VI. DESERT ARTHROPODS
373
these insects are active, spiracular water loss increases owing to the need
for greater 0 2 uptake, but not necessarily proportionately. In some insects
total transpiration is regulated according to water needs by spiracular control and by controlling the water content of fecal material.
In locusts, dry air seems to decrease cuticle permeability, and water loss
is further controlled by reducing ventilatory movements. Oxidation (metabolic) water is but one component of water balance, although an important
one, for desert arthropods. Its significance during the prolonged migratory
flight of locusts has been shown by Weis-Fogh ( 1967).
Excretion of waste nitrogen as uric acid involves little water loss, and
the Malpighian tubule-rectal gland complex regulates osmotic and ionic
concentrations. Water may be absorbed from the rectum against an overall
water activity gradient when necessary, and there is mounting evidence
that in such cases water follows the active transport and recycling of K
+
or other ions.
Uptake of water occurs with feeding and drinking, but it is doubtful
whether feeding or metabolism increase under water stress. Active uptake
of water vapor is known in a few arachnids and insects including the desert
cockroach, Arenivaga investigata, which, after partial dehydration, absorbs
water vapor from humidities down to 82.5% until its normal water content
is restored. The mechanism is not fully understood, but the energy involved
is relatively small. This insect also regulates its hemolymph osmotic pressure against changes in overall water content.
There is no convincing evidence that desert arthropods tolerate greater
water depletion than others—they probably rely on more efficient conservation. There is some evidence that they tolerate higher temperatures
than mesic forms do. Thus the camel spider survives for 24 hours at an
air temperature of 50°C, the tenebrionid beetle, Adesmia, tolerates a similar soil temperature, but measurements of body temperature are scanty.
Body temperature may be reduced for short periods by evaporative cooling, and depressions of 3°C have been measured in the desert isopod,
Hemilepistus. In tsetse flies, a small temperature depression at near lethal
temperature is achieved by opening the spiracles.
The significance of surface coloration is not clear, but other morphological adaptations are evident. Thus sand swimmers such as Arenivaga are
flat with short, spiny legs, while surface runners such as Onymacris have
long, spindly legs. Behavioral mechanisms are important in enabling
arthropods to avoid intolerable conditions. Many are nocturnal and some
are strongly seasonal in appearance. The appropriate relation of development to seasonal changes is exemplified by locusts, which achieve sexual
maturity only after feeding on fresh vegetation and which are carried, as
swarms, to areas where rain is likely.
373
these insects are active, spiracular water loss increases owing to the need
for greater 0 2 uptake, but not necessarily proportionately. In some insects
total transpiration is regulated according to water needs by spiracular control and by controlling the water content of fecal material.
In locusts, dry air seems to decrease cuticle permeability, and water loss
is further controlled by reducing ventilatory movements. Oxidation (metabolic) water is but one component of water balance, although an important
one, for desert arthropods. Its significance during the prolonged migratory
flight of locusts has been shown by Weis-Fogh ( 1967).
Excretion of waste nitrogen as uric acid involves little water loss, and
the Malpighian tubule-rectal gland complex regulates osmotic and ionic
concentrations. Water may be absorbed from the rectum against an overall
water activity gradient when necessary, and there is mounting evidence
that in such cases water follows the active transport and recycling of K
+
or other ions.
Uptake of water occurs with feeding and drinking, but it is doubtful
whether feeding or metabolism increase under water stress. Active uptake
of water vapor is known in a few arachnids and insects including the desert
cockroach, Arenivaga investigata, which, after partial dehydration, absorbs
water vapor from humidities down to 82.5% until its normal water content
is restored. The mechanism is not fully understood, but the energy involved
is relatively small. This insect also regulates its hemolymph osmotic pressure against changes in overall water content.
There is no convincing evidence that desert arthropods tolerate greater
water depletion than others—they probably rely on more efficient conservation. There is some evidence that they tolerate higher temperatures
than mesic forms do. Thus the camel spider survives for 24 hours at an
air temperature of 50°C, the tenebrionid beetle, Adesmia, tolerates a similar soil temperature, but measurements of body temperature are scanty.
Body temperature may be reduced for short periods by evaporative cooling, and depressions of 3°C have been measured in the desert isopod,
Hemilepistus. In tsetse flies, a small temperature depression at near lethal
temperature is achieved by opening the spiracles.
The significance of surface coloration is not clear, but other morphological adaptations are evident. Thus sand swimmers such as Arenivaga are
flat with short, spiny legs, while surface runners such as Onymacris have
long, spindly legs. Behavioral mechanisms are important in enabling
arthropods to avoid intolerable conditions. Many are nocturnal and some
are strongly seasonal in appearance. The appropriate relation of development to seasonal changes is exemplified by locusts, which achieve sexual
maturity only after feeding on fresh vegetation and which are carried, as
swarms, to areas where rain is likely.