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First published online June 15, 2006
Journal of Experimental Biology 209, 2486-2494 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02280

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Water vapour absorption in the penicillate millipede Polyxenus lagurus (Diplopoda: Penicillata: Polyxenida): microcalorimetric analysis of uptake kinetics

Jonathan C. Wright^1,* and Peter Westh²

¹ Department of Biology, 609 North College Avenue, Pomona College, Claremont, CA 91711, USA
² Department of Life Sciences and Chemistry, Roskilde University, DK-4000 Roskilde, Denmark

View larger version (20K): [in a new window]   Fig. 1. (A) Sample plot of a recording trace in which the relative humidity was increased stepwise from 84% to 94%. The left ordinate shows the measured heat signal (µW) and the right ordinate shows the relative humidity (RH), plotted as the broken line. Exothermic heat flows are negative. The black and red plots show data for two different animals; the green plot is a blank recorded with an empty calorimetric cell. The (negative) peaks at each RH increment represent the exothermic heat of water adsorption to the walls of the calorimetric vessel. Clear deviations from the blank in the equilibrated trace show exothermic heat flow attributable to WVA. The abrupt decrease in heat flow in the red trace at 23 h shows that the animal terminated WVA at this time and the black trace shows a similar termination of WVA at ca. 18 h. Further explanation is provided in the text. (B) The heat signals recorded in the second (water) channel for these same animals during the later period of data collection. Depression of the RH in Cell 1 during WVA results in a corresponding increase in evaporation from Cell 2 and augmented endothermic (positive) heat flow compared to the blank. The red trace shows the onset of WVA in this animal at 16 h. The black trace shows an animal that is initially absorbing, but ceases WVA at 18 h. Fuller discussion is given in the text.

View larger version (9K): [in a new window]   Fig. 2. Mean WVA uptake fluxes (± s.e.m.; vertical error bars), expressed both as the exothermic heat signal (HFWVA; µW) and the uptake flux (µg h–1), plotted as a function of the steady-state relative humidity in the calorimetric cell (RHs–s in %). RHs–s values are calculated as indicated in the text and the data points represent the means for each RH-step (± s.e.m.; horizontal error bars). Water vapour uptake in P. lagurus increases linearly with relative humidity up to 92% RH, but shows asymptotic kinetics above this value. N=4-16.

View larger version (15K): [in a new window]   Fig. 3. Graph showing the linear relationship between faecal pellet water exchange (µg) and the reciprocal of the vapour pressure deficit (kPa–1). The least-squares regression line (r2=0.755; N=31) is shown. The x intercept, at which faecal pellets show no net water exchange, is 0.00218, equivalent to a water activity of 0.855.