ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 421 
30°C.e <20% R.H. 
Dead 
34°63 Si7eG: 
Wy wy R.H. 
Ficure 2.—A set of observations in the habitat of Hemilepistus reaumuri (Isopoda), in the 
Algerian desert. 
passes, the surface of the soil and the air immediately above it 
undergo a very rapid rise in temperature (as much as 15° C. in 10 
seconds). In such crises, ability to transpire rapidly while seeking 
shelter is undoubtedly an advantage to the animal. 
It seems, then, that the physiology of arthropods in deserts, so far 
as it is known, conforms to expectation. They are too small 
to maintain a constant temperature by transpiration for long, and 
consequently they exist by avoiding true desert conditions. 
Ability to transpire rapidly is certainly of some immediate ad- 
vantage both for temperature depression and for respiration. But 
in the long run, it is a great disadvantage to small animals, for it 
restricts them to cryptozoic niches—moist, cool crevices which form 
only a small part of the terrestrial habitat. The most landworthy 
arthropods, not only in deserts but on land in general, have imper- 
meable cuticles. 
It may be permissible to speculate as to why woodlice, after living 
on land for at least 60 million years, have shown so little progress. 
Since some of the animals are still littoral, and their closest relatives 
are marine, there is strong evidence that the group migrated to land 
across the littoral zone. This is perhaps a more difficult route than 
that through estuaries and swamps because the problems of land 
locomotion, air breathing, and great temperature fluctuations are 
