Haemocytes and the metamorphosing tissues in Sarcophaga bullata, Drosophila melanogaster, and other cyclorrhaphous Diptera

Abstract

Changes occurring in the haemocytes during metamorphosis in Sarcophaga bullata, Drosophila melanogaster, and other Diptera Cyclorrhapha are considered. Phases in the formation of the familiar ‘spherules’ are described in detail for Sarcophaga: the granular haemocytes, in the late larval and pre-pupal stages, undergo nuclear division to form multi-nucleate haemocyte masses; these send out extensive cytoplasmic processes, which become attached to pieces of larval tissues; subsequent retraction of the processes and ingestion of the larval fragments by the multi-nucleate haemocytes results in the formation of the spherules. Haemocytes were found attacking only tissue-fragments. Cytoplasmic filaments from the spherules are, at later stages, closely associated with the developing adult tissues, particularly hypodermis and tracheal epithelium, eye ommatidia, and developing flight muscles. The spherules, at least in the appendages, do not appear to circulate; they occupy specific locations and their contents are digested and utilized in situ. Their later phases may represent the adipohaemocytes of other authors. Late in the pupal stage the spherules would appear to contribute material towards connective tissue formation by adjacent cells. It is thought possible that food transfer may be direct by way of the cytoplasmic filaments and not indirect by way of the haemolymph. Many of the multi-nucleate haemocytes, depleted of their inclusions, persist into the adult, where some are found fragmenting. Others may become the multi-nucleate adult fat body cells, but further evidence for this is required. It is suggested that the larval fat body cells, as with the spherules, remain stationary during pupation and that cells of specific areas contribute specific materials to particular developing tissues. The various plasmatocytes are present throughout pupation, some passing through motile and inclusion laden phases, and many reverting to their larval appearance after emergence. Certain of the ‘oenocytoid’ homologies are questioned in connexion with cuticle darkening. Preliminary results in Drosophila suggest that in general the haemocyte picture parallels that of other Cyclorrhapha, with typical spherules and spindle-shaped plasmatocytes present in the pupal stage.