TRACHEAL FILLING IN SCIARA LARVAE MARGARET L. KEISTER AND JOHX B. BUCK Laboratory of Physical Biology, National Institutes of Health, Bcthcsda 14, Maryland INTRODUCTION During insect development the gas-filled tracheae of a given instar become en-closed in larger, liquid-filled, coaxial ttihes which are to form the tracheal system of the next instar. At molting the old tracheae are withdrawn and shed with the body cuticle. The new system then fills with gas, either immediately or within a short time, the liquid probably passing through the tracheal wall into the blood or tissues (Weismann. 1863; Palmen, 1877;Keilin, 1924, 1944;Davies, 1927 ; Wiggles-worth. 1938; Keister, 1947, 1948). We shall call the process by which gas re-places the tracheal liquid "tracheal filling." It is frequently assumed that the gas which appears in the new tracheae of in-sects with open tracheal systems is atmospheric air which has entered through the spiracles. This assumption is supported by the fact that in some insects the tracheae do not fill with gas unless the spiracles are exposed to free air. In other species, however, the tracheae can fill with gas even when the larva or embryo is submerged in water or surrounded by amniotic fluid, and many aquatic insects with closed sys-tems normally fill their tracheae without apparent contact with an external source of gaseous gas (for literature see Keilin, 1924, and Sikes and Wigglesworth, 1931). It thus appears that in different insects there are at least two distinct mechanisms of tracheal filling, differing as to the source of the gas. A number of explanations of tracheal filling have been proposed. Weismann claimed that growth of tracheae (increase in diameter and number) continues after a change in permeability of the lining prevents entry of further liquid. The original liquid then retreats into the finer branches where it is absorbed, and gas enters from the outside or diffuses in from the tissues to occupy the increased tracheal volume. Major objections to this idea are the facts that, at least in Sciara, the tracheae have the same diameter when originally laid down as when they fill, and that no new branches are added near the time of filling (Keister, 1948). Stadtman-Averfeld (1923) suggested that the violent body movements at molt-ing expel the liquid from the tracheae, after which air enters through the spiracles. Tillyard (1916). from the equivocal observation that tracheae collapsed when dragonfly larvae were put in KOH, proposed that the gas in the tracheae was CO? which diffused in from the hemolymph and displaced the liquid. Keilin (1924) pointed out several difficulties with this hypothesis. An additional objection is that the hydrostatic pressure in the tracheal liquid would have to be less than in the body liquids in order for gas evolution to be confined to the tracheae. Filling has been attributed to reduction in body hydrostatic pressure at hatching (Davies) or after eclosion (Fraenkel, 1935). However, such reduction would not bring about the disappearance of the tracheal liquid unless the pressure had pre-323
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