The abdominal epidermis of the blowfly Calliphora erythrocephala. II. Imaginal histoblasts during larval life.

Abdominal histoblasts in Calliphora (Diptera) are situated within the larval epidermis, and secrete larval cuticle throughout the 3 larval instars. An optical and electron-microscopic study of histoblasts and epidermal sensilla suggests that the latter have been previously confused with histoblast Anlagen in the first instar. A steady rate of division until the mid-third instar leads to Anlagen growth, which is not seen in larval Drosophila. After a relatively quiescent period, division begins again, at a higher rate, at the time of pupariation (at the time when division first begins in Drosophila). During prepuparial larval life, the ventral, dorsal, and postero-dorsal Anlagen increase from approximately 6, 8 and 4 cells to about 350, 500 and 150 cells respectively. At the time of the second (puparial) mitotic phase there is also extensive reorganization of cellular fine structure in which large secondary lysosomes play an important part. The endoplasmic reticulum and Golgi subsequently present a different profile. At the same time numerous chromatic droplets appear in the rapidly dividing histoblast Anlagen. This division and the appearance of chromatic droplets are shown (and the subcellular reorganization is suggested) to depend upon the hormonal factors which are responsible for pupariation.