78 PHYSIOLOGICAL TRIGGERS 



cerned, to prolong the period of restricted spiracular valves (interburst) and 

 shorten the period of spiracular opening necessary to relieve CO2 accumulation 

 (burst). 



Whether the retention phase could be made so extreme as to more than com- 

 pensate for the exaggerated rate of water loss during the burst period is difficult 

 to predict. Computations using known and estimated parameters of the CO2 

 burst cycle in the moth Agapema indicate that only half as much water escapes 

 during the observed alternating periods of CO2 retention and release as would 

 be lost if the spiracles were held steadily in a partly open position just sufficient 

 to allow the metabolic CO2 to escape as fast as formed; but the uncertainties 

 of the measurements are sufficient that a larger differential would be desirable 

 before the case could be considered proved. The possibility is, however, at- 

 tractive, since it would on the one hand provide a neat example of adaptation 

 for water conservation, and on the other a rationale for what appears to be an 

 unusual or even illogical type of respiratory behavior. Actually, this type of 

 regulation may not turn out to be as unusual as it appears. Conceivably the 

 normal spiracular behavior of many resting insects, involving ostensibly the 

 same sorts of relatively long periods of valve closure and short intervals of open- 

 ing, may prove to be a much speeded-up version of the same behavior. In any 

 case it shares the same characteristic of 'regulation by averaged extremes,' or 

 'grading by alternating all-or-none responses.' 



SUMMARY 



The CO2 burst cycle in diapausing moth pupae and its underlying cycle of 

 spiracular opening and closure fail in many respects to measure up to an idea) 

 example of a triggered response. Not only is the response itself rather variable, 

 but the possible trigger is identified only indirectly. Furthermore, the valve 

 opening appears to involve muscular relaxation rather than contraction, as 

 would ordinarily be expected in a rapid response. In two respects, however, the 

 system has yielded quantitative evidence for triggering. First, it appears that 

 the triggering entity — probably COo concentration — increases very slowly and 

 uniformly up to the 'ignition point,' and second, the response, once started, 

 proceeds independently of the trigger. The cyclical nature of CO2 retention and 

 release, while not directly relevant to the trigger problem, is interesting in that 

 it suggests a possible physiological advantage obtainable from an exaggerated 

 alternation of overshoot and undershoot as compared with a steady state sort 

 of regulation. 



I am much indebted to Drs. Schneiderman, Beckel and Case for permission to refer to 

 unpublished data, and for suggestions. 



REFERENCES 



I. Beckel, W. E. and H. A. Schneiderman. The spiracle of the cecropia moth as an inde- 

 pendent effector. Anal. Rec. 125: 559-560, 1956. 



