1. The effects of insolation and evaporation upon the internal temperature of the woodlice, Ligia oceanica, Oniscus asellus, Porcellio scaber and Armadillidium vulgare, and of the cockroach, Blatta orientalis, were investigated.

2. During insolation, the temperature of a thoroughly dry Ligia remains within 0.5° C. of that of a freshly killed specimen covered with beeswax. Dry specimens do not, therefore, differ significantly in shape from living ones, and are valid as controls when estimating the effect of evaporation from living animals.

3. Living, freshly killed, and dead, dry animals were exposed together for periods of 30 min. or more upon insolated rock in the natural habitat, upon slate, and upon wooden blocks which could be moved from shade to sun. Their temperatures, and those of the surrounding air and ground, were measured by fine thermocouples; humidity was measured by a small electric hygrometer, and air speed was estimated approximately.

4. In the sun, the temperature of a dead, dry animal is usually near that of the ground, and always higher than that of a living or a freshly killed specimen. The air temperature, 5 mm. above ground, may be either above or below that of Ligia, but it is always below that of the other species.

5. The amount of the temperature differences between ground, air, living and dry specimens of any one species varied from one exposure to another according to the intensity of radiation, the temperature and himidity of the air, and its velocity.

6. When all species were exposed together, they showed, during a stable period, approximately the following temperature depressions (from ground temperature): Ligia, 8.0° C.; Oniscus, 4-5° C.; Porcellio, 2-3° C.; Armadillidium, 4° C. The species stand in the same order as they do in respect of rates of evaporation, except for Armadillidium, and it is suggested that this species absorbs less radiant energy per unit area by reason of its shiny surface and arched dorsum. Blatta shows a temperature depression between those of Porcellio and Oniscus.

7. Measurements made in the natural habitats of Porcellio and Ligia suggest that these animals are exposed to the sun, (a) when the resulting increase in body temperature brings it nearer the optimum for development, and (b) when insolation is incurred during locomotion from hot, saturated microcimates (e.g. under insolated stones) to more favourable ones. During exposures of type (b), rapid evaporation is advantageous, as a means of avoiding dangerously high temperatures.

8. Radiation, conduction, convection, evaporation and metabolism are considered with regard to their effect upon equilibrium temperature. Conduction and metabolism are shown to be negligible during exposure to direct sunlight. Heat exchange balances, applying to Ligia and Blatta during particular exposures, and involving radiation, convection and evaporation, are estimated in terms of milliwatts/cm.2. If evaporation is eliminated, the effect upon the two remaining terms, and the temperature at which a new balance is reached, can be calculated approximately. The temperature of a dry Ligia, so calculated, corresponds with the experimentally measured temperature.

9. The results are compared with what is known of the effects of insolation upon the temperature of insects. Evaporation is likely to play a more important part in determining the temperature of woodlice than of arthropods with an impermeable cuticle.

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