40 THE BIOLOGY OF INSECTS 



blood acts as a carrier of oxygen to the tissues and of com- 

 bustion-products from them, fresh oxygen being obtained 

 and the waste water vapour and carbon dioxide given up, 

 as the blood passes through the fine vessels or passages of 

 the breathing organs, such as the lungs of terrestrial and 

 aerial creatures, or the gills of aquatic animals that are 

 dependent for their oxygen-supply on the air dissolved in 

 the surrounding water. 



In all typical insects breathing is carried on by means 

 of a set of branching air-tubes, a tracheal system ; the fine, 

 thin-walled terminal branchlets lead into minute tracheoles 

 whose deHcate walls are in closest contact with the various 

 organs and tissues of the body. Thus gaseous exchange is 

 effected directly between the insect's Hving substance and 

 the air contained in the tracheoles, the oxygen passing in 

 from the atmosphere and the carbon dioxide and water 

 vapour passing out to it by means of a set of diffusion 

 processes accompanying alternate intakes and expulsions of 

 air. The air- tubes of an insect (Fig. 13, tr) are, as already 

 mentioned, due to ingrowths of the skin, and are naturally 

 therefore lined with an extension of the chitinous cuticle. 

 It is surely suggestive that the oxygen of the free air which 

 surrounds the insect's body should be brought into touch 

 with the creature's inner tissues by a series of actual ingrowths 

 of the skin with its overlying cuticle, each ingrowth dividing 

 and branching repeatedly, pushing thus its way ever more 

 deeply among the masses of living cells which vary greatly 

 in form and function but are all aHke hungry for oxygen. 



For the greater part of their course an insect's air-tubes 

 have their chitinous lining thickened spirally, so that when 

 broken the wall of the tube shows the appearance of a par- 

 tially unwound thread (Fig. 13, tr). Below the firm lining 

 is the epitheHum or cellular layer which is the continuation 

 of the skin ; outside this is a thin, supporting '* basement 

 membrane." The spirally thickened chitinous lining of 

 the air-tube has an important bearing on the function of 

 breathing. It gives such firmness to the tube-wall that this 

 cannot collapse, yet it yields to some extent to the pressure 



