Insects: structure, gas exchange and abundance
Insects as arthropods, the tracheal system, growth by ecdysis and metamorphosis, the uncertainty in insect numbers, pollination and insect decline, and a two-habitat pitfall-trap investigation.
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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.[5]
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.[5]
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).[4][5]
Numbers are uncertain. About 900,000 to over one million insect species have been described; estimates of undescribed species range from about 2 million to 30 million. An estimated 1019 (10 quintillion) insects are alive at any time, and insects probably have the largest biomass of all land animals.[2][6]
An estimated 85% of all known species are arthropods, and insects are the largest group of arthropods.[5]
About 84% of crops grown in Europe rely on pollinators. Long-term trapping in Germany recorded a fall of over 75% in flying insects over 27 years; suggested causes include climate change, agriculture, pesticides, artificial light and habitat loss.[6][8]
Try it: Comparing ground invertebrates in two habitats
You need
- 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.
What to do
- 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.
What happens, and why?
Pitfall traps sample animals active on the ground surface. Comparing habitats with the same number of identical traps, set and collected at the same time, keeps the test fair. The spread between traps shows how much the results vary; if the ranges overlap a lot, the difference may be due to chance. Pitfall traps under-sample animals that climb or fly, so they measure surface activity, not total abundance.[7][1][5]
Play: Insect, arachnid, crustacean or myriapod?
To which group of arthropods does this animal belong?
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Quiz
Other ages: Ages 6–7 · Ages 10–11