Cell differentiation during the ontogeny of larval salivary glands of the fly, Telmatoscopus albipunctatus

Abstract

Using the larvae, pharate pupa, and pharate adults of the moth fly, Telmatoscopus albipunctatus, histological and ultrastructural features of the salivary glands were investigated. The gland lumen contains a milky secretion from the first instar. This secretion continues to ccur at all subsequent developmental stages; with the onset of the pharate pupal stage, however, the secretion becomes transparent and rather viscous. Histochemical tests revealed that it is mainly proteinaceous. Glands from the same developmental stage may respond differently to PAS-reaction. Various cell organelles were compared at consecutive stages of larval development and of secretory activity of the salivary glands. In first and second instar larvae autophagic vacuoles are virtually absent in the salivary gland cells. They were occasionally found in the third instar, when they appear to be engaged in the process of organelle turnover. Histolysis of the larval glands is initiated towards the close of the fourth instar when the number of autophagic vacuoles starts to increase. Simultaneously, the cytoplasm, previously full of ribosomes and endoplasmic reticulum, starts losing these structures. At the beginning of the pharate adult stage, the cytoplasm becomes practically devoid of all structures other than those engaged in autophagy. Polyteny of the chromosomes during ontogeny of the larval salivary glands is also discussed.