Abstract
1. During the course of metamorphosis the heart of the Cecropia silkworm appears to undergo pronounced shifts in its sensitivity to cyanide.2. In the mature larva the heartbeat is promptly blocked by 10-3 M cyanide; in the adult moth it is even more sensitive and is brought to a standstill by 10-5 M cyanide.3. In the intervening pupal stage the heart is insensitive to acute poisoning by physiological concentrations of cyanide.4. This insensitivity is observed only in experiments of short duration. When the exposure to cyanide is continued for many hours, the pupal heartbeat is blocked by 10-2 or 10-5 M cyanide.5. The paradoxical response of the pupal heart can be accounted for in terms of a pronounced capacity for anaerobic metabolism which is peculiar to this particular stage. The pupal heart can beat for as long as 5½ hours in the complete absence of oxygen. During this same period the heart is insensitive to cyanide.6. While discounting any true insensitivity of the pupal heart to cyanide, the experime...
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