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

The aberrant millipedes of the order Polyxenida are minute animals that inhabit xeric microclimates of bark and rock faces. The lichens and algae that provide their main food substrates tolerate extensive dehydration, effectively eliminating a liquid water source during periods of drought. In this study, we used microcalorimetry to test whether Polyxenus lagurus (L.) exploits active water vapour absorption (WVA) for water replenishment. Individual animals were pre-desiccated to 10-20% mass-loss and heat fluxes then monitored using a TAM 2277 microcalorimeter. The calorimetric cell was exposed to an air stream increasing progressively in humidity from 84% to 96%. WVA was distinguishable as large exothermic fluxes seen in > or = 86% RH. Owing to very small and opposing heat fluxes from metabolism and passive water loss, the measured flux provided a good measure of water uptake. WVA showed an uptake threshold of 85% RH and linear sorption kinetics until >94% RH, when uptake became asymptotic. Uptake was rapid, and would allow recovery from 20% dehydration (by mass) in little over 5 h. The uptake flux scales proportional, variant mass (0.61), suggesting an area-limited mechanism. Polyxenus possesses a cryptonephric system, analogous to that of tenebrionid beetle larvae. Measurements of water absorption and desorption from faecal pellets voided in different humidities gave an estimated rectal humidity of 85.5%. The close congruence between this value and the WVA threshold provides evidence for a cryptonephric uptake mechanism derived independently from that of tenebrionids. Polyxenus represents the first documented example of WVA in the myriapod classes.