Drinking

Drinking is common in insects, and even those with substantial water intake in their food, as shown by Edney's (1977) comprehensive list of arthropods, are known to drink water. Drinking in terrestrial insects is usually measured by simple gravimetric methods, especially when prolonged drinking occurs after dehydration, as in dehydrated desert tenebrionid beetles, Onymacris plana (Nicolson 1980). Fog and dew are important water resources for desert insects (Broza 1972; Seely 1979). Loveridge

(1974) demonstrated that Locusta migratoria was able to utilize saline water sources, surviving when 0.25 and 0.5 M NaCl were provided with dry food, although initially reluctant to drink. An apparatus for measuring the timing and extent of drinking behaviour, from a plastic straw coated with conducting paint, showed that adult male cockroaches fed dry food consume 50-140 ml of tap water daily (Whitmore and Bignell 1990). Many laboratory studies have investigated the uptake of sugar solutions by nectar or fluid feeding insects, which probably seldom drink free water. Puddling behaviour by male butterflies and moths (see below) is not concerned with the acquisition of water.

Haemolymph appears to play an important role in the control of drinking, although investigations on blowflies and locusts have produced varying results (for discussion see Edney 1977). Responsiveness to water seems to depend on the CP concentration of the haemolymph in the sheep blowfly Lucilia cuprina but not in other blowflies (Barton Browne 1968). The initiation and termination of drinking have different controlling mechanisms in locusts: drinking readiness is associated with reduced abdominal volume, but the volume ingested depends on the increase in osmolality (Bernays 1977). Compensatory drinking occurs in locusts which exhibit larger drinking bouts on NaCl solutions than on distilled water (Raubenheimer and Gade 1993). Since phagostimulatory and volumetric effects can be ruled out, the locusts appear to be compensating for the decreased ability of the imbibed fluids to hydrate the haemolymph. There are strong interactions between hunger and thirst in locusts: the amounts of either water or food consumed are markedly affected by the absence of the other, on both a short and long-term basis, and all deprivation treatments increase the time spent in locomotion (Raubenheimer and Gade 1994, 1996).

Reproductive harvester termites Hodotermes mossambicus (Isoptera, Hodotermitidae) form pairs after swarming flights and drink large volumes of water: 82 and 42 per cent of initial wet mass in males and females, respectively (Hewitt et al. 1971). Dilute fluid is stored in large water sacs (salivary reservoirs) which extend into the abdomen, and may be an important water reserve for the founding of the new colony. Conditions are ideal for emergence after the first substantial summer rains, when desiccation is reduced, the soil is moist and soft for excavation, and free water is available (Hewitt et al. 1971). Water imbibition and transfer to the water sacs has also been described in workers and reproductives of fungus-growing termites Macrotermes michaelsoni (Termitidae, Sieber and Kokwaro 1982). Although salivary gland reservoirs are also used for water storage in cockroaches (Laird et al. 1972), in this case the fluid is a hypo-osmotic saliva (House 1980), rather than water stored after drinking.

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