ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 411 
connection the significance of size, which will increasingly concern 
us in what follows, becomes apparent. 
In large animals little importance need be attached to the preven- 
tion of water loss by evaporation, because a given rate of evaporation 
per unit area can proceed for a longer time in a large animal than 
in a small one before the total water content falls to a lethal level. 
A flea could tolerate a transpiration rate of 5 mg./cm.?/hour for about 
15 minutes before losing 10 percent of its water, but a man could 
tolerate 4,500 times this rate of loss for a similar period and suffer 
the same proportional loss of water. 
It is not surprising, therefore, to find that insects have developed 
a highly efficient method of waterproofing their integument. Insects 
have a very long terrestrial history, and their origins are obscure, 
but in this group physiological adaptations have been perfected and 
refined as far as possible within the general arthropod pattern. The 
integument has been waterproofed by means of a very thin but prob- 
ably continuous layer of a waxlike substance composed of a mixture 
of paraffins and alcohols with carbon chain lengths from about 8 
upward. Since Wigglesworth (1945) first demonstrated the presence 
of these cuticular waxes, there has been much work and speculation 
devoted to finding out their nature and properties. At first it was 
thought that they underwent a rather sudden physical change, which 
permitted much higher rates of transpiration above a characteristic 
critical temperature. This was held to represent a form of tempera- 
ture control. However, recent work suggests that there is no sudden 
increase in permeability with temperature (Holdgate and Seal, 1956; 
Mead-Briggs, 1956), but a gradual one over the range, and that at 
biologically significant temperatures the rate of transpiration is too 
low to contribute greatly to cooling. There is still very much to be 
discovered about the nature of these waterproofing substances. We 
do not even know whether the rise in permeability with temperature 
is due to a progressive “melting” of the constituents or to simple 
physical properties known to be associated with activated diffusion 
through inanimate membranes, or both. However, it is true to say 
that if the insect integument were permeable, no amount of water 
conservation by other means would be of any avail. 
It is instructive to compare the situation in insects with that in 
woodlice, whose evolutionary history is very different. Woodlice 
arrived on land comparatively recently—the earliest fossils date from 
the Upper Eocene, some 60 million years ago—while insects occur 
as far back as the Devonian, some 300 million years ago. Woodlice, 
though they are the only crustaceans whose entire life is spent on 
2Since this was first written, Beament (1959) has obtained further evidence in favor 
of a “‘critical temperature.”’ 
