Cycles of F-actin redistribution in the dermal glands of an insect relate to secretion

Abstract

Cells of the dermal glands in Calpodes ethlius Stoll (Lepidoptera, Hesperiidae) survive without division but increase in size from moult to moult. The secretory cells become especially large, measuring up to 6 mm long in the fifth larval stadium. The nucleus in these giant cells has a sponge-like profile, with the nuclear envelope indented around cytoplasmic intrusions, each of which comes to contain several secretory vacuoles. The secretory vacuoles coalesce just before the glands discharge at ecdysis. Rhodaminyl phalloin labelling shows that F-actin redistributes in cycles corresponding to ecdysial secretion at larval and pupal moults. Capsules of F-actin form around the vacuoles before ecdysis. After the vacuoles discharge, actin from the capsules survives in strands that aggregate into storage bundles during the intermoult as though the filamentous actin itself is redistributed and stored. The storage bundles disappear as the F-actin capsules re-form at the next moult.