Ultrastructural study of the prothoracic glands of Galleria mellonella L. in the penultimate last larval, and pupal stages.

The prothoracic gland (PGL) of Galleria mellonella is a Y-shaped, paired organ, consisting of 45-50 polyploid giant cells. The PGL cells are supplied by neurosecretory axons; release of neurosecretory granules (1000-1300 A in diameter) directly on the surface of PGL cells was frequently observed. Based on ultrastructure, the last two larval instars can be divided into three phases: 1) restitutive phase immediately after moulting; 2) gradual activation in mid-intermoult as indicated by the logarithmic cell growth, decrease of nucleo-cytoplasmic ratio, increase in the number of cell organelles participating in protein synthesis, and the structural changes of these organelles; 3) "release' period preceding moulting, characterized mainly by the extreme dilatation of peripheral invaginations. From the prepupal stage onward cellular activity is asynchronous. Part of the cells already show the signs of involution, while others histolyse only after the activation phase subsequent to moulting. PGL in G. mellonella is one of the larval tissues. In the course of activation its ultrastructure changes as a function of juvenile hormone (JH) cocentration, in the absence of which it histolyses. Accordingly, it has seemed to us to be a suitable model for the cytological study of JH activity.