412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
land, are still limited by crustacean anatomical and physiological 
systems. No wax layer has been successfully demonstrated in the 
integument; neither does the latter show any increase in permeability 
with rising temperature. Transpiration rates are generally a good 
deal higher than in most insects. 
KEEPING COOL 
Associated with transpiration is still another problem posed by life 
on Jand—that is the problem of existence at extreme temperatures. 
There are several ways of dealing with high temperatures. They 
may be avoided by living in a cryptozoic niche below the ground, 
under crevices, in caves, and so forth. This procedure is open only 
to small animals and is, in any case, a confession of failure. Alterna- 
tively, they may simply be tolerated, and this is one of the best solu- 
tions for small animals. Thus some insects can withstand 
temperatures as high as 60° C. (140° F.) for 20 minutes, and many 
can withstand temperatures well above 40° C. (104° F.) for long 
periods. Neither of these devices involves loss of water. 
The most satisfactory solution is to control the body temperature 
at a fixed level, as the higher vertebrates, birds and mammals, have 
done. This of course involves loss of water by transpiration, and 
the solution is open only to animals above a certain size, as the fol- 
lowing considerations show. Assuming that each square meter of the 
surface of an animal must evaporate 0.6 kg. of water per hour to 
maintain the temperature constant in warm dry air, a value which 
has been found for animals weighing from 96 to 16 kg. (Dill, Bock, 
and Edwards, 1938; Adolph and Dill, 1938), then the percentage of 
the total body weight lost per hour can be calculated approximately, 
and the answer varies from 0.77 percent (camel) to 105 percent (wood- 
lice) (fig. 1). Clearly no animal can lose 105 percent of its weight 
per hour for long, so that this method of keeping cool is available only 
for large animals. 
A rather unsatisfactory compromise, open to small animals with 
permeable skins, is to use the evaporation of water to cool the body 
for short, critical periods. This is in no sense temperature regula- 
tion; it is the inevitable result of having a permeable skin, for at 
high temperatures the rate of transpiration rises owing to the greater 
drying power of the air, and the difference between body tempera- 
ture and ambient temperature is therefore greater. Such an effect has 
been observed in several land animals, including amphibians, mol- 
lusks, worms, and woodlice. 
WATER ECONOMY—A SYNTHESIS 
Let us now attempt something of a synthesis, in order to appreci- 
ate the significance of these various sources of water loss. In partic- 
