‘Killer’-cell-mediated destruction of dipteran eclosion muscles

Abstract

After successful escape from the puparium, dipteran flies lose sets of eclosion muscles located within the head, thorax and abdomen. The thoracic eclosion muscles are described, with details of the degeneration of one of them. This muscle is found to be attacked by a macrophage-like cell that attaches itself to areas around the Z-discs and sends processes along the length of the muscle that apparently stimulate autolysis. A dramatic increase in sarcoplasmic reticulum is observed within muscle fibres, which spreads from the location of the macrophage and is coincident with degenerative changes. Neither nuclei nor mitochondria are observed to degenerate within the muscle although giant mitochondria in certain fibres are found to undergo degenerative changes. Phagocytosis was not observed. The system is not affected by hormones released before eclosion nor by bursicon, a hormone released after eclosion, and a direct neural trigger is discussed. The results are compared to the action of ‘killer’ cells in the vertebrate immune system and contrasted with degeneration processes resulting from disuse.