Hemolymph osmoregulation in several myriapods and arachnids

Cation regulation by the terrestrial isopod Armadillidium vulgare (Crustacea: Isopoda: Oniscidea) during dehydration in air

Many terrestrial arthropods display tight osmotic and ionic regulation of the hemolymph during dehydration. In this study, we sought to quantify the level of regulation of the major hemolymph cations in the terrestrial isopod Armadillidium vulgare (Isopoda, Oniscidea). Inulin space measurements showed that the hemolymph comprises 52 ± 2.2% of the hydrated water content but contributes 71 ± 9.8% of water losses during desiccation. Hemolymph concentrations of Na+, K+ and Ca²+ were measured in variably dehydrated animals using ion-selective microelectrodes and compared with predicted concentrations assuming no regulation. Na+ and Ca²+ are quite tightly regulated, showing respective concentration increases of 20.8% and 7.1% following a 50% reduction in hemolymph volume, but K+ showed no measurable regulation. The excreted cation fraction during desiccation is negligible. Sites of ion sequestration were examined by injecting ²²Na and ⁴⁵Ca into the hemolymph of hydrated animals and assaying tissue-specific activities following dehydration. Na+ is apparently sequestered non-specifically by an unknown mechanism. Ca²+ accumulates in the dorsal somatic tissues, possibly in the calcium pool of the cuticle. How A. vulgare avoids significant disruptions of E(m) and neuromuscular function in the absence of K+ regulation, and how it sequesters Na+, both pose intriguing challenges for future work.

Osmotic regulation in adultDrosophila melanogasterduring dehydration and rehydration

We have examined the osmoregulatory capacities of laboratory populations of the insect Drosophila melanogaster by measuring hemolymph osmotic concentration during desiccation and upon recovery from a bout of desiccation. Recovery treatments entailed allowing the flies access to distilled water, a saline solution or a saline+sucrose solution after a desiccation bout shown to reduce hemolymph volume by ∼60%. Prior to desiccation, the hemolymph osmotic concentration was 353±11 mOsm. We found that Drosophila display strict osmotic regulation under prolonged conditions of dehydration. Osmotic regulation continued during recovery from desiccation, regardless of the fluid provided. This result is evidence that this insect does not require an external source of osmolytes or energy to regulate its hemolymph osmotic concentration or to restore hemolymph volume,which is reduced during desiccation. We also examined populations that have been selected for over 250 generations for enhanced desiccation resistance to identify physiological characters that have evolved in response to the selection regime. The selected lines displayed a reduced pre-desiccation hemolymph osmotic concentration (315±7 mOsm) and a marginally improved capacity for osmoregulation.

Haemolymph osmoregulation and the fate of sodium and chloride during dehydration in terrestrial isopods

Osmoregulation of the haemolymph during dehydration was investigated in a selection of temperate oniscidean isopods. Inulin tracer studies show that the haemolymph contributes approximately 69% of water losses in Porcellio scaber, significantly more than predicted from the volume of this compartment (42% of total water). Haemolymph osmolality increases linearly as a function of haemolymph dehydration but at a significantly lower rate than predicted from the change in haemolymph fluid volume. Similar results for Oniscus asellus show that both species display efficient osmoregulation until lethal dehydration. Osmoregulation is associated with significant hyporegulation of haemolymph sodium and chloride. These findings indicate that: (1) cell water is conserved at the expense of the haemolymph; and (2) haemolymph dehydration is associated with the removal of Na(+) and Cl(-) contributing to net osmoregulation. During dehydration, accumulations of both Na(+) and Cl(-) are seen in the hindgut, with significant accumulations of electrolytes also seen in the luminal fluid of the hepatopancreas. Low fluid volumes in the foregut and hindgut suggest macromolecular association as the most plausible mechanism of ion sequestration. Evidence refutes ion excretion and haemocyte sequestration as osmoregulatory mechanisms. Sequestration of Na(+) as urate salts, as shown for Periplaneta and generally assumed for other insects, is insignificant in isopods.