Gaseous exchange in insects

• Life on land has a continual conflict with getting enough oxygen, and not losing too much water.
• Flatworms have a flattened shape which increases its surface area to volume ratio and allows a short diffusion path for diffusion. It lives in aquatic environments.
• Lugworms live in a sandy/muddy environment in u shaped burrows. This burrow is covered by seawater which circulates within it. Oxygen diffuses into the lugworm's blood from this water.
• When the tide goes out, lugworms do not have a supply of fresh oxygenated seawater so it uses up the oxygen from the water in the burrow.
• Terrestrial organisms are ones which live mostly on land.
• Water is always lost from gas exchange surfaces of terrestrial organisms as they are permeable to all molecules and there is a higher concentration of water inside the organism than out, causing a concentration gradient and diffusion.
• Earthworms are multicellular, terrestrial organisms that are restricted to damp areas. They have a moist body surface, a large surface area to volume ratio due to their elongated shape and a circulatory system transporting blood with haemoglobin.
• Earthworms have a low oxygen requirement as they are not very active organisms so have a low metabolic rate.
• Insects are covered in an impermeable structure called the cuticle which is made of chitin. This prevents water loss to the surroundings however oxygen and carbon dioxide are unable to diffuse through it.
• To overcome the impermeability of the cuticle, insects have holes in it called spiracles.
• Spiracles are paired holes running along the side of the insect body and lead into a system of tracheae: branched, chitin-lined air tubules which branch into smaller tubules called tracheoles.
• Muscle fibres in insects do not exceed 20 micrometers in diameter to ensure a short diffusion pathway to centre of muscles.
• Most gas exchange occurs in the end of the tracheoles and in cells with very high metabolic rates to ensure an adequate supply of oxygen.
• At rest, insects realty on diffusion across their spiracles, tracheae and tracheoles .
• During activity/flight, abdominal movements ventilate the tracheae, increasing rate of diffusion of gases into cells.
• Inspiration in insects is passive process.
• End of tracheoles contain tracheal fluid. During intense activity, insects anaerobically respire producing lactic acid in cells, lowering water potential. This means tracheal fluid moves into cells via osmosis thus reducing the volume of tracheal fluid in the tracheoles which causes air to be drawn further down the tracheoles.
• Tracheal fluid contains water and walls of tracheoles are moist meaning water can evaporate and be lost via spiracles. Each spiracle contains a muscular sphincter that allows spiracles to close stopping this water form being lost.
• When insect cells respire, oxygen is used up and its concentration in the tracheoles falls, this forms a concentration gradient leading to the cells. After respiration, carbon dioxide is produced , increasing its concentration forming a concentration gradient that forces it to move out of the tracheoles and into atmosphere.
• For larger more active insects, muscles can contract flattening out the body, decreasing the volume of the tracheal system, forcing air out in expiration.
• In some insects, tracheae contain expanded sections called air sacs. Changes in volume of the thorax and abdomen can squeeze the air sacs causing air to move from sacs into the tracheoles.
• Diffusion can occur effectively across surfaces up to 1cm thick.