Abstract
The young larvae of insects living on dry food produce large amounts of water by the metabolic combustion of dietary lipids. The metabolic production of water needed for larval growth, previously known as hypermetabolic responses to juvenile hormone (JH), is associated with a 10- to 20-fold increase in the rate of O2 consumption (10,000 µl O2/g/h in contrast to the usual rate of 500 µl O2/g/h). Growing and moulting larvae are naturally hypermetabolic due to the endogenous release of JH from the corpora allata. At the last, larval-pupal or larval-adult moult there is no JH and as a consequence the metabolic rate is much lower and the dietary lipid is not metabolized to produce water but stored in the fat body. At this developmental stage, however, a hypermetabolic response can be induced by the exogenous treatment of the last larval instars with a synthetic JH analogue. In D. vulpinus, the JH-treated hypermetabolic larvae survive for several weeks without moulting or pupating. In T. castaneum and G. mellonella, the JH-treated hypermetabolic larvae moult several times but do not pupate. All these larvae consume dry food and the hypermetabolic response to JH is considered to be a secondary feature of a hormone, which is produced by some subordinated endocrine organ. The organ is most probably the controversial prothoracic gland (PG), which is a typical larval endocrine gland that only functions when JH is present. According to our hypothesis, PG activated by JH (not by a hypothetical PTTH) releases an adipokinetic superhormone, which initiates the conversion of dietary lipid into metabolic water. This type of metabolic combustion of dietary lipid produces large quantities of endothermic energy, which is dissipated by the larvae in the form of heat. Thermovision imaging revealed that the body of hypermetabolic larvae of G. mellonella can be as hot as 43°C or more. In contrast, the temperature of cold normal last instar larvae did not differ significantly from that of their environment. It is highly likely that thermovision will facilitate the elucidation of the currently poorly understood hormonal mechanisms that initiate the production of metabolic water essential for the survival of insects that live in absolutely dry conditions.
Highlights
Previous studies revealed that insect hormones only affect the metabolism (O2 consumption) of particular target tissues indirectly, during specific developmental periods and according to a genetically programmed schedule (Sláma et al, 1974, for review)
In the wax moth (Galleria mellonella), implantation of active corpora allata into the last larval instar results in a great increase in respiratory metabolism, which is associated with an increase in feeding and growth of giant supernumerary larval instars (Sehnal & Sláma, 1966)
When the synthetic analogues of insect juvenile hormone (JH) became available, they were found to cause a great increase in respiratory metabolism in the last larval instar of different insect species (Sláma et al, 1974)
Summary
Previous studies revealed that insect hormones only affect the metabolism (O2 consumption) of particular target tissues indirectly, during specific developmental periods and according to a genetically programmed schedule (Sláma et al, 1974, for review). In the wax moth (Galleria mellonella), implantation of active corpora allata into the last larval instar results in a great increase in respiratory metabolism, which is associated with an increase in feeding and growth of giant supernumerary larval instars (Sehnal & Sláma, 1966). When the synthetic analogues of insect JH became available, they were found to cause a great increase in respiratory metabolism in the last larval instar of different insect species (Sláma et al, 1974). In the carpet beetle (Dermestes vulpinus), treatment of the last larval instar with JH analogues caused an incredible, 10- to 20-fold increase in the rate of O2 consumption (over 10,000 μl O2/g/h) and is referred to as the hypermetabolic response to JH (Sláma & Hodková, 1975)
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