Do ants even breathe? If they don't, how do they stay alive? On what resources do they depend upon to stay alive? How are they different form mammals?
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2 Answers
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The main difference between tracheal gas exchange, and other forms of gas exchange (except simple diffusion) is that it is generally a passive process. Organisms with lungs, gills, or other modified respiratory organs can actively pump the oxygen-containing medium (usually air or water) across their respiratory surface, and some also pump their blood across in the opposite direction to cause a countercurrent exchange to enable maximum efficiency of oxygen/CO₂ transfer.
The tracheal system is a more efficient mode of gas exchange than diffusion since it does not only involve the oxygen passing over the organism's body surface (a semi-permeable membrane) which limits the rate of movement. Instead the epidermis is invaginated to form tracheae (tubes) through which air can travel passively. The tracheal system is highly branched and terminates at tracheoles (fluid-filled sacs) which closely line respiring tissues to provide oxygen and allow release of CO₂.
Tracheal respiration limits the size of insects since their oxygen demands will not be able to be met if the air has to diffuse very far. Some insects require more oxygen than others: many larger insects and more active insects have evolved to supplement the tracheal system with pumps, gas gills (permanent or temporary bubbles of air which aquatic insects carry), or mechanically ventilated air sacs (honeybees have these, see Gullan & Cranston, 2005; and Snodgrass, 1956).
Furthermore, tracheal systems can be 'open', 'closed', or the insect can engage in discontinuous gas exchange. Insects with open tracheal systems have spiracles (holes in their cuticle). Closed tracheal systems are more common in aquatic insects, they come in two major designs: highly branched systems over the internal surface, allowing cutaneous gas exchange; and filamentous or lamellate arrangements, which are analogous to a primitive gill (see Wigglesworth, 1964 or Gullan & Cranston, 2005 chapter 10 for more on aquatic insect adaptations). Discontinuous gas exchange involves co-ordinated opening and closing of different spiracles which results in a uni-directional current allowing more effective ventilation.
References
Gullan, P.J. & Cranston, P.S. (2010) The Insects: An Outline of Entomology. John Wiley & Sons.
Snodgrass, R. (1956) Anatomy of the honey bee, Comstock Publ. Assoc. Ithaca, New York,.
Wigglesworth, S.V.B. (1968) The Life of Insects. New American Library.
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$\begingroup$ Comprehensive and logical answer, +1 $\endgroup$ Jun 27, 2012 at 16:30
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Insects breathe by simple diffusion through the cell walls. They do not breathe in the same sense that we humans do. They intake oxygen through spiracles in the insect's cuticle. It goes through the insect's trachea by dissolving into the tracheal liquid. From there it diffuses throughout the insect in tracheal tubes. At the ends of the tubes, the oxygen is taken by body cells.
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5$\begingroup$ Some insects are able to pump their abdomens in order to move air through the tracheae more effectively. $\endgroup$– jonscaMar 8, 2012 at 5:26
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$\begingroup$ And how are they different form mammals? $\endgroup$– user594Mar 9, 2012 at 10:51
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2$\begingroup$ That is, by the way, one of the constraints that limit the size of insects. $\endgroup$– nicoJun 27, 2012 at 18:02
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2$\begingroup$ @Zenon as an example: pnas.org/content/104/32/13198.short and: rspb.royalsocietypublishing.org/content/277/1690/1937.abstract $\endgroup$ Jun 27, 2012 at 22:37