VI. DESERT ARTHROPODS
321
as indeed they may well be, for these inactive stages cannot replenish lost
water by feeding.
The cuticle of tsetse fly pupae (Glossina morsitans) is 630 times less
permeable to water than that of caterpillars of the swift moth Hepialus;
the desert isopod Hemilepistus loses water at only one-fifth the rate of
Porcellio, a mesic species; the cuticle of the blowfly Calliphora is about
10 times more permeable than that of the mealworm Tenebrio; and the
tropical forest scorpion Pandinus is about 100 times more permeable than
the desert form, Androctonus. The overall picture is clear enough. We
should beware, however, of applying the relationship universally, for it does
not hold at all levels. Desert scorpions, for example, lose water at something like one-quarter to one-tenth the rate for desert beetles when both
are expressed as percentages of total weight per unit time (Hadley,
1970a), and when Bursell (1959b) made comparisons among species
within the genus Glossina, no correlation appeared between resistance to
desiccation and habitat in adult flies, although such correlation was strong
in the case of pupae (Bursell, 1958).
It is often difficult to make valid comparisons between different arthropods in regard to water loss because workers have expressed their results
in different, perhaps nonconvertible, terms. Ahearn (1970a) for example,
expressed his results as percentages of original weight lost per unit time.
This is certainly an ecologically significant measure, but because of the
differences in size (about which no information was provided), the measure says nothing about cuticle permeability. For example, a scorpion may
weigh 100 times as much as a thysanuran, and if both lose an equal percentage of their weight in unit time, the rate of loss per unit area will
be five times greater in the scorpion than in the thysanuran (for further
discussion, see Edney, 1971a).
Comparative rates of weight loss—largely water—by a number of species of beetles all living in the same general habitat, the Namib Desert,
are shown in Table III. Here differences in rates of transpiration are
adapted to the habits of the organisms, since, in general, those species
which show low transpiration rates, such as Omymacris plana, are active
by day while those showing high rates of loss, such as Lepidochora porti,
are active by night. For comparison, two other tenebrionid species from
a more mesic environment (Grahamstown in the Eastern Cape Province)
are included, and they clearly lose water much more rapidly. Finally, with
respect to its size, the thysanuran Ctenolepisma pauliana seems to be remarkably well waterproofed, and it lives in one of the driest and hottest
habitats—under granite rocks in the stony desert.
It seems, then, that the arthropod integument is organized in such a way
as to permit a high degree of waterproofing if necessary, and that by and
321
as indeed they may well be, for these inactive stages cannot replenish lost
water by feeding.
The cuticle of tsetse fly pupae (Glossina morsitans) is 630 times less
permeable to water than that of caterpillars of the swift moth Hepialus;
the desert isopod Hemilepistus loses water at only one-fifth the rate of
Porcellio, a mesic species; the cuticle of the blowfly Calliphora is about
10 times more permeable than that of the mealworm Tenebrio; and the
tropical forest scorpion Pandinus is about 100 times more permeable than
the desert form, Androctonus. The overall picture is clear enough. We
should beware, however, of applying the relationship universally, for it does
not hold at all levels. Desert scorpions, for example, lose water at something like one-quarter to one-tenth the rate for desert beetles when both
are expressed as percentages of total weight per unit time (Hadley,
1970a), and when Bursell (1959b) made comparisons among species
within the genus Glossina, no correlation appeared between resistance to
desiccation and habitat in adult flies, although such correlation was strong
in the case of pupae (Bursell, 1958).
It is often difficult to make valid comparisons between different arthropods in regard to water loss because workers have expressed their results
in different, perhaps nonconvertible, terms. Ahearn (1970a) for example,
expressed his results as percentages of original weight lost per unit time.
This is certainly an ecologically significant measure, but because of the
differences in size (about which no information was provided), the measure says nothing about cuticle permeability. For example, a scorpion may
weigh 100 times as much as a thysanuran, and if both lose an equal percentage of their weight in unit time, the rate of loss per unit area will
be five times greater in the scorpion than in the thysanuran (for further
discussion, see Edney, 1971a).
Comparative rates of weight loss—largely water—by a number of species of beetles all living in the same general habitat, the Namib Desert,
are shown in Table III. Here differences in rates of transpiration are
adapted to the habits of the organisms, since, in general, those species
which show low transpiration rates, such as Omymacris plana, are active
by day while those showing high rates of loss, such as Lepidochora porti,
are active by night. For comparison, two other tenebrionid species from
a more mesic environment (Grahamstown in the Eastern Cape Province)
are included, and they clearly lose water much more rapidly. Finally, with
respect to its size, the thysanuran Ctenolepisma pauliana seems to be remarkably well waterproofed, and it lives in one of the driest and hottest
habitats—under granite rocks in the stony desert.
It seems, then, that the arthropod integument is organized in such a way
as to permit a high degree of waterproofing if necessary, and that by and