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Journal of Experimental Biology 161,439-453 (1991)
Published by Company of Biologists 1991

Simultaneous Measurements of Spiracular and Cuticular Water Losses in Periplaneta Americana: Implications for Whole-Animal Mass Loss Studies

J. MACHIN ¹, P. KESTLER ², and G. J. LAMPERT ¹

¹ Department of Zoology, University of Toronto, Toronto, Ontario, Canada M5S 1A1
² Department of Biology, University of Osnabrück, D-4500 Osnabrück, Germany

Spiracular and cutaneous water loss through the cuticle and spiracles of Periplaneta americana was measured in animals of differing initial water contents under controlled temperature, humidity and airflow conditions, by continuous weighing (resolution ±10µg). Stable water loss rates (2.09x10^-8 to 3.47x10^-8gh^-1cm^-2Pa^-1) were as much as an order of magnitude less than those reported in earlier studies employing intermittent weighing techniques. We suggest that increased water loss caused by substantial increases in ventilatory water loss during the episodic disturbances required by intermittent weighing is the principal contributor to this discrepancy.

Water losses, as well as the interval between spiracular ventilations, decreased with water content. However, greater variation was primarily due to interruptions of the regular cyclic pattern of ventilation by highly variable, activity-related bouts of increased ventilatory loss. Variations in water loss appear to be too large and too rapid to be convincingly explained in terms of cuticular conductance. Our results suggest that previous experiments using 5% CO₂ anaesthesia, linking changes in total water loss to the conducting properties of the cuticle, were not valid. There seems to be no adaptive value for the proposed voluntary increases in cuticular permeability in an animal where ventilatory water losses can be so high.

Key words: Periplaneta americana, cuticular water loss, cuticle permeability, spiracular water loss, ventilatory cycles

Accepted on July 15, 1991

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