152 Prof. R. von Lendenfeld on the Physiological 



Apart from tliose air-cavities that (in birds) extend in the 

 bones, and the object of which — the reduction of the weight 

 of tlie bones without impairing their strength — is distinctly 

 evident, all air-chambers are readily contractile and dilatable. 

 The spiral thread of chitin that stiffens the tracheae in insects 

 is absent from the walls of their larger air-chambers, and in 

 birds the walls of the air-sacs of the body are very delicate 

 and soft membranes. 



It appears certain that flying animals have the power, by 

 means of voluntary contraction of the body-muscles, to alter 

 very ra])idly the degree of fullness of these air-chambei-s (of 

 course with the exception of those that are situated in the 

 bones), and that thus they are enabled to displace their own 

 centre of gravity, and to change the specific gravity as well as 

 the size of their parts. But we now have to ask how changes 

 of this kind can be of so great an advantage to flight as to 

 explain the extraordinarily high degree of the development 

 of these cavities in good fliers. 



In insects the air-chambers have manifold connexions one 

 with another by means of various tracheal tubes ; in birds an 

 intercommunication between them does not appear to exist. 



The most spacious air-bladders, in birds as well as in insects, 

 are met with for the most part in the abdomen. In certain 

 cases (in dragonflies and the condor) tiie air-chambers of the 

 head are also very large. 



In the case of insects it might be supposed that by means 

 of a sudden discharge of considerable quantities of air from 

 the stigmata of one side, or from only one particular stigma, 

 a rebound might be produced which would entail a definite 

 and advantageous lateral or turning movement of the body. 

 A process such as this would be materially facilitated by the 

 connexion of the air-chambers of the two symmetrical halves 

 of the body. In the case of birds, however, an explanation 

 of this kind is naturally impossible. 



Since those birds that for the most part soar — tliat ia, 

 without strokes of their wings move, maintain themselves 

 aloft, and even ascend — possess quite peculiarly well-developed 

 air-chambers (I would remind the reader only of the pelican 

 and certain large Kaptores), it is natural to suppose that it is 

 precisely in soaring that these structures come most into play. 

 Since soaring requires only slight muscular labour, increasing 

 the intensity of breathing but little, their high degree of 

 development in birds whicii soar is an argument against the 

 assumption that the air-sacs are nothing more than accessory 

 respiratory organs. 



In order to be able to give a decision as to their mechanical 



