Gas Exchange in Insects Instead of having lungs and a circulatory system to carry O2 and CO2 like you and me, insects have a tracheal system to carry air directly to and from every cell in their body.

Gas exchange in insects – the tracheal system Insects do not breathe through their mouths like other animals. Instead, they breathe through tiny holes along the sides of their body called spiracles. The spiracles connect to tubes called tracheae or tracheal tubes and these branch into microscopic tubes called tracheoles that reach right in amongst their body cells. O2 and CO2 travel in air through this tracheal system to and from the body cells by diffusion.

air sacs air rich in O2 air rich in CO2 air chitin bands in tracheal tubes

The Tracheal System Spiracles – tiny holes along the sides of an insect’s body (up to 10 pairs in total). These are the openings that let air in and out of the tracheal system. Tiny hairs lining the spiracles trap dirt and dust, preventing them from entering and blocking up the fine tubes in tracheal system. Valves, controlled by tiny muscles, allow the insect to open and close its spiracles to reduce water loss. This has enabled insects to live successfully on land, particularly in dry habitats.

The spiracles are lined with tiny hairs that trap dust and dirt preventing them from entering the tracheal system.

The spiracle mechanism The spiracles can be opened and closed to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. Relaxing the muscles opens the spiracles.

The Tracheal System Tracheae – tubes leading from the spiracles and branching throughout the insects body. The tracheae or tracheal tubes are strengthened and held open by bands of chitin. This ensures that air can always pass freely through the tracheal system.

Tracheoles – microscopic tubes at the ends of the tracheae that reach in amongst the body cells. Tracheoles are the gas exchange surfaces. They are thin-walled and do not have chitin bands. This enables O2 and CO2 to diffuse easily through the tracheoles to and from the body cells. The tracheoles have fluid at their ends to keep them moist, as O2 and CO2 must dissolve in water as they pass through the gas exchange surface. The millions of microscopic tracheoles in an insects body provide a very large surface area for gas exchange.

Air sacs – are present in the tracheal system of some insects, particularly larger ones. As the insect moves, particularly during flight, its body muscles squeeze on the air sacs making them expand and contract. This increases air flow through the tracheal system. Many insects also make pumping movements with their abdomen to move air in and out of their tracheal system. Pumping air through the tracheal system is called ventilation and is the insect’s way of “breathing”. Ventilation helps to make gas exchange more efficient and is particularly important for larger or more active insects.

Air movement through the tracheal system Air enters spiracles and passes along tracheae and tracheoles Air rich in CO2 passes out of the tracheal system through the spiracles. O2 from the air dissolves in fluid at the ends of the tracheoles. CO2 passes into the air in the tracheae. Dissolved O2 passes from a high conc. in tracheole to a low conc. in cells by diffusion. Dissolved CO2 passes from a high conc. in cell to a low conc. in tracheole by diffusion.

Insects in different habitats – dry climates Muscles controlling the spiracle valves enable the insect to open and close them. In dry climates insects avoid losing valuable body moisture by controlling the size of their spiracle openings to match their O2 requirements. During flight the spiracles are fully open but at rest, with less oxygen demand, the spiracles partially close.

Aquatic habitats Aquatic life poses particular challenges for this primitive breathing system. Some insects, eg-mosquito larvae, obtain air from above the water surface with a specialized breathing tube called a siphon that connects to their tracheal system. Other insects, like water boatmen and some aquatic diving beetles, stay submerged for long periods of time by trapping a bubble of air against tiny hairs on their thorax and abdomen.

Gas exchange in aquatic insects Insects that are fully aquatic eg – mayfly nymphs, have tracheal gills that extract O2 from water by diffusion. Instead of spiracles, they have gills which are leaf-like, thin-walled extensions of the body along the sides of the abdomen. Tracheal gills

Gas exchange in aquatic insects Tracheal gills do not contain blood vessels like the gills of fish, but contain fine tracheal tubes which absorb O2 from the water by diffusion. Some insects can move their tracheal gills like paddles for ventilation and can hold them flat against their body to protect them from damage. Tracheal gills provide a very large surface area for gas exchange. This is important because water contains a much lower % of dissolved O2 than air.

Advantages of a tracheal system The tracheal system is a very efficient system for gas exchange because: - it is simple – air is delivered directly to all the body cells without the need to carry O2 and CO2 in the blood. - it is internal so the tracheoles are protected from damage and kept moist for gas exchange. - the microscopic tracheoles provide a very large surface area for gas exchange. - ventilation enables air to be pumped through the tracheal system to meet the needs of larger or more active insects. - valves in the spiracles allow the insect to control the entry and exit of air to meet its needs and to reduce water loss in dry habitats.

Limitations of a tracheal system The tracheal system limits the size that an insect can grow to because: - The diffusion of gases in a tracheal system, even with ventilation, is effective over short distances only so insects cannot be too large or the gases would not reach all of their cells. - The tracheal system has a large mass so if insects get too large it becomes too heavy. - Because insects rely solely on diffusion of air for gas exchange and do not carry O2 and CO2 in their blood, this restricts the size they can grow to.

Insect Gas Exchange websites http://www.szgdocent.org/resource/ff/f-arth2a.htm (exoskeletons) http://www.earthlife.net/insects/anatomy.html (insect morphology and anatomy) http://everest.ento.vt.edu/~carroll/insect_video_dissection.html (Cockroach dissection) http://www.agsci.ubc.ca/courses/agro/327/AGRO327.lecture.07.pdf (spiracles and trachea) http://www.mic-d.com/gallery/oblique/insectspiracle.html (spiracles)