442 THE INVERTEBRATA 



hoppers (Fig. 308), the first four pairs are open at inspiration and 

 closed in the expiratory phase, while the last six pairs are open in the 

 expiratory phase and closed at inspiration. It follows that an air 

 circulation through the main trunks is set up, aiding considerably in 

 the diffusion of gas through the whole system. Air sacs (as mentioned 

 above) in the form of thin-walled diverticula of the main tracheae 

 occur in many insects (Fig. 308), particularly those, such as bees, 

 migratory locusts and house-flies, with the power to fly for prolonged 

 periods. These also assist considerably in the circulation of air 

 through the tracheal system owing to the ease with which they can 

 be compressed. 



Thus to assist respiration in typical insects a neuro-muscular 

 mechanism has been evolved which ensures some control of the 

 ventilation of the tracheal system. Spiracular closing mechanisms 

 and compressible air sacs are important in this process. But though 

 a circulation of air certainly does take place in some, there are forms, 

 such as lepidopterous larvae, which exhibit no respiratory movements 

 and so, it may be inferred, possess no means of ventilating the air 

 tubes. Forces of diffusion have been shown to be adequate to supply 

 oxygen to the tissues of such examples as have no ventilating mechan- 

 ism. These same forces will also explain the transfer of oxygen from 

 the wider to the narrower air-containing tracheae. With regard to the 

 ultimate problem concerning the way in which the air reaches the 

 cell, modern theory on insect respiration assumes that the blind ends 

 of the tracheolar tubes are bounded by a membrane which is im- 

 permeable to lactic acid and such metabolites. Each tracheole con- 

 tains a variable amount of fluid, the height of the column of which is 

 determined in a state of equilibrium by hydrostatic pressure and 

 capillarity on the one hand and by forces of osmotic pressure in the 

 tissue fluids and of atmospheric pressure on the other. If now the 

 osmotic pressure of the tissue fluids, for any reason, increases, water 

 will then be absorbed from the tracheole tubes and the column of air 

 will be made to extend more deeply into the tissue. It has been shown 

 that muscular activity of insects is associated with such withdrawal 

 of water from the tracheoles. The evidence points to the conclusion 

 that the change from glycogen to lactic acid which accompanies 

 muscle contraction would provide the necessary osmotic changes to 

 withdraw water from the tube and so bring the column of air to the 

 tissues when and where their need is greatest (Fig. 310). From the 

 air column, thus brought deeply into the tissues, the oxygen must 

 diffuse into the surrounding tissue fluids. 



The control of respiratory movements by nerve centres is of in- 

 terest. Though each nerve ganglion of the ventral chain serves as a 

 centre for the respiratory movements of its own segment, there are 



