VI. DESERT ARTHROPODS
347
of exchange of water between a mite and the surrounding air, and calculated rate constants for transpiration and absorption. They conclude that
a hypothetical cyclical pump moving water against the known gradient,
would have to pump only 0.0001 pg of water once a second. Maddrell,
(1971), using Noble-Nesbitt's (1969) data for Thermobia, concluded that
even if the process were only 10% efficient the energy involved would
be much less (per unit body weight) than that used by a flying insect;
although, if all the energy were used in the rectum, that tissue would have
to be one of the most active known in any animal.
To show that little energy is involved by no means explains the mechanism of course, and this is still essentially a mystery. Up to this point
there have been two main suggestions regarding mechanisms, one by
Beament (1964, 1965), the other by Locke (1964, 1965). Beament's proposal involves the cyclical exposure of polar groups on protein molecules
which would strongly attract water, coupled with a rectifying mechanism
provided by the epicuticular lipid molecules. Locke's involves the depolymerization of glycoprotein molecules to produce locally low water activities
(high osmotic pressure). Both proposals have in common the generation
of water activity in the cuticle that is low enough to produce an inward
movement of water from the outside air. This is not the place to consider
what is essentially a biophysical problem in further detail. The mechanisms
are well discussed by Berridge (1970) and by Maddrell (1970).
IV. Tolerance of Water Depletion and of High Temperatures
For all arthropods, and for desert ones particularly, it would clearly be
advantageous to be able to withstand considerable loss of water and to
tolerate high temperatures. So far as water is concerned, the "normal" content varies greatly among species, from over 90% of the total body weight
in soft-bodied caterpillars to less than 50% in highly sclerotized beetles.
Such figures are often imprecise since the amount of fat present, and the
gut contents, may seriously affect the measurements. A figure generally
acceptable for the majority of insects would be about 75% (Bursell,
1964a) and this may be reduced by transpiration to about 60% without
lethal effects. Values in this vicinity, based on fat-free weights, have been
found for tsetse flies (Bursell, 1959a).
In the desert sand roach, Arenivaga, the normal water content is about
67%, and the insects can generally survive a loss of water by transpiration
equal to 30% of their wet weight during 1 week. Allowing for oxidation
water produced during this time, the final water content would be about
60%. It has been reported that the desert grasshopper, Poecilocerus hiero-
347
of exchange of water between a mite and the surrounding air, and calculated rate constants for transpiration and absorption. They conclude that
a hypothetical cyclical pump moving water against the known gradient,
would have to pump only 0.0001 pg of water once a second. Maddrell,
(1971), using Noble-Nesbitt's (1969) data for Thermobia, concluded that
even if the process were only 10% efficient the energy involved would
be much less (per unit body weight) than that used by a flying insect;
although, if all the energy were used in the rectum, that tissue would have
to be one of the most active known in any animal.
To show that little energy is involved by no means explains the mechanism of course, and this is still essentially a mystery. Up to this point
there have been two main suggestions regarding mechanisms, one by
Beament (1964, 1965), the other by Locke (1964, 1965). Beament's proposal involves the cyclical exposure of polar groups on protein molecules
which would strongly attract water, coupled with a rectifying mechanism
provided by the epicuticular lipid molecules. Locke's involves the depolymerization of glycoprotein molecules to produce locally low water activities
(high osmotic pressure). Both proposals have in common the generation
of water activity in the cuticle that is low enough to produce an inward
movement of water from the outside air. This is not the place to consider
what is essentially a biophysical problem in further detail. The mechanisms
are well discussed by Berridge (1970) and by Maddrell (1970).
IV. Tolerance of Water Depletion and of High Temperatures
For all arthropods, and for desert ones particularly, it would clearly be
advantageous to be able to withstand considerable loss of water and to
tolerate high temperatures. So far as water is concerned, the "normal" content varies greatly among species, from over 90% of the total body weight
in soft-bodied caterpillars to less than 50% in highly sclerotized beetles.
Such figures are often imprecise since the amount of fat present, and the
gut contents, may seriously affect the measurements. A figure generally
acceptable for the majority of insects would be about 75% (Bursell,
1964a) and this may be reduced by transpiration to about 60% without
lethal effects. Values in this vicinity, based on fat-free weights, have been
found for tsetse flies (Bursell, 1959a).
In the desert sand roach, Arenivaga, the normal water content is about
67%, and the insects can generally survive a loss of water by transpiration
equal to 30% of their wet weight during 1 week. Allowing for oxidation
water produced during this time, the final water content would be about
60%. It has been reported that the desert grasshopper, Poecilocerus hiero-