336
E. B. EDNEY
the organs and mechanisms concerned with excretion in true desert species
would probably be very profitable.
F. GAIN OF WATER BY EATING AND BY DRINKING
We have seen that a good deal of information is now available concerning the sources of water loss and their regulation. The other side of the
balance has not been so well explored. Water is taken in with food by
all insects, whether they feed on blood, plant juices, or "dry" material.
Dry grain, for instance, usually contains about 15% water, and in some
cases, where this is the only source of water, insects seem to make do
with remarkably low water contents. For example, the flour moth, Ephestia, and the mealworm, Tenebrio, can both exist on food containing only
1% water (Fraenkel, 1929). The question is whether or not an insect
controls its water content by eating more or less food simply for the sake
of the water content, and evidence on this point is scarce.
According to Schultz (1930) Tenebrio larvae do just this, consuming
more food in dry conditions than in moist. Buxton (1922) many years
ago suggested that tenebrionid beetles in the North African Desert might
feed at n%ht on dead plant material which had a moisture content of 60%
owing to hygroscopic absorption from humidities above 80%, but he does
not say that he ever saw this happen. Pierre (1958) does not believe that
tenebrionid beetles obtain water in this way since they are mostly carnivorous. The process might be important for lepismatids, he suggests, although the water content of the dry plant material available was only 2%
after a night in summer. Taylor (1968), on the other hand, found that
leaves of a desert shrub, Disperma sp., have a water content as high as
40% after about 8 hours in air at 85% relative humidity, and form an
important source of water for oryx and Grant's gazelle in Kenya. Further
observation on these points is most desirable.
Intake of liquid water directly by drinking is widespread among insects
when such water is available, and here there is little doubt that the amount
drunk is related to need at the time. Bees consume more water at lower
humidity than at higher humidity (Altmann, 1956), and the amount they
consume can be increased by exposing them to C0 2 , which causes the
spiracles to open and thus leads to greater water loss. It seems that blood
volume is important in determining how much the blowfly, Calliphora,
drinks (DethSer and Evans, 1961), and injection of strong (0.4 M) solutions of NaCl into the hemocoel of Lucilia also stimulates the drinking
response (Barton-Browne, 1964).
The desert cockroach, Arenivaga, when in water shortage, seeks out
and drinks water when this is available (Hawke and Farley, 1973).
E. B. EDNEY
the organs and mechanisms concerned with excretion in true desert species
would probably be very profitable.
F. GAIN OF WATER BY EATING AND BY DRINKING
We have seen that a good deal of information is now available concerning the sources of water loss and their regulation. The other side of the
balance has not been so well explored. Water is taken in with food by
all insects, whether they feed on blood, plant juices, or "dry" material.
Dry grain, for instance, usually contains about 15% water, and in some
cases, where this is the only source of water, insects seem to make do
with remarkably low water contents. For example, the flour moth, Ephestia, and the mealworm, Tenebrio, can both exist on food containing only
1% water (Fraenkel, 1929). The question is whether or not an insect
controls its water content by eating more or less food simply for the sake
of the water content, and evidence on this point is scarce.
According to Schultz (1930) Tenebrio larvae do just this, consuming
more food in dry conditions than in moist. Buxton (1922) many years
ago suggested that tenebrionid beetles in the North African Desert might
feed at n%ht on dead plant material which had a moisture content of 60%
owing to hygroscopic absorption from humidities above 80%, but he does
not say that he ever saw this happen. Pierre (1958) does not believe that
tenebrionid beetles obtain water in this way since they are mostly carnivorous. The process might be important for lepismatids, he suggests, although the water content of the dry plant material available was only 2%
after a night in summer. Taylor (1968), on the other hand, found that
leaves of a desert shrub, Disperma sp., have a water content as high as
40% after about 8 hours in air at 85% relative humidity, and form an
important source of water for oryx and Grant's gazelle in Kenya. Further
observation on these points is most desirable.
Intake of liquid water directly by drinking is widespread among insects
when such water is available, and here there is little doubt that the amount
drunk is related to need at the time. Bees consume more water at lower
humidity than at higher humidity (Altmann, 1956), and the amount they
consume can be increased by exposing them to C0 2 , which causes the
spiracles to open and thus leads to greater water loss. It seems that blood
volume is important in determining how much the blowfly, Calliphora,
drinks (DethSer and Evans, 1961), and injection of strong (0.4 M) solutions of NaCl into the hemocoel of Lucilia also stimulates the drinking
response (Barton-Browne, 1964).
The desert cockroach, Arenivaga, when in water shortage, seeks out
and drinks water when this is available (Hawke and Farley, 1973).