TERRESTRIAL INSECTS -GAS EXCHANGE

==Adaptations for Gas Exchange== [image:http://i.imgur.com/D0SJaSg.png?3] ===Spiracles=== Gasses enter / leave the insect through small openings called spiracles. Spiracles are often lined with fine hairs / bristles that '''filter air''' as it enters to prevent the gas exchange system getting clogged which would reduce the '''surface area''' available for gas exchange. This is an adaptation to the often dry and dusty environments in which insects live. The spiracles contain valves that can close in order to prevent water loss, although this also limits gas exchange. The tiny '''hairs''' surrounding the spiracles also help to trap humid air reducing the concentration gradient of water vapour which '''reduces water loss'''. Preventing water loss helps to ensure that gas exchange surface remains moist (a requirement for gas exchange). ===Tracheal Tubes=== Insects have an impermeable exoskeleton and internal gas exchange system in order to prevent water loss (dessication). This is an adaptation to their terrestrial (land) lifestyle. Insects are found in some of the driest places in the world. Because the exoskeleton is impermeable respiratory gases must move into the insect through a series of internal tubes called tracheae or tracheal tubes. These tracheae are highly '''branched''' helping to increase the '''Surface area : volume ratio''' available for gas exchange. ===Tracheoles=== The tracheal tubes tubes end in tiny '''fluid lined''' tubes called tracheoles. Respiratory gasses '''can dissolve''' into this fluid and '''easily diffuse''' into surrounding tissues because the tracheoles have such '''thin''' walls. The fluid lining the tracheoles normally fills the ends of these small tubes. During muscle contractions when more oxygen is being used / carbon dioxide is being produced the '''fluid is drawn into surrounding tissues'''. This further increases the '''surface area'''available for gas exchange. The small size of insects mean that they are often lower in the food chain. Flight is often an essential mechanism to escape predation, however, it demands considerable amounts of energy. The increased surface area helps the insect meet the respiratory demands associated with an increased rate of cellular respiration. ===Taenidia (Chitin Rings)=== The surface of the tracheal tubes is lined with a '''spiral fold of chitin''' (a strong but light-weight material). This acts like a reinforcing wire that '''keeps the airways open''' during body movements, while allowing some flexibility. This ensures that the gas exchange surface is ventilated - that a concentration gradient is maintained. Because of the short distance between the insects exterior surface and internal tissues, ventilation can often occur by passive diffusion of air in / out of the tracheal tubes. Without the taenidia external forces (due to gravity / body movements) would compress the tracheal tubes and prevent gas exchange. ===Air Sacs=== In some insects '''air sacks''' help to '''ventilate''' (push air in / out of) the tracheal tubes when the insect is moving. The muscular contractions associated with body movements (especially flying) also rhythmically compresses the tracheal tubes facilitating ventilation. Ventilation ensures that fresh, oxygen rich / carbon dioxide poor air reaches the gas exchange surface (tracheoles). This ensures an adequate '''concentration gradient''' for gas exchanges is maintained especially during vigorous activity. The small size of insects mean that they are often lower in the food chain. Flight is often an essential mechanism to escape predation, however, it demands considerable amounts of energy. The increased surface area helps the insect meet the respiratory demands associated with an increased rate of cellular respiration. ==Advantages== *An internal gas exchange system with an impermeable exoskeleton and spiracles that can open and close helps to protect gas exchange system and prevent water loss, allowing insects to live in some of the driest places on earth. *Short diffusion distance for gasses to move through tracheoles means that no transport system is required (saves space, weight and energy). Oxygen moves down tracheoles by passive diffusion, dissolves into the fluid at the end of each tube and diffuses directly into each cell. Therefore oxygen reaches the respiring cells more quickly. *Rhythmic contractions of abdominal muscles compress air sacs increasing ventilation which helps to maintain a concentration gradient during vigorous activity such as flying. This increases the rate of gas exchange when the insect is more active and has an increased metabolic demand (needs more oxygen). ==Disadvantages== *Limits size / is only efficient in smaller organisms. In larger organisms the number and length of tracheal tubes needed would significantly increase the organisms weight / volume, restricting movement. Also, tracheoles are not extensively ventilated and the diffusion distance would be too great and it would simply take too long for gases to diffuse down the tracheoles. Because insects must be small in size they tend to be primary consumers (eat plant materials) rather than larger predators. *Does not work under water. Diffusion of oxygen is slower through water than air. Oxygen would move too slowly through fluid filled tracheal tubes. Water is denser and more viscous and could not easily be ventilated (forced in and out of the tracheal tubes). *Air flow is '''tidal''' (in / out) and not all of the air can be forced out of the tracheal tubes. Some oxygen poor air always remains and mixes with fresh air coming into the tracheal tubes, slightly reducing the concentration gradient and making gas exchange less efficient.