54 PHYSIOLOGICAL TRIGGERS 



pressure respirometer and their oxygen uptake recorded in air upon which 5 

 atmospheres of CO had been superimposed. Occasional experiments were per- 

 formed at atmospheric pressure. 



From a large number of inhibition experiments we learned that the respira- 

 tion of the diapausing pupa is completely insensitive to phenylthiourea, and is 

 inhibited less than 15 per cent by high concentrations of carbon monoxide 

 (CO/O2 = 25:1) or cyanide (io~^ m). This minor inhibition was accounted for 

 in terms of the cyanide- and carbon monoxide-sensitivity of the intersegmental 

 muscles of the pupal abdomen. The other tissues of the insect, or at any rate 

 those that contribute more than a trivial amount to the overall respiration, 

 were not inhibited (35). Indeed, simultaneous exposure of pupae to both lo"^ m 

 cyanide and 16:1 CO/O2 had only trivial effects on respiration although this 

 maneuver certainly left less than 5 per cent of the cytochrome c oxidase system 

 functioning (29). 



The situation in the developing adult is quite different. The termination of 

 the pupal diapause and the progress of adult development are accompanied 

 by a marked increase in respiratory sensitivity to cyanide and carbon monoxide. 

 The effect of these agents becomes no longer limited to the muscular tissue 

 but extends to the insect as a whole. Cyanide and carbon monoxide appear to 

 act exclusively on the extra metabolism accompanying development and reduce 

 the over-all metabolism to the old diapausing level (35). 



The onset of diapause immediately after pupation is accompanied by a rever- 

 sal of the above events: 6 hours after pupation, 30 40 per cent of the respiration 

 is inhibited by 16: i CO/O2; 48 hours after, 20 per cent is inhibited, as the insect 

 gradually loses the carbon monoxide- and cyanide-sensitive system. 



The insensitivity to carbon monoxide and cyanide in most tissues of the 

 diapausing pupa argues in favor of the presence and utilization of a terminal 

 oxidase other than cytochrome oxidase or tyrosinase. Cytochrome oxidase is the 

 principal terminal oxidase of the somatic musculature of the diapausing pupa. 

 Months later, with the termination of the pupal diapause, cytochrome oxidase 

 becomes the principal terminal oxidase of the growing post-diapausing insect 

 as a whole. Cytochrome oxidase is likewise the principal terminal oxidase of the 

 growing larva and prepupa and gradually disappears from most tissues of the 

 insect after pupation and the onset of diapause. 



These qualitative changes in the insect's metabolism from cytochrome oxidase 

 to non-cytochrome oxidase and back to cytochrome oxidase are synchronized 

 with the secretion of PGH. When PGH titer falls at the time of pupation the 

 cytochrome oxidase system disappears within a week. When PGH titer rises 

 at the onset of adult development the system reappears. However, these obser- 

 vations in themselves provide only circumstantial evidence that the coupling 

 of metabolism to cytochrome function is causally related to PGH action and 

 its biological end rcsuH, the termination of diapause and the initiation of devel- 



