E. Homeostasis · HomeostasisInvestigation: How Invertebrates Respond to Stimuli
E2.4 E2.1 A1.1 A1.2 A1.3 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11 A1.13About 75 minutes
Learning goals- I can distinguish innate behaviours such as taxes and kineses and explain how they help invertebrates maintain homeostasis.
- I can plan an ethical, controlled investigation of an invertebrate's response to a stimulus such as light or moisture.
- I can collect, organize and graph behavioural data and use a control to check for bias.
- I can analyse results using percentages and (optionally) a chi-square test, and evaluate sources of error.
- I can communicate my investigation in a formal lab report.
Words to know
stimulus
A change in the environment that an organism can detect, such as light, moisture, temperature or a chemical.
innate behaviour
Behaviour that an animal performs correctly the first time, without learning; it is inherited.
taxis (plural taxes)
A directed movement toward (positive) or away from (negative) a stimulus, such as phototaxis (light) or chemotaxis (chemicals).
kinesis
A non-directed change in the speed of movement or rate of turning in response to a stimulus; the animal ends up spending more time in favourable conditions.
choice chamber
A container with two or more areas that differ in one condition, used to see which an animal prefers.
control trial
A trial in which both sides are the same, used to check that the animals have no bias for one side of the apparatus.
chi-square test
A statistical test that compares observed counts with the counts expected by chance.
Why do pill bugs hide under logs?
Lift a log or a flat stone in a damp corner of a Canadian backyard between spring and the first frost, and you will often find pill bugs or sow bugs (woodlice) scurrying away from the light. Is it because they "don't like" light? Or is it really about water?
In this lesson you will design and carry out a real investigation of how a small invertebrate responds to a stimulus. This is exactly the kind of investigation the Ontario curriculum asks for (expectation E2.4), and it links to homeostasis: for a small animal, behaviour is often the first line of defence for keeping its internal conditions stable.
Innate behaviour: taxes and kineses
Many invertebrates respond to stimuli with innate (inborn) behaviours. Two important kinds:
- Taxis: a directed movement toward or away from a stimulus. The prefix tells you the stimulus: phototaxis (light), chemotaxis (chemicals), thigmotaxis (touch), geotaxis (gravity). Moths flying toward a porch light show positive phototaxis; earthworms moving away from light show negative phototaxis.
- Kinesis: a non-directed change in activity. A pill bug in dry air moves faster and turns more often; in damp air it slows down and stops. It is not steering toward the damp; it just stays longer wherever conditions are good, so it ends up there.
Why does this matter for homeostasis? Pill bugs are not insects; they are crustaceans, related to shrimp and lobsters. They breathe through gill-like structures and their exoskeleton does not have the waxy waterproof layer insects have, so they lose water quickly in dry air. Moving away from light and dryness keeps them in damp, dark places where they can control their water balance. Behaviour is doing the job that ADH does in your kidneys!
Other good study animals: mealworms (beetle larvae, sold in pet stores all year, useful in winter), earthworms (after a rain) and, in a school lab, planaria (flatworms with simple eyespots).
Planning a good investigation
1. Question. Make it specific and testable: "Do pill bugs prefer dark or light conditions?" or "Do mealworms prefer damp or dry paper towel?"
2. Hypothesis. A prediction with a reason: "If pill bugs are given a choice between dark and light, then more than half will move to the dark side, because darkness is usually associated with damp places where they lose less water."
3. Variables.
- Independent variable: the condition you change (light vs dark).
- Dependent variable: what you measure (number of animals on each side after a set time).
- Controlled variables: everything else: moisture, temperature, surface, number of animals, time, species, how they are released.
4. Control trial. Run the chamber with both sides the same (both dark). If the animals still pile up on one side, something else (a draft, a slope, a smell, warmth from the lamp) is affecting them.
5. Repeated trials and sample size. Use at least 10 animals, repeat several times and switch which side is dark each trial to cancel out any side bias.
6. Ethics and safety. Animals in research must be treated with respect:
- handle gently with a soft paintbrush or spoon, never squeezing;
- keep them in a container with damp soil or leaves, out of direct sun and heat, and test for short periods;
- never use a hot lamp close to the animals;
- return wild animals to exactly where you found them on the same day;
- wash your hands after handling animals or soil.
Worked exampleExample 1: Designing a moisture investigation
Design an investigation to test whether mealworms prefer damp or dry conditions. Include the question, hypothesis, variables, control, procedure outline and one ethical consideration.
- Question: Do mealworms prefer a damp or a dry surface?
- Hypothesis: If mealworms are given a choice, more will be found on the damp side after 10 minutes, because moist conditions reduce water loss through their body surface.
- Variables: IV = moisture (damp vs dry paper towel). DV = number of mealworms on each side at 10 minutes. Controlled = light (whole box covered), temperature, type of paper towel, number of mealworms (10), time, release point.
- Control: a trial with dry towel on both sides, to check for side bias.
- Procedure outline: line each half of a shoebox lid with a paper towel; dampen one with 10 mL of water. Release 10 mealworms in the centre, cover the box, and count each side every 2 minutes for 10 minutes. Swap the damp side between trials; repeat 5 times.
- Ethics: keep the trials short, handle the larvae with a spoon, and return them to their container with food (oat flakes and a slice of carrot) afterward.
A complete plan has a testable question, a reasoned hypothesis, clearly named variables, a control trial, repeated trials with sides swapped, and humane handling.
Sample class data (means of several trials). In the experiment (one side dark), the percentage on the dark side rises to about 80%. In the control (both sides dark), it stays near 50%, showing there was no bias toward one side of the box.Worked exampleExample 2: Is the result more than chance? (A chi-square test)
In one trial, 15 of 20 pill bugs were on the dark side and 5 on the light side. If they were moving randomly, we would expect 10 on each side. Use a chi-square test to decide whether the difference is likely to be due to chance. (For two categories, if χ² is greater than 3.84, there is less than a 5% probability that a difference this large would happen by chance alone.)
- Formula: χ² = Σ (observed − expected)² ÷ expected.
- Dark side: (15 − 10)² ÷ 10 = 25 ÷ 10 = 2.5.
- Light side: (5 − 10)² ÷ 10 = 25 ÷ 10 = 2.5.
- χ² = 2.5 + 2.5 = 5.0.
- 5.0 is greater than 3.84, so the result is statistically significant at the 5% level: it is unlikely to be just chance. The data support the hypothesis that pill bugs prefer the dark. (One trial is still limited; repeated trials make the conclusion stronger.)
χ² = 5.0 > 3.84, so the preference for the dark side is unlikely to be due to chance.
Writing it up
A formal lab report has: title, purpose/question, hypothesis, materials, procedure (numbered, in enough detail to repeat), observations (data table with units and a graph), analysis (percentages, means, and the chi-square test if used), evaluation (sources of error, how to improve), and conclusion (answer the question; does the evidence support the hypothesis?). University biology courses expect this structure, and you'll use it again in the Population Dynamics unit.
Hands-on activityInvestigation: light preference in pill bugs (or mealworms)
Grown-up note: use a desk lamp with an LED bulb (which stays cool) at least 50 cm away, or simply use daylight from a window. Supervise collecting outdoors, and make sure the animals go back where they were found. Everyone washes hands afterward.
You need: 10 to 20 pill bugs or sow bugs (from under logs or stones in a damp area, spring to fall) or mealworms from a pet store; a shoebox lid; black construction paper; 2 damp paper towels; a soft paintbrush or spoon; a timer; a pencil and paper.
- Line the whole lid with damp paper towel (same dampness everywhere). Cover one half with black paper as a roof; leave the other half open to the light.
- Control trial first: cover both halves. Release the animals in the centre. Count each side every 2 minutes for 10 minutes.
- Experimental trials: one half covered. Release the animals in the centre and count every 2 minutes for 10 minutes.
- Repeat the experimental trial at least 3 times, switching which half is dark each time. Let the animals rest in their container for 5 minutes between trials.
- Record all data in a table. Calculate the percentage on the dark side at each time, and the mean across trials.
- Graph % on the dark side against time for the experimental and control trials (like the sample graph above).
- Optional: do a chi-square test on your 10-minute counts.
- Return the animals to where you found them (or to their container if from a store).
Analyse and evaluate: Did the control show a side bias? Do your results support your hypothesis? Is the response a taxis or a kinesis, and how could you tell? (Watch: do the animals walk straight to the dark, or wander and slow down once they are there?) Identify two sources of error (for example, uneven dampness, animals clumping together) and how to fix them.
What to look for: most groups find that pill bugs and mealworms gather on the dark side, and that individual animals often slow down and stop once in the dark, which suggests kinesis plays a part. A good report explains the result in terms of water balance.
Just for funFunny-but-true: pill bugs are crustaceans, more closely related to shrimp and lobsters than to insects. So the little grey creatures under your garden stones are, technically, land shrimp. (Please don't put them on the barbecue.)
Your turn
1Distinguish a taxis from a kinesis.
A taxis is a directed movement toward or away from a stimulus. A kinesis is a non-directed change in speed or turning rate, so the animal spends more time in favourable conditions.
2An earthworm moves away from a flashlight beam. Name this behaviour precisely.
3Why do pill bugs need to stay in damp places?
They are crustaceans with gill-like breathing structures and no waxy waterproof layer, so they lose water quickly in dry air.
4In a light-preference experiment, name the independent, dependent and two controlled variables.
IV: light vs dark. DV: number (or %) of animals on each side. Controlled (any two): moisture, temperature, surface, number of animals, time, species, release point.
5What is the purpose of a control trial with both sides dark?
To check for side bias: if animals still gather on one side, another factor (slope, draft, warmth, smell) is affecting them.
6Why should you switch which side is dark between trials?
To cancel out any bias toward one side of the box, so the result reflects the stimulus only.
7Give two ethical practices for investigations with live invertebrates.
Any two: gentle handling with a brush or spoon; short trials; keep them damp and cool; avoid hot lamps; return them where found; wash hands.
More practice: warm-up
A change in the environment that an organism can detect.
Movement toward or away from light.
4Where might you find pill bugs?
Under logs, stones or leaf litter in damp places.
5Why use 10 or more animals rather than 1?
A larger sample size reduces the effect of individual differences and makes results more reliable.
More practice: on your own
1In a trial, 14 of 20 mealworms are on the damp side. What percentage is that? What would you expect by chance?
14 ÷ 20 × 100 = 70%. By chance you'd expect about 50% (10 of 20).
2A student uses an incandescent lamp 15 cm above the light side. Identify the problem with the design and suggest a fix.
The lamp makes the light side warmer (and drier), so temperature and moisture are not controlled; animals might move away from heat, not light. Use an LED lamp farther away or daylight, and check the temperature of both sides with a thermometer.
3How could you tell whether pill bugs show a taxis or a kinesis toward the dark?
Observe their paths: in a taxis they turn and walk directly to the dark side; in a kinesis they move randomly but slow down or stop when they reach the dark (and move faster in the light). You could measure walking speed on each side.
4Explain how a kinesis can lead animals to gather in favourable conditions even though they aren't steering toward them.
They move quickly and turn often in poor conditions, but slow down or stop in good ones. Over time, they spend more time where conditions are good, so most of them end up there.
5Calculate χ² for 12 dark and 8 light (expected 10 and 10). Is it significant at the 5% level?
(12 − 10)² ÷ 10 + (8 − 10)² ÷ 10 = 0.4 + 0.4 = 0.8. This is less than 3.84, so not significant: the difference could easily be due to chance.
Challenge
1A class pools 5 trials of 20 pill bugs each (100 animals total): 68 dark, 32 light. Calculate χ² and interpret. Why is pooling helpful?
Expected 50 and 50. χ² = (68 − 50)² ÷ 50 + (32 − 50)² ÷ 50 = 324 ÷ 50 + 324 ÷ 50 = 6.48 + 6.48 = 12.96, much greater than 3.84, so the preference is highly significant. Pooling increases the sample size, so a real but modest effect becomes easier to detect and less affected by random variation. (Strictly, the same animals in repeated trials are not fully independent, which is a limitation to mention.)
2Design an experiment to separate the effects of light and moisture on pill bug distribution (they are usually linked in nature).
Use four conditions (a 2 × 2 design): dark + damp, dark + dry, light + damp, light + dry, either as four quadrants of a chamber or as pairwise choices with one variable changing at a time (e.g., dark-dry vs light-dry tests light alone; dark-damp vs dark-dry tests moisture alone). Control temperature, release point and time, switch positions between trials, and repeat. Comparing the results shows which factor has the stronger effect and whether they interact.
3Explain how pill bug behaviour in this lesson is an example of homeostasis, and compare it with osmoregulation in humans.
Pill bugs keep their internal water balance steady mainly through behaviour: kinesis and negative phototaxis keep them in damp, dark places where they lose little water. Humans live in drier environments but have waterproof skin and kidneys with ADH control, adjusting urine output to keep water balance. Both are negative feedback responses to water loss, one behavioural and one physiological (humans also use behaviour, like drinking when thirsty).
Quiz
1. A moth flying toward a porch light shows:
2. A kinesis is best described as:
3. In a light-preference experiment, the number of animals on each side is the:
4. Why run a trial with both sides dark?
5. For 15 dark and 5 light (expected 10 each), χ² equals:
6. Which practice is NOT ethical when working with pill bugs?
Big idea: this lesson is a full scientific investigation (curriculum expectation E2.4) of an invertebrate's response to a stimulus. Students practise question, hypothesis, variables, controls, repeated trials, ethical treatment of animals, data analysis (including an optional chi-square test) and evaluation.
How to help: collect pill bugs together from under a log or stone in a damp spot (from spring until the first hard frost), or buy mealworms at a pet store in winter. Be the timer and the second counter. Make sure the animals go back where they came from.
Common difficulties:
- Forgetting the control trial. Without it, you can't rule out side bias.
- Uncontrolled heat from lamps. Use daylight or a cool LED far away.
- Over-claiming from one trial. Encourage repeated trials and talk about chance.
Safety: wash hands after handling animals and soil; supervise outdoor collecting.