420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
ture is the most efficient adaptation that they can possess. Neverthe- 
less the ability to prevent undue rise in temperature for a limited 
period would be a distinct advantage in certain critical situations, 
and there is evidence that this does occur. 
There are plenty of desert-living arthropods, including insects, 
spiders, and their allies, and even a few crustaceans. Rather less is 
known about the physiology of desert arthropods than desert mam- 
mals. What information exists, suggests that there are no striking 
physiological adaptations (Cloudsley-Thompson, unpublished), but 
this may be a mistaken impression because nobody has looked very 
carefully. Desert beetles have a rather less permeable integument 
than those from temperature climates, and the same is true of wood 
lice. On the whole, however, arthropods exist by avoiding extreme 
conditions. Because of their small size and mobility they are able 
to do so very well. 
This focuses attention on an aspect of the problem which we have 
not so far considered in detail: the microclimates available for escape. 
There is by now a considerable amount of information on this sub- 
ject, and I should like to consider two examples. Many years ago 
Williams investigated the variation in climate in a small area of the 
Egyptian desert. His conclusions, recently summarized (Williams, 
1954), show how an animal, by moving through a very short dis- 
tance, can avoid extremes of temperature and dryness. When the 
ground surface temperature was 56° C., 10 cm. below the surface it 
was only 384° C., and by moving up and down through a distance 
of 30 cm. in the soil, an animal could live in a constant temperature 
throughout the daily cycle. 
A second example provides a rather interesting comment on Wil- 
liams’s work. It concerns some observations (Edney, 1958) on 
a desert woodlouse, Hemilepistus reaumurt. It is strange enough 
to find land crustaceans at all in desert conditions, and this species 
seems, so far as we know, to differ but slightly in physiological mat- 
ters from its temperate relatives. It is larger than most woodlice, 
it runs with its body held well above the ground by longish legs, 
thereby avoiding contact with the hot surface, and its cuticle is less 
permeable to water than that of other woodlice. But it still breathes 
by what are essentially gills, and survives by digging small vertical 
holes about 30 cm. deep in which it spends the hot part of the day. 
Temperatures and humidities measured in the habitat of these ani- 
mals show the efficacy of the retreat holes (fig. 2). These measure- 
ments also show that Hemélepistus transpires rapidly enough to 
reduce its body temperature significantly for short periods—a fact 
of considerable practical importance, for the animals emerge from 
their holes if the sun is covered by a cloud. As soon as the cloud 
