The Deposition of the Third Instar Larval Cuticle of Calliphora erythrocephala

Abstract

ABSTRACT A histological study has been made of the formation of the cuticle of the third instar of Calliphora. Increase in size of the epidermal cells and nuclei, particularly in the spiny regions, preceded the release of the moulting fluid. The cytoplasm first became vacuolated and this was followed by the appearance of many granules. The basement membrane is constituted in part by stellate tracheoblast cells. The membrane breaks down just before cuticle deposition and reappears after ecdysis. The outer epicuticle is formed from coalesced droplets aligned at the cell surface to form a delicate continuous, folded, acidophil, lipoprotein layer. Further, protein and lipoid (cuticulin) is added beneath this layer from cytoplasmic processes to form the inner epicuticle. These processes later became the pore canals. Endocuticle is secreted as filaments from the epidermal cells between and around the processes. During moulting, phenolic substances and oxidase are transported to the inner epicuticle only in the cuticular spines. Wax and cement layers are not formed and there are no dermal glands. The peak in the secretory cycle of the oenocytes corresponds to the time of deposition of the epicuticle and the secretory material shows similar staining properties to epicuticular material. The oenocyte groups are connected with each other and with the epidermis by cytoplasmic extensions, through which secretory granules pass. The muscle insertions are not attacked by the moulting fluid. They are thought to contain protein-bound sulphur. The separation of the muscle insertions and sensory organs from the exuvial cuticle occurs just before ecdysis. The nature of the epicuticle and the theory that the oenocytes are associated with its formation is discussed.