68 THE BIOLOGY OF INSECTS 



(p. 27). Similarly contractions of the thin muscular wall 

 of the heart are effective in propelling the blood, while by 

 the action of the abdominal muscles changes in the form and 

 capacity of the body are brought about so that through 

 varying pressure on the tracheal tubes air is drawn in and 

 expelled, the spiracles being rhythmically opened and closed 

 by means of their special musculature. Only a glance at 

 the highly complicated muscular system of an insect has been 

 possible, but it will be realised that the actions of the various 

 parts of this system must be co-ordinated if the creature's 

 movements are to have purposeful relation to the needs of 

 its Hfe. Some discussion of the mode of such co-ordination 

 will be found in the succeeding chapters. 



