The Evaporation of Water from Woodlice and the Millipede Glomeris

Abstract

Comparative studies have been made on seven woodlice and the millipede Glomeris to determine: (i) the site of water loss, (ii) the effect of temperature and humidity on the rate of evaporation of water, (iii) the effect of humidity on their upper temperature tolerance, and (iv) their capacity to regain water after desiccation. An apparatus was constructed in which several animals can be exposed simultaneously but separately to a slowly moving stream of air of any required humidity and temperature. There is no difference between the sexes as regards rate of water loss, and no difference was found in this respect between living and dead animals for exposures up to 1 hr. In Armadillidium and Porcellio the pleopodal area loses water some ten to twenty times faster per unit area than the dorsal or ventral surface (some five times only in Ligia), but the absolute amount of water lost per unit time by the pleopods is less than that from either of the other areas mentioned. Tables and graphs are given showing the rate of loss of water in mg./cm.2/hr. in dry air at various temperatures for each species studied, both for 15 min. and 1 hr. exposures. These show a linear relationship between rate of evaporation and saturation deficit except for Armadillidium and Glomeris above 40° C. (where the rate falls off), and Ligia. Reasons for these exceptions are suggested : curling in the first two, absolute size in the last. The animals studied can be placed in a series as regards rate of loss of water, from Ligia (which shows the greatest rate) through Philoscia, Oniscus, Porcellio, Cyclisticus and Armadillidium nasatum to A. vulgare, which shows the lowest rate of loss. Oniscus, Porcellio and Cylisticus do not differ very greatly from one another in this respect. Glomeris loses water less rapidly than the majority of species of woodlice if exposed for 15 min., but, because the average rate of loss from woodlice falls if they are exposed for an hour, Glomeris appears to lose more rapidly than they do for an hour’s exposure. The highest temperature tolerated by woodlice for short exposures (15 min. or 1 hr.) does not vary greatly with humidity—temperatures are somewhat above 40° C. for 15 min. exposures—it is slightly higher at 50% R.H., due to a cooling effect, than in dry or saturated air; but for 24 hr. exposures, the animals die of desiccation and consequently the highest tolerable temperature is very much lower in dry air than in moist. These findings are also true, but to a less marked extent, of Glomeris. The species can be arranged in a series as regards temperature tolerance which corresponds with the rate of evaporation series, A. vulgare having the highest temperature tolerance, Ligia the lowest. After exposure to desiccation for 45 min. in dry air, or for 23 hr. in 50% R.H., both at room temperature, no species could five or recover weight if subsequently kept at 95 % R.H. Armadillidium (but no others) could live and recover weight by absorption of water vapour if kept at 98 % R.H., and all species could live and recover weight if kept in saturated air. Only living animals recovered weight by absorption of water vapour or liquid water. Animals which died after a period in the ‘recovery ‘dishes always lost water very rapidly before doing so. The power to recover water from air at 98% R.H., possessed by Armadillidium, is limited by relative humidity not by saturation deficit. The above results are discussed and the need for microclimatic data is stressed.