The importance of metabolic water production in counteracting respiratory water loss in flying insects depends on factors such as body mass, the level of activity, and ambient temperature. At high temperatures, honeybees ferry water to the hive for cooling purposes, carrying up to 65 per cent of their body mass, and the increased metabolic demand produces enough metabolic water to almost balance evaporative losses (Louw and Hadley 1985). Larger bees were discussed in Section 4.1.2. On a more modest scale, aphids maintain water balance during long flights by production of metabolic water, metabolizing glycogen in the early phase and then lipid (Cockbain 1961). Fat-based and carbohydrate-based metabolisms differ considerably in yields of metabolic water (Corbet 1991). This is because these substrates differ not only in the amount of water released per unit weight of energy, but also in the amount of water associated with them in storage (Weis-Fogh 1967b). Because 1 g dry weight of stored glycogen is associated with about 2.5 g water of hydration, metabolism of glycogen yields six times as much water as does that of lipid. This does not happen, however, if sugars in the crop are used directly as fuel for flight.
Metabolic water is also significant in inactive insects which are not feeding or drinking, and in those consuming dry food. Doubly labelled water (DLW) has been used to measure field water balance of tenebrionid beetles in Northern and Southern hemisphere deserts, and either food, metabolism or drinking dominated water influx, depending on season (Cooper 1982, 1985). Metabolic water suffices for water balance of inactive Onymacris unguicularis in the Namib Desert, but active beetles must drink fog water. The DLW technique (see Butler et al. 2004) has been considered less accurate for arthropods than for vertebrates, and there have been few recent applications to insects. However, a validation study on bumblebees Bombus terrestris has shown excellent agreement between values obtained using respirometry and DLW (Wolf et al. 1996).
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