The Waterproofing Mechanism of Arthropods

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Abstract
1. The firebrat, Thermobia, is very impermeable to the outward flow of water, losing only 0.013 mg. of water/mm. Hg saturation deficit/hr./standard animal of 30 mg. between 19 and 26° C. 2. The permeability increases to a higher level at a critical temperature just below 30° C. and well below the optimal temperature for the insect as a whole. 3. The waterproofing is mobile, for it regains its lower permeability on cooling below the transition temperature and is also restored a short time after abrasion. 4. Extractions support the hypothesis that the impermeable layer is a stiff, epicuticular grease but further suggest that an imperforate lamella, perhaps a cement, lies over the grease. 5. The live firebrat can absorb water from moist air, reaching an equilibrium between absorption and transpiration at a saturation deficit of approximately 10 mm. Hg at a temperature of 21° C. 6. Although abrasion of the integument decreases the net rate of water uptake and lowers the saturation deficit at which equilibrium is possible, it may act only by increasing temporarily the rate of transpiration.