JOHN BUCK 77 



the entire interburst period (which may last several hours) and that the spiracle 

 performs its opening within about a minute (fig. 3), the triggering change must 

 be of the order of hundredths of a per cent. The blood equilibration experi- 

 ments show likewise that the hydrogen ion concentration changes linearly with 

 ambient CO2 concentration and to the extent of only about 0.02 pn unit per 

 I per cent change in COo concentration. The fact that the spiracles, once open, 

 remain so until the internal CO2 concentration has fallen many times the 

 amount of the postulated critical increment, provides another point of similarity 

 with triggered responses. 



POSSIBLE RATIONALE OF CYCLIC REGULATION 



Almost all insects are highly vulnerable to desiccation, and it has long been 

 known that one of the major — perhaps the principal — function of the spiracular 

 valves is to reduce evaporatory water loss from the tracheal system. Spiracular 

 aperture must therefore always be a compromise between the necessity for 

 minimizing transpiration and the necessity for permitting entrance of ade- 

 quate O2 and escape of adequate CO2. In most insects the rate of O2 uptake is 

 not limited by ambient Oo until its concentration has fallen very markedly 

 below atmospheric. This means that respiration can be maintained with only 

 a low percentage of O2 in the tracheae and that the trans-spiracular O2 gradient 

 can be very steep. A comparably steep COo gradient in the opposite direction 

 could not, presumably, be established because the necessary tracheal CO2 

 concentration would be narcotic. At any rate, the spiracles of most insects open 

 when the external CO2 concentration reaches 3-7 per cent, and almost certainly 

 open also when the internal concentration reaches such levels, thus preventing 

 buildup above that tracheal concentration. Consequently the fact which limits 

 the reduction in spiracular valve aperture is tracheal COo concentration: O2 

 limitation is never a problem. 



A priori, it might appear that the relative rates of loss of water vapor and 

 COo would be the same for all possible spiracular areas. This is presumably 

 true for all conditions in which COo escapes as fast as it is formed. When COo 

 is actually impounded, however, tracheal Pcoo rises, while tracheal PH2O re- 

 mains constant, being a function only of the temperature. This means that 

 when tracheal PcOo is relatively high, CO2 will escape faster, in proportion to 

 the escape rate of water vapor through the same spiracle, than when no CO2 

 is being impounded and tracheal Pcoo is lower. In other words, restricting the 

 spiracles below the area which will permit COo to bleed off as fast as it is formed 

 will reduce water loss not only absolutely, but also relative to CO2 loss. 



However, COo accumulation cannot continue indefinitely, and when the 

 spiracles are finally forced to open the situation is reversed; the internal COo 

 concentration will fall and the relative rate of water loss will increase. It should 

 therefore be advantageous to the insect, as far as water conserv^ation is con- 



