Effects of Sub-lethal High Temperature on an Insect, Rhodnius Prolixus (stal.) Ii. Mechanism of Cessation and Delay of Moulting

It is now well established that moulting in insects is under hormonal control; a hormone secreted by the brain activates the thoracic (prothoracic) glands which in turn secrete the moulting hormone (for review, see Wigglesworth, 1954, 1964). Investigating the influence of temperature on moulting in Rhodnius, Wigglesworth (1952a) stated that as the temperature rises above 35—36 C. the secretion of the moulting hormone fails, although the other metabolic processes in the insect are not visibly affected; whether it is the 'activating' component from the brain or the 'moulting' factor from the thoracic glands which is lacking has not been determined. Later, Wigglesworth (1955) found that if Rhodnius larvae were kept after the critical period at 36° C. they would moult. Conversely, when 4th-stage larvae were decapitated 1 day after feeding, injected with an effective amount of ecdyson and then kept at 36 C , they failed to moult. He suggested that although high temperature (36 C.) may have an adverse effect on the secretion of the brain factor, since the thoracic glands show no signs of activity, it may in addition arrest moulting by interfering with the thoracic-gland hormone or its action upon the epidermis. According to Mellanby (1938), exposure of Lucilia sericata larvae to 35 C. and 37 C. prevents pupation. Investigating pupation in prepupae transferred from normal temperature to and from low and high temperatures, Mellanby suggested that low temperature appears to have no harmful effects on the 'pupating' hormone or the pupation process, while high temperature seems to destroy the hormone. Later, Mellanby (1954) working on Aedes aegypti stated that brief exposures to high temperature interrupt development. Following Wigglesworth, he suggested that sublethal exposures to heat may destroy the internal secretions whiek control the processes of development and moulting. Similar results were also obtained by Church (1955) on Cepkus cinctus. This paper describes experiments which were designed to elucidate the mechanism involved in the arrest or delay of moulting induced by high temperature.