Teacher sheet · Ages 15–16 · about 45–50 minutes
Insects: structure, gas exchange and abundance
Lesson page: echoesof.science/school/insects/ages-15-16/ · Pupil worksheet: echoesof.science/school/insects/ages-15-16/print/
Learning aims
- Describe the arthropod and hexapod body plan and distinguish insects from other arthropods.
- Explain gas exchange through spiracles and tracheae, and growth by ecdysis.
- Handle large numbers and percentages, and discuss uncertainty in species estimates.
- Plan, carry out and evaluate a two-habitat pitfall-trap comparison.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: how could you count every insect on Earth? Discuss why you can't. |
| 10 min | Read the text; quiz calculations. |
| 20 min | Set traps in two habitats (day 1). |
| 40 min | Sort, count, calculate and evaluate (day 2). |
| 5 min | Arthropod-group game. |
Materials for the experiment
- 10 identical plastic cups with small drainage holes
- Trowel, dead leaves, 10 small roofs (tiles or wood) with stones
- White trays, spoons, paintbrushes, hand lenses, simple invertebrate key
- Recording sheet
Safety: Follow the school's fieldwork risk assessment: check sites for hazards, wear gloves when digging, do not leave traps for more than one night, check them before the day warms up, handle animals gently and release them, and wash hands afterwards. Do not use any killing or preserving fluid.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Insects have lungs and blood that carries oxygen like ours. | Gases are exchanged through tracheae that reach the cells directly. |
| Woodlice, spiders and centipedes are insects. | They are other arthropods: crustaceans, arachnids and myriapods. |
| We know how many insect species exist. | Described species are about 0.9–1 million; the total is very uncertain. |
Worksheet answers
- Air enters through spiracles on the body surface and travels through tracheae, which branch into fine tubes that deliver oxygen directly to the muscle cells.
- The exoskeleton cannot expand, so the insect sheds it (ecdysis) and forms a larger one; the stage between two moults is an instar, so growth happens in steps.
- Accept class data with means and ranges; overlapping ranges mean the difference may be due to chance, so confidence is limited.
- Limitations e.g. only samples active ground-dwelling animals; climbers and fliers under-sampled; weather affects activity; few traps. Improvement e.g. more traps per habitat, repeat on several nights, same weather conditions.
Quiz answers
- Which structure carries oxygen to an insect's cells? The tracheal system. Tracheae exchange gases directly with the cells.[5]
- What is an instar? The stage between two moults. Ecdysis separates successive instars.[4]
- Which feature separates insects from spiders? Insects have antennae and three tagmata. Both are arthropods; chelicerates lack antennae and have two body regions.[5]
- If 10¹⁹ insects are alive and there are 8 × 10⁹ people, about how many insects are there per person? 1.25 × 10⁹. 10¹⁹ ÷ (8 × 10⁹) = 1.25 × 10⁹.[2]
- A trap catches 40 animals, of which 26 are insects. What percentage are insects? 65%. 26 ÷ 40 × 100 = 65%.[1]
- Why are estimates of the total number of insect species so uncertain? Most species are probably not yet described. Estimates of undescribed species range from about 2 to 30 million.[2]
Going further
Pool the class data from several groups and test whether the difference between habitats is significant, or repeat the survey in a different season.
Sources used in this lesson
- [1] What is an insect? (Smithsonian National Museum of Natural History): reliability High
- [2] Numbers of insects (species and individuals) (Smithsonian Institution (Encyclopedia Smithsonian)): reliability High
- [4] Insect life cycles (fact file) (Amateur Entomologists' Society): reliability Medium-high
- [5] Biology 2e, 28.6 Superphylum Ecdysozoa: arthropods (OpenStax (Rice University)): reliability High
- [6] What is entomology? (Royal Entomological Society): reliability High
- [7] How to make a pitfall trap to catch insects (Natural History Museum, London): reliability High
- [8] Speaking up for the little ones: why bugs need our help more than ever (Natural History Museum, London): reliability Medium-high
Full list with links and a note on each source's reliability: echoesof.science/school/insects/sources/
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