372
E. B. EDNEY
among different groups of arthropods—the result of very different evolutionary histories—so that adaptations to desert living open to one are not
open to all, and we should not expect to find the maximum possible development of adaptive features in an arthropod simply because it occurs in
areas marked as deserts on maps. Rather we should expect a relatedness
between one adaptation and another and between these and the basic structure and physiology of the groups concerned. Most adult insects fly, but
other arthropods do not, and their adaptations to desert life will differ correspondingly. Desert beetles have very impermeable integuments and tolerate high body temperatures, while desert cockroaches Jive below the
sand, have more permeable integuments, and absorb water vapor. In fact,
there is probably no single aspect in which all desert arthropods differ from
all others.
Certainly the arthropod organization, in some of its forms at least, is
capable of producing highly efficient desert species, and investigation of
the adaptations involved is an absorbing field of research. At the same
time, it may be well to recognize another side to the picture which is of
general biological interest; this concerns the question as to whether the
evolution of adaptations to desert environments necessarily involves loss
of adaptation to more mesic habitats. If indeed deserts are just extremely
"harsh" environments, and if our animals have solved the problems of living there, should they not be much better at living in "easier" environments? Why, for example, are such hardy insects as Glossina morsitans
not found in tropical forests? Sometimes there are obvious answers—sand
roaches need sand dunes to live in—but more often there are not.
IX. Summary
Most classes of arthropods, including terrestrial crustaceans, are represented in the desert fauna, although insects predominate. Their small size
and large relative surface forbids the use of evaporative cooling as a longterm measure and necessitates an impermeable integument. The insect organization, which is already very suitable for terrestrial existence, lends
itself to further quantitative changes in adaptation to desert environments.
There is a broad correlation in arthropods between cuticle permeability
and habitat. Impermeability of the cuticle is conferred largely by lipid material in the epicuticle, and this reduces transpiration into dry air from
tsetse fly pupae to as little as 0.3 μg/cm
2 /mm Hg/hour (0.9 X 10
-4
cm/second). Loss of water from the respiratory membranes is inevitable but is
controlled by spiracular mechanisms so that in resting tsetse flies and desert
locusts this amounts only to about one-third of the total water loss, When
E. B. EDNEY
among different groups of arthropods—the result of very different evolutionary histories—so that adaptations to desert living open to one are not
open to all, and we should not expect to find the maximum possible development of adaptive features in an arthropod simply because it occurs in
areas marked as deserts on maps. Rather we should expect a relatedness
between one adaptation and another and between these and the basic structure and physiology of the groups concerned. Most adult insects fly, but
other arthropods do not, and their adaptations to desert life will differ correspondingly. Desert beetles have very impermeable integuments and tolerate high body temperatures, while desert cockroaches Jive below the
sand, have more permeable integuments, and absorb water vapor. In fact,
there is probably no single aspect in which all desert arthropods differ from
all others.
Certainly the arthropod organization, in some of its forms at least, is
capable of producing highly efficient desert species, and investigation of
the adaptations involved is an absorbing field of research. At the same
time, it may be well to recognize another side to the picture which is of
general biological interest; this concerns the question as to whether the
evolution of adaptations to desert environments necessarily involves loss
of adaptation to more mesic habitats. If indeed deserts are just extremely
"harsh" environments, and if our animals have solved the problems of living there, should they not be much better at living in "easier" environments? Why, for example, are such hardy insects as Glossina morsitans
not found in tropical forests? Sometimes there are obvious answers—sand
roaches need sand dunes to live in—but more often there are not.
IX. Summary
Most classes of arthropods, including terrestrial crustaceans, are represented in the desert fauna, although insects predominate. Their small size
and large relative surface forbids the use of evaporative cooling as a longterm measure and necessitates an impermeable integument. The insect organization, which is already very suitable for terrestrial existence, lends
itself to further quantitative changes in adaptation to desert environments.
There is a broad correlation in arthropods between cuticle permeability
and habitat. Impermeability of the cuticle is conferred largely by lipid material in the epicuticle, and this reduces transpiration into dry air from
tsetse fly pupae to as little as 0.3 μg/cm
2 /mm Hg/hour (0.9 X 10
-4
cm/second). Loss of water from the respiratory membranes is inevitable but is
controlled by spiracular mechanisms so that in resting tsetse flies and desert
locusts this amounts only to about one-third of the total water loss, When