1. 1.

    The argasid tick Ornithodorus moubata Murray was fed on modified blood meals, the ionic strength and/or osmotic pressure of which was varied by additions of NaCl or glucose, and by dilution with distilled water or isosmotic glucose. The following parameters were monitored: total volume increase of tick, total volume of meal ingested, net increase in haemolymph volume, volume of coxal fluid excreted and ion concentrations and osmotic pressures of the gut contents, haemolymph and coxal fluid, as well as potential differences across the gut lumen.

  2. 2.

    Absorption of fluid from the gut into the haemolymph appeared to be linked to the active transport of Na+ and Cl, although the rate of absorption was an inverse function of the prevailing osmotic gradient across the gut epithelium.

  3. 3.

    The tick can maintain a reasonably constant [Na+] and [Cl] in the haemolymph when fed meals in which the concentration of these ions varies up to 6-fold, but only if the meal remains isosmotic with blood.

  4. 4.

    The rate and volume of coxal fluid production are direct functions of the rate and volume of fluid absorption by the gut epithelium. Thus, during the feeding cycle, the coxal organ appears to function chiefly in an osmo- and volume-regulatory capacity in this species of argasid tick.

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