echoesof.science · Ages 15–16
Insects: structure, gas exchange and abundance
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Insects are hexapods: arthropods with a chitinous exoskeleton, jointed appendages, three pairs of legs and three tagmata (head, thorax, abdomen), with one pair of antennae and a pair of compound eyes. Spiders and other chelicerates have two body regions and no antennae.
Insects exchange gases through a tracheal system: tracheae open at the surface through spiracles and branch into fine tubes that carry gases directly between the air and the cells. They excrete with Malpighian tubules.
Because the exoskeleton cannot expand, insects grow by ecdysis (moulting); the stage between two moults is an instar. Development is either complete (egg, larva, pupa, adult) or incomplete (egg, nymph, adult).
Experiment: Comparing ground invertebrates in two habitats
- Choose two contrasting habitats (for example a hedge base and a mown lawn). Write a hypothesis about which will have more insect types.
- Set 5 traps in each habitat at least 2 m apart, rims level with the soil, leaves inside and a roof above.
- Collect all traps early next morning, before it warms up.
- For each trap, sort the catch by leg number and type, and count individuals.
- For each habitat calculate: total individuals, number of insect types, and the percentage of individuals that are insects.
- Calculate the mean and range of catch per trap for each habitat. Do the ranges overlap? What does that say about how confident you can be?
- Release all animals near where they were caught and fill in the holes.
| Measure | Habitat A | Habitat B |
|---|---|---|
| Total individuals (5 traps) | ||
| Insect individuals | ||
| Percentage that are insects (%) | ||
| Number of insect types | ||
| Mean catch per trap | ||
| Range of catch per trap |
Questions
Describe how oxygen reaches the muscles in an insect's thorax.
Explain why insects grow in steps. Use the words exoskeleton, ecdysis and instar.
Using your results, compare the two habitats. Do the ranges overlap? How confident are you in the difference?
Give two limitations of pitfall traps for measuring insect abundance, and one improvement.
Sources for every fact on this sheet: echoesof.science/school/insects/sources/