Figure 3: Spiracle use in Polypterus air breathing.

(a) Kinematic events during spiracular air breathing in P. delhezi (numbered black dots, means±s.d.) in relation to the pressure oscillation in a 70-ml closed air chamber showing an initial pressure rise caused by air-breath release followed by an abrupt pressure drop during the onset of inhalation. Kinematic events: (1) Body wall begins to compress as air is forced out of the lungs (time=0); (2) Buccopharyngeal chamber begins to expand; (3) Exhaled breath begins to leave the opercular openings; (4) Buccopharyngeal chamber begins to compress to force additional air out the opecula; (5) Spiracles open; (6) Exhaled breath has completely left the opercula; (7) Body diameter begins to increase as air is drawn in through the spiracles and enters the lungs through recoil aspiration; (8) Buccopharyngeal chamber begins to expand drawing air entering the spiracles into the buccopharyngeal chamber; (9) Spiracles close; (10) Buccopharyngeal chamber begins to compress forcing additional air into the lungs; (11) Buccopharyngeal chamber compression ends signaling the end of lung inflation. Kinematic events are the mean of 20 analysed air breaths; for visual simplicity only a single representative pressure trace is shown. (b) Percentage of total air breaths taken through the spiracles for four Polypterus species without a blind (190 air breaths observed) and behind a blind (542 air breaths observed).