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TRACHEAE & Oxygen Diffusion

Insects, in general, do not have an oxygen-carrying chemical in their blood so oxygen reaches cells by other means.

Most insects have a waterproof cuticle but some insects live in moist areas and are sedentary. Their cuticle is permeable to water and they obtain sufficient oxygen by diffusion across their cuticle.

However, most insects have a special respiratory system comprising a system of internal tubes, called trachea, which branch and rebranch. Very fine branches, tracheoles, penetrate individual cells.

Trachae have spiral stiffening - like vacuum cleaner hose - to prevent collapse.

Air enters from the outside through a series of openings, spiracles. Typically there are 2 pair of spiracles laterally on the thorax and 8 pair laterally on the abdomen.

O2 from spiracles --> tracheae --> tracheoles --> cells

Rate of gas diffusion is determined by

So most of the path for oxygen to diffuse from the spiracle to the cell is through the air-filled tracheal tubes.



Insects need to avoid water loss through their spiracles and also to prevent contamination by dust etc.

Spiracles are therefore usually equipped with opening and closing devices and filtering lobes or hairs in an atrium before the beginning of the trachae.

Very active insects have internal air sacs, as extra reservoirs, as part of their tracheal system. They also may employ mechanical ventilation along the larger trachae. Bees and wasps may extend and telescope their abdomens to pump air along.



Many aquatic insects must return periodically to the surface to breathe.

Mosquito larvae have tubes connecting to the air at the water surface. There are hydrofuge hairs (water resistant) associated with the spiracles, to break the surface tension of the water and to keep water out when the insect is underwater.

Other insects, such as stone fly, dragonfly and mayfly nymphs have tracheal gills whereby oxygen diffuses from the water to fine, surface trachae.

Water beetles take a bubble of air with them underwater. Their spiracles open into the bubble As the oxygen in the bubble is used up, the oxygen pressure in the bubble drops to that of the surrounding water so that more oxygen diffuses into the bubble from the water. In this way the bubble remains usable for a long time.

Bloodworms, (midge larvae) live in the mud of stagnant pools where there is very little oxygen. These insect larvae are one of the few which have haemoglobin in their blood and this haemoglobin is more efficient than ours.

Small internal parasites may diffuse oxygen across their cuticle. Others may have spiracles at the end of siphons which open into the trachae of their host or penetrate the skin of their host into the air.


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