Primary 4 Science Life Cycles Quiz

Primary 4 Science Quiz - Life Cycles: Answer Key

Total Marks: 40 marks

Section A: Multiple Choice (2 marks each)

Section A Total: 20 marks

Section B: Short Answer

Question 11 (3 marks)

(a) Nymph [1 mark]

Key concept: In incomplete metamorphosis, the young stage between egg and adult is called a nymph. It resembles a smaller version of the adult.

(b) Incomplete metamorphosis [1 mark]

Key concept: This life cycle has three stages (egg → nymph → adult) with no pupal stage. The nymph gradually develops into the adult through moulting.

(c) Any one of: [1 mark]

• The nymph has no wings (or small wing buds) while the adult has fully developed wings
• The nymph is smaller in size than the adult
• The nymph is not sexually mature (cannot reproduce) while the adult can
• The nymph may be different in colour from the adult

Common mistake: Students sometimes say the nymph "looks completely different" from the adult—this is incorrect for incomplete metamorphosis. That description applies to complete metamorphosis (e.g., caterpillar vs. butterfly).

Question 12 (4 marks)

(a) [2 marks]

• A: Egg [1]
• D: Adult [1]

Key concept: Mosquito life cycle: Egg → Larva → Pupa → Adult (complete metamorphosis). All except the adult are aquatic stages.

(b) The eggs need water to develop / The larvae that hatch need to live in water [1 mark]

Reasoning: Mosquito eggs are laid on water because all immature stages (larva and pupa) are aquatic. Eggs may even be laid in dry areas that flood later, but typically they require water for the larvae to survive.

(c) Any sensible suggestion:** [1 mark]

• Remove stagnant water from containers
• Cover water tanks and containers
• Clear clogged drains
• Add larvicidal fish to ornamental ponds
• Regularly change water in vases

Key concept: Mosquitoes need standing water for their immature stages. Eliminating breeding sites breaks the life cycle.

Question 13 (4 marks)

(a) Tadpole [1 mark]

Key concept: The earliest active stage of a frog after hatching from the egg is the tadpole (also called a polliwog). It is fully aquatic.

(b) Any two: [2 marks]

• The tail becomes shorter (or shrinks)
• The tail gradually disappears (is absorbed into the body)
• The tail is reabsorbed to provide nutrients during metamorphosis

Biological detail: During frog metamorphosis, the tail is not simply lost—it is resorbed (broken down and recycled) by the body to provide energy and nutrients while the froglet cannot eat efficiently during transformation. This is why the tail gets progressively shorter.

(c) The adult frog needs to be in water to reproduce (lay eggs, sperm fertilises eggs externally) and on land to catch insects/food and avoid aquatic predators [1 mark]

Or acceptable alternative: Needs moist skin for breathing through skin; needs water for reproduction but land for food/foraging.

Question 14 (4 marks)

(a) [2 marks]

Working: $\frac{50 \text{ mm}}{2 \text{ mm}} = 25$

Answer: 25 times longer [1 mark for correct working, 1 mark for correct answer]

Verification: The caterpillar grows from 2 mm to 50 mm—this is massive growth necessary to store enough energy for metamorphosis. If a student writes "50 - 2 = 48 times longer," this is incorrect (subtraction rather than division). The correct operation is division of lengths.

(b) No, Mary is not correct [1 mark]

Explanation: The pupa is alive even though it does not move or eat. Inside the pupa case, the body is actively changing and reorganising—breaking down larval tissues and building adult structures (wings, legs, antennae, reproductive organs). This requires energy and is a very active biological process [1 mark]

Teaching note: The pupa may look inactive externally, but internally there is massive cellular activity. The stored energy from the caterpillar's feeding powers this transformation.

Question 15 (3 marks)

Marking: 1 mark per correct row (both ticks/crosses must be correct for that row)

Key concept summary:

• Butterfly (complete metamorphosis): 4 stages, pupa present, young (caterpillar) looks completely different from adult
• Grasshopper (incomplete metamorphosis): 3 stages, no pupa, young (nymph) resembles small adult

Question 16 (3 marks)

(a) The insect is more likely to be a cockroach [1 mark]

Reason: The young looks like the adult but without wings—this is characteristic of incomplete metamorphosis, which cockroaches undergo [1 mark]

Butterflies would be incorrect because: Butterfly caterpillars look completely different from adult butterflies (complete metamorphosis).

(b) Moulting [1 mark]

Key concept: Moulting (also called ecdysis) is the process of shedding the hard outer covering (exoskeleton). Insects with exoskeletons must moult because the rigid exoskeleton does not grow with the animal. A new, larger exoskeleton forms underneath, and the insect expands before the new covering hardens.

Section C: Application and Reasoning

Question 17 (3 marks)

(a) Any one: [1 mark]

• Duck
• Fish (most species)
• Tortoise/turtle
• Crocodile/alligator
• Snake (most species)
• Most insects (e.g., ants, bees, butterflies)

Note: Must be an animal that lays eggs outside the mother's body (oviparous). Do not accept "bird" if Animal P is already a chicken/bird unless a specific different bird is named.

(b) Any one: [1 mark]

• Human
• Elephant
• Dog/cat
• Cow/goat
• Monkey
• Rabbit
• Kangaroo

Note: Must be a mammal or other viviparous animal whose young resembles a small adult (not a larval stage).

(c) Any sensible answer:** [1 mark]

• The hard shell protects the developing embryo from physical damage
• The shell prevents drying out (desiccation) in air
• The shell is semi-permeable, allowing gas exchange while protecting the contents
• Parent does not need to carry the young, allowing mobility

Question 18 (4 marks)

(a) Box B at 25°C [1 mark]

Reading data: At 25°C, the eggs hatched, caterpillars grew actively, and pupae formed—showing complete, healthy development.

(b) The temperature was too low (5°C) for the eggs to develop [1 mark]

Biological explanation: Metabolic processes and enzyme activity needed for embryonic development are slowed or stopped at cold temperatures. The eggs are dormant but may remain viable if warmed.

(c) The temperature was too high (35°C) / The heat caused water to evaporate from the eggs, drying them out [1 mark]

Explanation: At high temperatures, eggs lose moisture. Unlike reptile eggs with leathery shells, insect eggs have protective coatings but can still desiccate. The proteins and enzymes also begin to denature (break down) at excessive heat.

(d) The eggs would likely not develop successfully / would dry out and die / have very low hatching success [1 mark]

Prediction based on pattern: Box C at 35°C showed drying/shrivelling. At 38°C+, this effect would be worse. The temperature exceeds the optimal range for butterfly egg development.

Question 19 (4 marks)

(a) [2 marks — 0.5 each]

(b) [2 marks]

• Chemical sprays would kill the ladybirds as well as the aphids [1]
• Without ladybirds, the aphid population would grow again when the spray effect wears off / ladybirds are natural predators that provide long-term control [1]

Alternative valid answers:

• Ladybirds are sensitive to pesticides
• Chemicals might accumulate in the food chain
• Natural biological control is more sustainable than repeated spraying

Key concept: This demonstrates biological pest control using natural predators rather than chemicals.

Question 20 (5 marks)

(a) [2 marks]

• It takes about 10 years for a Horseshoe Crab to become adult and able to reproduce [1]
• If many adults die, there are too few mature crabs to produce enough eggs to replace the population [1]
• The species cannot recover quickly from population losses due to this slow replacement rate [1]

Any two points for full marks.

(b) [2 marks]

• Moulting is when the animal sheds its hard outer shell (exoskeleton) [1]
• The exoskeleton is rigid and cannot stretch or grow; moulting allows the animal to grow larger by forming a new, bigger shell [1]

Connect to concept: This is the same reason insects moult—arthropods with exoskeletons must periodically shed their restricted covering to increase in size.

(c) Any sensible suggestion:** [1 mark]

• Protect coastal mudflats from development
• Avoid polluting coastal waters where they live
• Educate the public not to collect or disturb them
• Create marine protected areas or nature reserves
• Reduce coastal reclamation that destroys breeding beaches

Singapore context: Horseshoe Crabs are found at places like Kusu Island and the Mandai mudflats. Conservation efforts include habitat protection and public education.

Marking Summary

Common errors to watch for in marking:

• Confusing "nymph" with "larva" — nymphs resemble adults; larvae look completely different
• Forgetting that the pupa is alive and active internally
• Thinking all insects have the same type of metamorphosis
• Misunderstanding that moulting occurs in incomplete metamorphosis, not just complete metamorphosis
• Confusing the reasons for egg-laying locations (food vs. protection)