VI. DESERT ARTHROPODS
337
Drinking from moist surfaces has also been observed in several arthropods. Spiders may drink against a suction pressure of as much as 600 mm
Hg (Parry, 1954), and both oral and anal drinking from moist surfaces
(although at very much lower suction pressures) occurs in isopods (Spencer and Edney, 1954). These animals were, in fact, able to replace all
the water they lost through transpiration (and their integuments are rather
permeable) by such drinking, provided that the relative humidity was at
least 80%. Drinking from moist surfaces may prove to be of considerable
significance, particularly for desert forms, where water in bulk is rare, and
moist soil is more often available.
G. OXIDATION WATER
It is important to distinguish clearly between water taken in with the
fcod and oxidation water. Oxidation of hydrogen-containing food materials
necessarily leads to the production of water, and indeed this is a valuable,
sometimes a sole, source of water supply. But it is not the case, as is sometimes implied, that some animals "use" oxidation water while others do
not. Oxidation water simply enters the balance on the input side—whether
or not this results in an overall positive or negative balance depends upon
the size of all other constituents of the balance, such as loss by transpiration
and excretion, which are themselves dependent to a greater or less extent
on environmental conditions. The subject has been reviewed by Edney
(1957).
Information concerning the amounts of oxidation water derivable from
various classes of substrate is readily available (Schmidt-Nielsen, 1964),
and need not be repeated here, but brief reference to two further points
may be useful. First, we want to know whether insects metabolize more
food material in conditions of water stress and thus derive more oxidation
water than they otherwise would. The early work in this field (Buxton,
1930; Fraenkel and Blewett, 1944; Mellanby, 1932a) was held to demonstrate that this did occur. In Tenebrio larvae the metabolic rate was indeed
higher in dry than in moist air, although such an effect was not apparent
in other insects such as Cimex (Mellanby, 1932b, 1934).
In tsetse flies, Buxton and Lewis (1934) found an increase in metabolic
rate in dry air. But as Bursell (1957b) pointed out, tsetse flies are more
active in dry air than in moist air, and this, rather than an attempt to
regulate the water balance, would account for the increased metabolism.
In any case (and perhaps this is the main point) an increase in metabolic
rate can by no means be relied upon to increase the water reserves, since
more 0 2 is needed and this could result in a greater loss of water from
337
Drinking from moist surfaces has also been observed in several arthropods. Spiders may drink against a suction pressure of as much as 600 mm
Hg (Parry, 1954), and both oral and anal drinking from moist surfaces
(although at very much lower suction pressures) occurs in isopods (Spencer and Edney, 1954). These animals were, in fact, able to replace all
the water they lost through transpiration (and their integuments are rather
permeable) by such drinking, provided that the relative humidity was at
least 80%. Drinking from moist surfaces may prove to be of considerable
significance, particularly for desert forms, where water in bulk is rare, and
moist soil is more often available.
G. OXIDATION WATER
It is important to distinguish clearly between water taken in with the
fcod and oxidation water. Oxidation of hydrogen-containing food materials
necessarily leads to the production of water, and indeed this is a valuable,
sometimes a sole, source of water supply. But it is not the case, as is sometimes implied, that some animals "use" oxidation water while others do
not. Oxidation water simply enters the balance on the input side—whether
or not this results in an overall positive or negative balance depends upon
the size of all other constituents of the balance, such as loss by transpiration
and excretion, which are themselves dependent to a greater or less extent
on environmental conditions. The subject has been reviewed by Edney
(1957).
Information concerning the amounts of oxidation water derivable from
various classes of substrate is readily available (Schmidt-Nielsen, 1964),
and need not be repeated here, but brief reference to two further points
may be useful. First, we want to know whether insects metabolize more
food material in conditions of water stress and thus derive more oxidation
water than they otherwise would. The early work in this field (Buxton,
1930; Fraenkel and Blewett, 1944; Mellanby, 1932a) was held to demonstrate that this did occur. In Tenebrio larvae the metabolic rate was indeed
higher in dry than in moist air, although such an effect was not apparent
in other insects such as Cimex (Mellanby, 1932b, 1934).
In tsetse flies, Buxton and Lewis (1934) found an increase in metabolic
rate in dry air. But as Bursell (1957b) pointed out, tsetse flies are more
active in dry air than in moist air, and this, rather than an attempt to
regulate the water balance, would account for the increased metabolism.
In any case (and perhaps this is the main point) an increase in metabolic
rate can by no means be relied upon to increase the water reserves, since
more 0 2 is needed and this could result in a greater loss of water from