74 



PHYSIOLOGICAL TRIGGERS 



gas is continuous with the ambient atmosphere (4), d) the spiracles show a cycle 

 of long closed and brief open periods corresponding to the timing of the CO2 

 release cycle (13, 14), e) the frequency and size of the bursts are influenced pre- 

 dictably by ambient CO2 and O2 concentrations (15, 4). 



In the present connection two facets of the CO2 release cycle are of special 

 interest: i) Is it a triggered response, and if so what is the trigger?; and 2) what 

 is the physiological significance of this system in which diffusion appears to 

 be controlled by alternating extremes rather than by maintained intermediacy? 



THE CO2 BURST CYCLE IN RELATION TO TRIGGERING 



Superficially considered, the CO2 burst appears to resemble certain triggered 

 events. First, the burst takes place in the absence of any environmental cue 

 or obvious abrupt physiological change within the pupa. Second, the start of 

 the burst, considered either volumetrically (fig. i) or from the standpoint of 



plHOUR^ 



Fig. 2. CO2 release pattern in pupa of Papilio macliaon. Time course from left to right, 

 with top trace continuous at .1 with bottom trace at A'. (Punt, 1950, fig. 12, p. 68.) 



the duration of the actual period between the start of opening and the fully 

 open position of the tracheal valve (fig. 3), is very sudden in comparison with 

 the interburst period of near closure which may last 20-1000 times as long 

 (15, 4). Third, the burst shows a rather marked constancy in volume (4). How- 

 ever, as has become apparent upon closer study of many ostensibly all-or-none 

 responses, the 'event' which occurs at the start of the burst of CO2 release is 

 considerably more complex than at first appears. In some records of spiracular 

 valve movement (fig. 3), it is clear that the valve is not totally closed through- 

 out the entire interburst period, nor does the change which occurs at the start 

 of the burst consist either of a simple shift from a closed to an open position or 

 of repeated full oscillations between the two positions, as in the chattering of 

 a relay. Neither, however, does it show a smooth transition through inter- 

 mediate rest positions as in a graded response. Rather, we see wide-amplitude 

 oscillations from the 'closed' position giving way rather abruptly to similar 

 oscillations from or around the open position. Spiracular oj^ening thus differs 

 from irreversible events, such as spore activation or infection. Although it is 



