A comparative study of woodlice which show different degrees of fitness for terrestrial life provides information about the evolution of land faunas and underlines the significance of water relations in this respect.

Species of woodlice differ as regards rates of transpiration and survival in dry air. They may be considered as physical bodies so far as the effects of temperature and humidity upon evaporation are concerned, and they probably lack an epicuticular wax. No simple relation has been found between those climatic factors which affect evaporation and period of survival; neither is it expected on theoretical grounds.

Transpiration from the region of the pleopods is greater, per unit area, than from elsewhere. Pseudotracheae assist respiration in dry air. In moist air oxygen is absorbed through regions of the integument other than the pleopods; but such integumental respiration decreases in importance in more terrestrial species.

The only adaptation to land as regards excretion is a general suppression (compared with aquatic isopods) of nitrogen metabolism. Uric acid in small quantities is retained in the tissues. The faeces are moist. The tegumental glands are not an important source of water loss. The pleopods are not moistened by glandular secretions.

Desiccated woodlice can restore their original weight by absorption of moisture, by mouth and anus, from free water surfaces, and by mouth from moist surfaces, even though the ambient air is unsaturated. The higher forms possess external capillary channels which assist in irrigation of the pleopods.

The osmotic pressure of the blood of Ligia oceanica is higher than that of sea water; that of the other species measured is somewhat lower. Osmotic regulation is possible for Ligia in dilute sea water down to 50%, but adaptation to land on the part of most woodlice seems to be secured by osmotic tolerance rather than regulation.

Woodlice can withstand higher ambient temperatures, for short exposures, in air at 50% R.H. than in saturated air, and this is the result of rapid transpiration. In the field transpiration plays a significant part in determining body temperature, and may be of survival value during exposure to direct insolation.

All species require saturated air or a moist substrate in their permanent habitats, and it is probable that they differ as regards tolerance of suboptimal conditions during wandering rather than dryness of the normal retreat. With this proviso the families which have been investigated stand substantially in the following order of increasing terrestrialness of habitat: Ligiidae, Trichoniscidae, Oniscidae, Porcel-lionidae, Armadillidiidae. They also stand approximately in the same order as regards various morphological and physiological specializations associated with life on land (Table 1).

What success the group has achieved on land may be ascribed to avoidance of the rigours of true terrestrial conditions by means of behavioural mechanisms which retain them in the cryptozoic niche, rather than to morphological or physiological adaptations.

Further information is needed along three lines: (i) precise microclimatic measurements linked with observations of behaviour in the field, (ii) laboratory analysis of orientation mechanisms, (iii) neurophysiological investigation of the sensory mechanisms involved.

The geological age of the group is uncertain, but there were probably land-living oniscoids before the Tertiary. Their comparative lack of progress subsequently may be the result of retaining a water-permeable integument, which is necessary for cooling during brief exposures to high temperatures and also for respiration.