Triggering of Insect Spiracular Valves 



JOHN BUCK 



National Institute of Arthritis and Metabolic Diseases, National Institutes 

 of Health, Bethesda, Maryland 



IN MOST INSECTS the spiracles, or external openings of the respiratory system, 

 are provided with valves. The anatomy of the valves differs greatly in 

 different species, but the basic closing mechanism usually involves a chitinous 

 lever which, in one position, holds one lip of a valve across the opening or else 

 flattens a tubular chamber leading inward from the external opening of the 

 spiracle (9). A common arrangement for moving the lever involves an elastic 

 ligamentary opener and a muscular closer. Thus the insect must do work to 

 keep the spiracles closed, whereas the opening is a passive process involving 

 relaxation of the muscle. In accord with this fact, the spiracles are ordinarily 

 found open in anesthetized, hypoxic or dead specimens. 



Oxygen lack or carbon dioxide excess in the atmosphere surrounding an in- 

 sect stimulates spiracular opening, and each gas modifies the other's effect in 

 a definite and predictable fashion (10, 16, 6, 14). It therefore seems reasonable 

 to infer that these gases could be involved in the normal control of the spiracles. 

 The rapid and readily reversible responses to carbon dioxide and oxygen sug- 

 gest that at least their immediate effects are due to direct local action at the 

 spiracles rather than to general systemic alterations such as tissue hypoxia, 

 narcosis, etc. (6). However, it has not been shown directly that the valves re- 

 spond to normal changes in C(\ and O^ concentrations in the internal gaseous 

 environment as well as to abnormal changes in the external gas phase. 



In point of fact, the mechanism of even the external gas effects is obscure. 

 Central innervation of the spiracles has been described in various insects, and 

 in some spiracles the tonic contraction of the closer muscle is maintained by 

 neural stimulation and its relaxation brought about by diminution of nervous 

 activity (ref. 8 and personal communication). In species which normally have 

 a marked ventilatory rhythm it seems clear that the observed synchronous 

 and coordinated activity of the dozen or more spiracles must be centrally con- 

 trolled (cf. also ref. 8). Even in such species, however, the valves may also act 

 individually under certain conditions, a single spiracle, for example, responding 

 to a localized jet of CO2 (10). In still more striking experiments it has been 

 shown that denervated spiracles close in response to increase in ambient CO., 

 concentration (10, 16, 17, 8, i). The mechanism of the (X).. effect is itself un- 

 clear, quite aside from the question of whetlier it is direct or indirect. Spiracular 



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