AQA Biology AS/A-level Year 1
Fig 1 The tracheal system in insects. Air enters tiny tracheoles that pass very close to each respiring cell. The distances involved are so short that diffusion is rapid and efficient enough to meet the insect’s needs.
More complex multicellular organisms have evolved specialist gas exchange organs such as lungs. Three more gas exchange solutions are studied here: gas exchange in insects, fish and plants.
Gas exchange in insects Insects solve the gas exchange problem in a unique way. They have a tracheal system that consists of a network of tubes that branch out and take air directly from the outside and into the respiring tissues (Fig 1). air-sac
air-sacs
spiral rings of chitin
mitochondria O2
nucleus body cells CO2
tracheae body surface
tracheole spiracles spiracle
trachea
Air enters through holes in the insect’s exoskeleton called spiracles. It then passes into tubes called tracheae (sing. trachea), which branch out into smaller tracheoles. The key difference between these two types of tube is that tracheae have thicker supportive walls whereas tracheoles have thin permeable walls that allow gas exchange. The tracheoles form a fine network of tubes that pass within a short diffusing distance of all the respiring cells in the insect’s body. Some tracheoles actually enter the cells, delivering oxygen directly to the mitochondria.
Essential Notes Compared to lungs and gills, this may seem a strange way to breathe air, but diffusion of O2 and CO2 through air is very rapid, making the insect’s system very efficient. However, it doesn’t work in large organisms – this is one reason why insects are limited in size.
Insects can open or close their spiracles to alter the level of ventilation, and to minimise water loss. Some insects with a high metabolic rate are able to increase ventilation by contracting the muscles of their abdomen.
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