly, distinguish them from interpretations, and state limits on the evidence.

Check your understanding

Question 1

In a damp-versus-dry choice chamber, which is the independent variable?

The moisture condition
The number of pill bugs counted
The observation time, even if it is kept constant
The conclusion written after the trial

Show answer and explanation

The moisture condition

The independent variable is the factor deliberately changed. Here, that factor is moisture condition.

Question 2

A student counts 5 pill bugs in the damp half, 4 in the dry half, and 1 in the centre. What percentage of all counted pill bugs are in the damp half?

Show answer and explanation

50%

There are 10 animals in total. Five divided by 10, multiplied by 100, is 50%.

Question 3

Which conclusion is best supported by a few trials in which more pill bugs were counted on damp paper?

Every pill bug always moves toward dampness.
The results are consistent with more pill bugs being found on damp paper under the tested conditions.
The investigation proves exactly how pill bugs detect moisture.
The pill bugs moved because the damp paper was darker.

Show answer and explanation

The results are consistent with more pill bugs being found on damp paper under the tested conditions.

This conclusion describes the observed pattern and limits the claim to the tested conditions. The counts do not establish a universal response or reveal the exact detection process.

Key terms

Stimulus: A change in the surroundings that may affect an organism.
Response: An observable change in an organism’s activity or behaviour.
Invertebrate: An animal without a backbone.
Independent variable: The factor deliberately changed in an investigation.
Dependent variable: The measured outcome in an investigation.
Controlled variable: A condition kept the same to make a comparison fair.
Choice chamber: A container that lets an organism move between areas with different conditions.
Interpretation: An explanation of what recorded observations suggest.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation E2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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