The Evaporation of Water from Spiders

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Abstract
1. The purpose of the present work was to investigate and compare the water-retaining properties of the integument in a number of species of spider. Subsidiary investigations concerned the anatomy and function of the ‘tracheae’ as respiratory organs, and the significance of these organs and of the lung-books in total water loss. 2. The anatomy of the tracheae was investigated by means of the cobalt-sulphide injection technique. In Lycosa amentata they consist of four unbranched tubes, and their surface area is approximately one-thirtieth of that of the lung-book leaflets. Injection of reduced indigo blue demonstrates that O2 enters via these tracheae, but the amount is too small to be measured by a standard Warburg manometer, and is insufficient to keep the animal alive if the lung-books are blocked. At 30° C. intact spiders absorb approximately 0.6µl./mg./hr. 3. If the lung-book spiracles are kept open by exposing living spiders to 10% CO2 in air, evaporation increases by nearly 50% (from 16 to 23% of body weight in 24 hr.). There is no significant increase if dead spiders are exposed, possibly because the spiracles do not open. 4. The rate of evaporation into dry air moving at ca. 5.0 cm./sec. was measured from dead and living Lycosa with the spiracles either blocked or free. The spiders were exposed for 15 min. at 10°C. intervals from 10 to 60°C. Up to 30°C. the rates in mg./cm3./hr. were low, never more than 1.6 (dead spiders with free spiracles) and usually Meta segmentata, Tegenaria derhami, Zilla atrica and Z. x-notata. Lung-book spiracles were blocked, only females were used, and the same individuals were exposed at each temperature except for the Zilla spp. As in Lycosa, the rate of evaporation from all these spiders increases abruptly at a critical temperature, and the shape of the curves is similar to that found in insects. 6. The species stand in the following order as regards critical temperatures (lowest to highest): Zilla atrica, Lycosa, Meta, Tegenaria. Zilla x-notata shows a less well-defined critical temperature, and a lower rate of evaporation than any other species at higher temperatures. 7. Abrasion with an inert dust produces an approximately sixfold increase in the rate of evaporation from Lycosa. 8. The above results are compared with similar measurements in other arthropods. Spiders resemble insects and ticks, and differ from isopods and myriapods, so far as the effect of temperature upon evaporation is concerned, and it is suggested that a discrete wax layer is probably present in the spider cuticle. 9. The suggestion that evaporation is resisted by active secretion of the epidermal cells (as in ticks) is put forward to account for somewhat greater rates of evaporation from dead than from living spiders in similar conditions.

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