The Thoracic Gland in Rhodnius Prolixus (hemiptera) and Its Role in Moulting

It is now generally accepted that the periodic phases of growth in insects, which terminate in the deposition of a new cuticle and the casting of the old skin, are initiated by chemical changes in the circulating blood. For these growth-stimulating factors the general term 'moulting hormone' has been used (Wigglesworth, 1934), although it has been realized for some time that this hormone ' might ultimately prove to be made up of more than one constituent' (Wigglesworth, 1940) and that a succession of secretions might in fact be concerned (Williams, 1947; Wigglesworth, 1949). The source of these active secretions appeared to differ in different insects. The original view of Kopec (1922) that in Lepidoptera the secretion came from the brain was confirmed in Rhodnius (Wigglesworth, 1940) when it was shown that the neurosecretory cells of the pars intercerebralis were the apparent source of the 'moulting hormone'. But about the same time Fukuda (1940, 1941, 1944) found that in both the larva and pupa of the silkworm the prothoracic gland was the immediate source. So far as the Lepidoptera are concerned this controversy has been resolved by Williams (1947,1948 a), who has proved that secretions from the neurosecretory cells in the brain are necessary to activate the prothoracic glands. In cyclorrhaphous Diptera it has long been recognized that the hormone causing pupation is produced in the ring gland of Weismann (Burtt, 1937, 1938; Hadorn, 1937) and almost certainly in the large lateral cells of this gland (Vogt, 1943) which are now commonly regarded as homologous with the 'pericardia! glands' (or prothoracic glands) of other insects (Thomsen, 1941). It has recently been proved by Possompes (1950) that these lateral cells (called by him the ' peritracheal gland') are induced to secrete the pupation hormone by a factor liberated from the brain. In Dixippus, 'ventral glands' and 'pericardial glands', again perhaps homologous with the prothoracic glands (Williams, 1948 6, 1949; Pflugfelder, 1949), have been thought on histological grounds to be concerned in the control of moulting (Pflugfelder, 1947). Experiments on Odonata make it appear probable that the 'ventral glands' or 'intersegmental glands' are concerned (Deroux-Stralla, 1948). And in SiaMs the moulting hormone appears to come from some centre in the thorax (Geigy & Ochse, 1940). In Gryllus the brain is necessary for moulting (Sellier, 1949), but the histological changes in the prothoracic glands (Sellier, 1951) show that these also are probably concerned. In Periplaneta the immediate source of the 'moulting hormone' is the prothoracic gland (Bodenstein, 1953).