VI. DESERT ARTHROPODS
327
No general statement can be made regarding the relative amounts of
water lost from respiratory and integumental surfaces, because spiracular
loss increases greatly when the animal is active and because the cuticles
of different species vary greatly in permeability. In tsetse flies the total
loss by transpiration rises from 0.12mg/hour for a resting fly to
0.21 mg/hour if the fly is active for only 30% of the time, and this reflects
the need to open the spiracles in order to obtain more 0 2 for activity
(Bursell, 1957a). Flies with blocked spiracles lose only 0.09 mg/hour;
thus in this insect about 75% of water loss occurs through the cuticle,
but the proportion drops to 40% as a result of partial activity. In the
desert locust, Schistocerca gregaria, about 65% of the water loss takes
place through the spiracles during flight, while in the resting insect very
little indeed is so lost (Church, 1960). In flying Locusta, Loveridge
(1968b) found that trachéal water loss was about four times greater than
loss through the cuticle.
Ahearn (1969, 1970b) has made a useful analysis of the situation in
three desert tenebrionids. He found that at all temperatures up to 40°C
loss from the respiratory surfaces was very low, but above this temperature
spiracular loss rose rapidly and reached about % of total loss, probably
as a result of greater 0 2 need causing the spiracles to open.
In the scorpion, Hadrurus arizonensis, overall water loss is low
(0.028% of weight per hour at 30°C), and, as in Ahearn's beetles referred
to above, Hadley (1970a) found respiratory water loss to be negligible
up to 35°C and to rise steeply at higher temperatures, until at 45 °C it
was about five times as great as cuticular loss. Once again the great increase probably results from ventilation necessary to cope with the high
0 2 needs.
Recently, in an interesting study on the whip scorpion, Mastigoproctus
giganteus, Ahearn (1970c) found a large increase in rate of water loss
at temperatures above 37.5°C, an increase which seems to result from a
change in cuticle permeability, since 0 2 uptake increased more regularly
with temperature. Mastigoproctus is not a desert arthropod, but it is interesting to compare its water balance mechanisms with others that are. In
fact, its water loss rate is some ten times greater than in some similarly
sized desert scorpions (see Table II).
The relation between spiracular water loss and 0 2 uptake is not necessarily a simple one. In tsetse flies, for example, the metabolic rate is increased 22 times during flight, but the water loss increases only six times
(Bursell, 1959b). In any case, the extent of water loss incurred as a result
of 0 2 absorption depends upon the relative humidity of the inspired air.
Furthermore, some such device as the intermittent C0 2 release described
above could permit greater 0 2 uptake without a concomitant increase
327
No general statement can be made regarding the relative amounts of
water lost from respiratory and integumental surfaces, because spiracular
loss increases greatly when the animal is active and because the cuticles
of different species vary greatly in permeability. In tsetse flies the total
loss by transpiration rises from 0.12mg/hour for a resting fly to
0.21 mg/hour if the fly is active for only 30% of the time, and this reflects
the need to open the spiracles in order to obtain more 0 2 for activity
(Bursell, 1957a). Flies with blocked spiracles lose only 0.09 mg/hour;
thus in this insect about 75% of water loss occurs through the cuticle,
but the proportion drops to 40% as a result of partial activity. In the
desert locust, Schistocerca gregaria, about 65% of the water loss takes
place through the spiracles during flight, while in the resting insect very
little indeed is so lost (Church, 1960). In flying Locusta, Loveridge
(1968b) found that trachéal water loss was about four times greater than
loss through the cuticle.
Ahearn (1969, 1970b) has made a useful analysis of the situation in
three desert tenebrionids. He found that at all temperatures up to 40°C
loss from the respiratory surfaces was very low, but above this temperature
spiracular loss rose rapidly and reached about % of total loss, probably
as a result of greater 0 2 need causing the spiracles to open.
In the scorpion, Hadrurus arizonensis, overall water loss is low
(0.028% of weight per hour at 30°C), and, as in Ahearn's beetles referred
to above, Hadley (1970a) found respiratory water loss to be negligible
up to 35°C and to rise steeply at higher temperatures, until at 45 °C it
was about five times as great as cuticular loss. Once again the great increase probably results from ventilation necessary to cope with the high
0 2 needs.
Recently, in an interesting study on the whip scorpion, Mastigoproctus
giganteus, Ahearn (1970c) found a large increase in rate of water loss
at temperatures above 37.5°C, an increase which seems to result from a
change in cuticle permeability, since 0 2 uptake increased more regularly
with temperature. Mastigoproctus is not a desert arthropod, but it is interesting to compare its water balance mechanisms with others that are. In
fact, its water loss rate is some ten times greater than in some similarly
sized desert scorpions (see Table II).
The relation between spiracular water loss and 0 2 uptake is not necessarily a simple one. In tsetse flies, for example, the metabolic rate is increased 22 times during flight, but the water loss increases only six times
(Bursell, 1959b). In any case, the extent of water loss incurred as a result
of 0 2 absorption depends upon the relative humidity of the inspired air.
Furthermore, some such device as the intermittent C0 2 release described
above could permit greater 0 2 uptake without a concomitant increase