The diverticulated crop of adult Phormia regina

Abstract

The crop of adult Phormia regina consists of a duct that diverges from the esophagus, just in front of the cardia, and extends ventrally and posteriorly into the thorax and abdomen where it forms a bilobed sac. Flattened epithelial cells produce the cuticular lining of the crop. When empty, or partially full, the epithelial cells and cuticular lining form folds extending into the lumen, thus providing for expansion as the crop sac fills. Covering the sac on the hemolymph side is a layer of anastomosed, intrinsic muscles connected to one another by intercellular cytoplasmic bridges. Mitochondria are located at the periphery of the sarcomere. Also inside the sarcomere are glycogen, sarcoplasmic reticula, and transverse tubular systems (T-system). I, A, and Z-bands are present and the Z-bands are not in register making the muscle-type supercontractile. Important structures, not previously researched and associated with the crop muscles, are the crop nerves. Coming off the corpora cardiaca, and running down each side of the crop duct, is a pair of nerves, each housing several axons. These nerves extend to and branch over the crop sac. Here they penetrate the muscle mass and form neuromuscular junctions where electron-dense droplets of neurosecretion are released. Based on the literature, and research in our laboratory, it has now been shown that these nerves carry adipokinetic hormone, Drosophila insulin-like peptide, and a dromyosuppressin-like neuropeptide.