Hydration Maintenance by Geof Gaiser

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Mechanisms for maintaining hydration
1.       Increasing body water content is a survival strategy in some mosquitoes. Identical water use and excretion rates by two strains of Anopheles mosquitoes, shows higher survival rate for the Arabian species with a higher normal body hydration level than the Gambian species with a lower average body hydration.

Ref.  E.M.Gray and T.J.Bradley “Physiology of dessication resistance in Anopheles gambiae and Anopheles arabeinsis” American Journal of Tropical Medicine and Hygiene 73(3):553-559

2.       Some fruit fly larvae Drosphila melanogaster store more more water by producing high levels of glycogen, a polysaccharide of glucose, that fixes five times it weight in water molecules.

Ref.  J.L.Graves etal (1992) “Dessication, flight, glycogen and postponed senescence in Drosophila melanogaster” Physiological Zoology 65:268-286

3.       Larger hemolymph volume in some varieties of Drosophila melanogaster is accomplished by production of high levels  of trehalose (the α 1-1 linked glucose dimer, similar to maltose, which is α 1-4 linked). This results in a 300% increase in hemolymph volume relative to other varieties of Drosphila melanogaster.

4.       More frequent feeding in order to acquire more food contained water.

5.       Production of higher levels of lipoproteins and fatty acids in the epicuticle (the protein-polyphenol complex that is the primary defense against water loss, made by the dermal glands.) Responding to temperature extremes, neuroendocrine hormones stimulate a protective effect on the cuticle that results in a sharp reduction in transpiration.

6.       Many insects have their trachea covered with responsive spiracles that shut when needed to prevent water vapor from escaping.

7.       Excretory water loss can be managed by insects living in arid conditions. Tse-tse flies raised in arid locations have excrement with less than half the moisture content as those raised in non-arid zones.

8.       Toleration of grater water loss is an adaptation to dry environments resulting in survivability to 20% more water loss than the same insects in wetter areas.

9.       Relocation to better microenvironments is exhibited by:
Fruit flies moving to areas of higher humidity
Dung beetles burying food in moister underground locations
Caterpillars feeding on the underside of leaves where there is higher relative humidity

10.   Cryptobiosis is exhibited by the larvae of the African chironomid, Polypedilum vanderplanki that completely dehydrate when rock-pools dry up. Their survival depends on the manufacture of trehalose sugar.
Water Vapor Absorption

Some ticks uptake vapor phase water in structures adjacent to their mouth, tow bladder-like extensions of the hypopharynx (the lower portion of the throat).

Ref.  M.J.O’Donnell in Proceedings of the National Academy of Sciences of the U.S.A. V74 No.4 (Apr 1977) pp 1757-1760

Thermobia Flea beetles (Coleoptera: Chrysomelidae) larvae and adults are hosted in the Sand dune willow. They have bladders inflated by hemolymph pressure made by contraction of the thoracic and abdominal musculature. Densely packed microscopically fine hairs on the surface of these bladders hold a layer of fluid exuded under this pressure. Condensation of high relative humidity on these fluid coated hairs is rapid enough to exhibit a measurable localized temperature increase due to the change-of-state
vapor condensation.

Ref.  Catherine Bach  “Plant Successional Stage and Insect Herbivory: Flea Beetles on Sand Dune Willow” Ecology 7:(2) 1990 pp 598-609

Atmospheric water vapor absorption has been shown in 22 species of Psocoptera (booklice, barklice, barkflies) which are the most primitive hemipteroids, but it appears to be a property of the entire order of Psocoptera. From 58-85% relative humidity, the water uptake occurs in only 3% of the insect’s body mass. This process also occurs in Mallophaga (chewing lice, biting lice, bird lice) Order: Phthiraptera.

Ref.  D.Rudolph in Journal of Insect Physiology (1982) V 28 (2) pp 111-121

The route for trans-epithelial water movement during fluid secretions are a passive response to osmotic gradients produced in the cells of the lumen of the Malphghian tubules by the active process of salt transport utilizing an energy expenditure to result in water transport into cells. Hormonal stimulation of the cell membrane permeability is triggered by ADH (Anti-diuretic hormone) and 5-HT (Serotonin precursor).

Ref.  M.J.O’Donnell and S.H.Maddrell “Paracellular and transcellular routes for water and solute movements across insect epithelia” Journal of Experimental Biology (1983) 106: 231-253

Water Vapor Absorption was first noted in 1932 by Kenneth Mellanby B.A. in mealworm larvae. The hydrophilic cuticle in the Desert cockroach (Arenivaga investigate) helps accomplish this process when the humidity is > 82.5%. Note to two bladders adjacent to the pharynx.


The same organisms with osmotic control mechanisms also utilize this for freeze adaptability. 

Ref. M.J. O’Donnell and J.Machin “Advances in Comparative and Environmental Physiology 2” pp 47-90 in Water Vapor Absorption by Terrestrial Organisms (1988).



A hydrophilic cuticle is an important water source for:
                Psocoptera
                Mallophaga
                Dust mites
                Ticks
                Tenebrionid larvae
                Lepismatid Thysanura
                Blattodea

Four major mechanisms require the maintenance of a fluid compartment of correspondingly reduced dilution (vs. the hemolymph) where the water activity is less than that of air. This permits water to enter along an osmotic gradient.
1.       Elevate air  pressure to the dew point. Insects are directed downhill to accomplish this.
2.       Alter the radius of a curved surface, such as surfaces of micro-hairs on the oral bladder wetted surfaces. This curvature changes the surface tension allowing water to flow into chambers, permitting osmotic absorption passively.
3.       Changes in water affinity of proteins by salt incorporationcaused by changing the proteins’ ionic strength occurs in the desert cockroach.
4.       Use of an uptake fluid of elevated osmolality. This is the moset frequently employed mechanism for Water Vapor Absorption.

To effectively hydrate, the uptake rate must be greater than the losses due to respiration + ventilation + excretion + salivation. Problems that are overcome to accomplish this are:
1.       Generating elevated osmotic pressure (solute concentration)
2.       Transporting fluids to the absorbing surface without dilution on the way
3.       Resorption of absorbed water following  vapor condensation.

Most successful terrestrial arthropods for Water Vapor Absorption
                Crustaceans – hermit crabs
                Isopods – sowbugs
                Oniscidea – pill bugs and woodlice

Pumping of the pleopod is always observed during uptake of water vapor, which gives a greater degree of ventilation.

Ref.  J.C. Wright and J.Machinn, Journal of Experimental Biology 154: 13-30 (1990) “Water Vapor Absorption in Terrestrial Isopods”



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