In “higher” flies (e.g. Musca dornestica L.), a pair of large air-sacs is found in the anterior region of the abdomen (Hewitt 1914). The main function of these air-sacs appears to be to maintain the extended external shape of the abdomen in newly emerged flies, because as the fat-bodies in both sexes, and also eggs in the female, grow, these air-sacs decrease in size, thus allowing room for these growing organs (Evans 1935). These air-sacs, and those elsewhere in the thorax and head, were believed to give buoyancy for insect in flight, to reduce the volume of circulating blood, and(or) increase the volume of the “tidal air” which is renewed at each respiration (see Wigglesworth 1965 for review). In our recent studies on the feeding of S. calcitrans, we noted that the change in the size of the abdominal air-sacs, besides making room for the growing abdominal organs, may minimize change in abdominal volume during feeding and digestion.