Abstract
Dehydration of Onymacris unguicularis (Haag) for 10 days at 27°C resulted in a weight loss of 14.9%, and a 37% decrease in haemolymph volume. Although there was an overall decrease in the lipid content during this period, metabolic water production was insufficient to maintain total body water (TBW). Rehydration resulted in increases in body weight (6.2% of initial weight), TBW (to normality), and haemolymph volume (sub-normal at the end of rehydration). Despite an increase of 44.0 mg in the wet weight of O. unguicularis after drinking for 1h, there was little change in the water content at this time, although the total lipid content increased significantly. Increases in haemolymph osmolality, sodium, potassium, chloride, amino acid and total sugar concentrations during dehydration were subject to osmoregulatory control. No evidence of an active amino acid-soluble protein interchange was noted during dehydration or rehydration. Haemolymph trehalose levels were significantly increased at the end of rehydration (relative to immediate pre-rehydration values), indicating de novo sugar synthesis at this time. Osmotic and ionic regulation was evident during rehydration, but control of OP during haemolymph-dilution is poor and accomplished largely by the addition to the haemolymph of free amino acids and solute(s) not measured in this study. There was little mobilization of sodium and chloride ions from storage sites at this time. The lesser osmoregulatory ability of Onymacris unguicularis and perhaps earlier susceptibility to osmotic stress, a significantly high normal blood glycerol level (relative to other diurnal adesmiine tenebrionids), and a water storage mechanism associated with synthesis of fat, probably all contribute to the development of fog-basking behaviour in this species. Water gain in O. unguicularis during periods of relative drought is probably largely accomplished by a greater food consumption.
Highlights
Onymacris unguicularis are unique amongst diurnal Namib tenebrionids in the water-trapping behaviour they display during fogs
At the end of 10 days of dehydration, the mean weight of O. unguicularis had decreased by 14.92% ± 0.76% (P < 0.05)
O. unguicularis occurs in the vegetationless dune fields of the Namib Desert, are ordinarily day-active, (Holm & Edney, 1973; Hamilton & Seely, 1976; Seely et al, 1983) are ultrapsammophilous (Holm & Edney, 1973), and swim through the sand as if it were a fluid medium, not forming true burrows (Louw et al, 1986)
Summary
Onymacris unguicularis are unique amongst diurnal Namib tenebrionids in the water-trapping behaviour they display during fogs. O. unguicularis are ordinarily diurnal, demonstrate a bimodal foraging regime (feeding mainly on wind-blown plant detritus – Seely et al, 1983; Louw et al, 1986), and maintain elevated day-time body temperatures ranging from 30–40oC (Hamilton, 1975; Hamilton & Seely, 1976; Seely et al, 1988). They emerge from the sand of dune slipfaces (where they normally remain buried overnight) and climb to the crests to fog-bask during nocturnal fogs, when ambient temperatures may be lower than 3oC and surface temperatures as low as 1°C (Hamilton & Seely, 1976; Robinson & Seely, 1980; Cooper, 1982; Seely, 1983). In it’s response to fogs, the species appears to be facultatively crepuscular
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