Insects

• Insects living on land have to cope with obtaining oxygen from the air without dying of dehydration; two conflicting needs: need for exchange of respiratory gases and need to reduce water loss
• Insects have a rigid exoskeleton and are covered by an impermeable cuticle which prevents water loss from body cells to the surroundings but also means that oxygen and carbon dioxide are unable to diffuse through it
• It’s vital for insects, especially those in deserts, to be able to exchange respiratory gases without losing too much water, so the insect has developed small holes in the cuticle called spiracles
• Spiracles are connected to the insect’s internal organs and tissues by a network of tubes called tracheae which then branch into narrower tracheoles (the tracheal system), these tubes are reinforced with rings of chitin
• Most gas exchange takes place at the tips of tracheoles and in cells with very high metabolic rates to ensure an adequate supply of oxygen for respiration
• When an insect is less active, the ends of the tracheoles contain fluid; as activity increases, fluid is removed because muscle cells produce lactic acid, which lowers the cell’s water potential
• Means that the gas exchange occurs nearer the cells and happens quicker; when cells actively respire, O_2 is used up and so its concentration at the end of the tracheoles falls creating a diffusion gradient for O_2 towards the cells
• CO_2 is produced by respiring cells, creating a diffusion gradient with the atmosphere and so CO_2 moves out of the insect
• Insects have the ability to close spiracles using valves in the cuticle to reduce water loss (some even have hairs) this reduces water loss or prevents excess oxygen from entering their cells (particularly during periods of rest/inactivity)
• They have no transport system, such as blood, is involved in moving oxygen or carbon dioxide around the body
• Although the tracheal system is a highly effective means of gaseous exchange, in most insects it relies on diffusion of O_2 through the body
• For larger, more active insects: Expiration; flight muscles contract and flatten the body, tracheal system volume decreases and air is forced out, Inspiration; achieved passively as the elastic nature returns them to their original shape