Certain arthropods lacking access to conventional sources of water from feeding or drinking have developed the ability to extract water from unsaturated air (Machin 1983; O'Donnell and Machin 1988). Different oral or rectal structures are utilized for this purpose, indicating that the mechanisms have evolved independently, often based on pre-existing capabilities. In Tenebrioni-dae, the rectal complex (Section 4.1.3, Fig. 4.9) has evolved for efficient drying of faeces and water vapour uptake is probably a secondary function. Ticks use oral uptake involving salivary secretions. Most mechanisms for water vapour absorption are based on the colligative lowering of vapour pressure by accumulating solutes, with the exception of oral uptake in the desert cockroach Arenivaga (reviewed by O'Donnell and Machin 1988).
Water vapour absorption is a regulated process which only occurs above a critical equilibrium activity (CEA) and only when the animal is dehydrated. The CEA (water activity at which vapour absorption balances passive losses) is discussed by Wharton (1985) and O'Donnell and Machin (1988). Net gain of water does not occur below a particular av that is characteristic of the species and stage in question. Steep gradients are involved in vapour uptake, because the aw of insect haemolymph is usually about 0.995 (equivalent to an osmolality of 300 mOsmolkg-1 or 99.5 per cent RH). In Tenebrio molitor, the importance of steep solute gradients in absorption is shown by the good agreement between the CEA (av = 0.88) and the maximum osmolality (6.8 Osmolkg-1) observed in the rectal complex (Fig. 4.9) (Machin 1983).
Uptake thresholds and absorption kinetics are the main factors determining the physiological and ecological significance of water vapour absorption in different groups: together, they determine the av range over which absorption is possible and the water deficits which can be recovered in a given time. The finding that water vapour absorption occurs in terrestrial isopods in moist environments has altered perspectives concerning its ecological significance (Wright and Machin 1993). The entire order Psocoptera possesses this ability, regardless of habitat and flight status, so the adaptive value is not readily apparent (Rudolph 1982). Recently, the common soil collembolan Folsomia Candida has been shown to absorb water vapour from the atmosphere by accumulating myo-inositol and glucose in order to raise its haemolymph osmolality above ambient av (Bayley and Holmstrup 1999). These authors point out that water vapour absorption at an av of 0.98 is as ecologically relevant for this soil insect as that occurring at much lower humidities in a desert insect.
Eggs of many insects, especially Orthoptera and Coleoptera, take up water in liquid form during development (Tanaka 1986), but diapausing eggs of the tropical stick insect Extatosoma tiaratum (Phasmida) can absorb water vapour from vapour activities down to 0.30 (Yoder and Denlinger 1992). Vapour uptake also occurs in the diapausing first instar, although the CEA is then 0.60. To date, this is the only instance of water vapour absorption in insect eggs, although the mechanism remains unknown.
Wharton (1985) has stressed that the atmosphere is a source of water for all insects, not only those capable of active water vapour absorption. It is a misconception that insect cuticle is asymmetrical to the diffusion of water. Passive absorption (sorption) of water vapour from the atmosphere serves to reduce net losses (Wharton and Richards 1978). For example, when atmospheric av is 0.50, roughly half of the water lost by transpiration will be regained without expending energy on absorbing mechanisms. The significance of passive sorption is seldom considered in studies of insect water balance, in part because isotopic techniques are
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