ABSTRACT
In Sciara larvae exposed to total anoxia before moulting, all visible movement and all visible change in the content of the tracheal system cease. Moulting and tracheal gas-filing can be postponed at least
hr. beyond normal time.In most third-stage larvae exposed to 0·3-0·75 % O2 before the third moult, the future fourth-stage tracheal system, which is present fully-formed in the body, fills with gas. This shows that although moulting invariably precedes gas-filling under normal circumstances it need not do so.
In premoult larvae which have filled their trachea with gas upon exposure to 0·3-0·75 % O2, the tracheae fill again with liquid when the larvae are put back into atmospheric air. This reversal of gas-filling can be alternated with gas-filling several times in the same individual.
The fact that in reversal of gas-filling an increase in pO2 promotes liquidfilling, whereas in moulted larvae it not only never leads to liquid-filling but actually accelerates gas-filling, indicates that some basic, but at least temporarily reversible physiological or chemical change occurs in the tracheae or in the metabolism of the peritracheal tissue, near the time of moulting. A partial explanation of the observed phenomena can be made in terms of a combination of active uptake and physical uptake of tracheal liquid. Evidence for the existence of both types of mechanism, separately, has been adduced by Wigglesworth in other material.
Although this is a logical and attractive proposal, changes in tracheal gas content are not necessarily linked to movements of liquid. Herford (1938), for example, has shown that flea tracheae may alternately inflate with gas and deflate, due, probably, to respiration, and in the Phormia larva Buck & Keister (1956) have demonstrated both tracheal collapse and inflation caused purely by gas diffusion.
Such larvae pupate apparently normally but do not complete adult development. The fact that they can live as fourth-stage larvae for 5 or 6 days restricted to cutaneous respiration indicates that the tracheal system may not be important in normal respiration. However, the tracheal system is not of the closed type but has the normal dipteran complement of open spiracles, as shown by Keister (1948) and Keister & Buck (1949a). The fact that gas appears first in the interior of the tracheal system, therefore, is not because entry of air through the spiracles is impossible.
This deduction is not confined to active uptake mechanisms, nor can we conclude from it that the liquid is taken up through the walls of the large tracheae (though instances of gas-filling from two ends of an unbranched trunk show that this can occur: Wigglesworth, 1938 ; Keister & Buck, 1949a). A further point indicating physical influences on locus of first filling is our observation that filling often seemed to start at a tracheal fork. Keilin’s fig. 2 suggests the same thing.
A further indication of non-effect of muscular activity per je was obtained in putting a drop of ethyl ether on the cover-glass of the gas chamber so as to chill the cover and renew by condensation the water film around occasional larvae which had begun to become dry from long sojourn in flowing gas. The chilling caused the larvae to contract suddenly and extremely, after which they slowly reelongated as the cover-glass warmed. Even repeated chilling, however, did not induce gas-filling in anoxic larvae. When larvae with partially gas-filled trunks were so chilled the tracheal gas first contracted (i.e. retreated), then expanded again to its original position.
