VI. DESERT ARTHROPODS
333
critical osmotic pressure difference was established. If the difference was
larger still, water then moved in the opposite direction (i.e., into the lumen
of the rectum). The critical difference was not constant, however, but depended upon the water reserves of the locust. Under water stress, imposed
by keeping the insects at 60% relative humidity with saline water to drink,
the critical osmotic pressure difference was about twice that for insects
with access to tap water. Clearly, the excretory system of the desert locust
is physiologically well adapted to regulate the insect's water content. We
cannot legitimately claim this as an adaptation found only in desert insects,
however, since Phillips found similar properties in the blowfly, Calliphora.
As regards the actual mechanism of water uptake, Phillips found that although a small amount of xylose was also taken up, he obtained similar
results with trehalose, which is a nonpenetrating solute, so that it is not
the case that water simply followed an active absorption of solute. Neither
can the slight increase in hydrostatic pressure following injection account
for the movement of water, for this was negligible (about 12 cm of water)
compared with the large osmotic pressure difference in the opposite direction. Finally, the possibility of electro-osmosis as an explanation was ruled
out since the establishment of contrary potential differences across the wall
did not prevent uptake. He was left, then, with the firm impression that
water itself is moved by an active process against a strong osmotic gradient.
However, recent work on the blowfly, Calliphora (Gupta and Berridge,
1966; Berridge and Gupta, 1967, 1968), and on the cockroach, Periplaneta
(Oschman and Wall, 1969), suggests another mechanism which does not
involve active transport of water, but only of K
+
or other ions. By means
of electron microscopy, Gupta and Berridge found that within each rectal
papilla in the blowfly there is a complex system of intercellular spaces and
sinuses which are in connection, by means of a one-way valve, with the
hemocoel, and that the walls of the cells lining this system are themselves
complexly infolded to provide innumerable stacks of plasma membranes.
The authors believe that the plasma membrane stacks are sites of active
transport of KCl into the morphologically intercellular spaces, thus raising
the osmotic pressure there and causing the withdrawal of water from the
cytoplasm and ultimately from the rectal lumen. The resultant increase in
volume causes a mass flow of water and solutes through the valve into the
hemocoele. Energy-releasing enzyme systems exist on the intracellular surfaces of the plasma membranes, where they could well supply energy for
the active transport of ions (Berridge and Gupta, 1968).
A similar mechanism, also based on the more general model proposed
by Diamond and Tormey (1966), has been suggested for the transport
of water and ions across the tubule wall in isosmotic proportions during
the formation of urine (Berridge and Oschman, 1969).
333
critical osmotic pressure difference was established. If the difference was
larger still, water then moved in the opposite direction (i.e., into the lumen
of the rectum). The critical difference was not constant, however, but depended upon the water reserves of the locust. Under water stress, imposed
by keeping the insects at 60% relative humidity with saline water to drink,
the critical osmotic pressure difference was about twice that for insects
with access to tap water. Clearly, the excretory system of the desert locust
is physiologically well adapted to regulate the insect's water content. We
cannot legitimately claim this as an adaptation found only in desert insects,
however, since Phillips found similar properties in the blowfly, Calliphora.
As regards the actual mechanism of water uptake, Phillips found that although a small amount of xylose was also taken up, he obtained similar
results with trehalose, which is a nonpenetrating solute, so that it is not
the case that water simply followed an active absorption of solute. Neither
can the slight increase in hydrostatic pressure following injection account
for the movement of water, for this was negligible (about 12 cm of water)
compared with the large osmotic pressure difference in the opposite direction. Finally, the possibility of electro-osmosis as an explanation was ruled
out since the establishment of contrary potential differences across the wall
did not prevent uptake. He was left, then, with the firm impression that
water itself is moved by an active process against a strong osmotic gradient.
However, recent work on the blowfly, Calliphora (Gupta and Berridge,
1966; Berridge and Gupta, 1967, 1968), and on the cockroach, Periplaneta
(Oschman and Wall, 1969), suggests another mechanism which does not
involve active transport of water, but only of K
+
or other ions. By means
of electron microscopy, Gupta and Berridge found that within each rectal
papilla in the blowfly there is a complex system of intercellular spaces and
sinuses which are in connection, by means of a one-way valve, with the
hemocoel, and that the walls of the cells lining this system are themselves
complexly infolded to provide innumerable stacks of plasma membranes.
The authors believe that the plasma membrane stacks are sites of active
transport of KCl into the morphologically intercellular spaces, thus raising
the osmotic pressure there and causing the withdrawal of water from the
cytoplasm and ultimately from the rectal lumen. The resultant increase in
volume causes a mass flow of water and solutes through the valve into the
hemocoele. Energy-releasing enzyme systems exist on the intracellular surfaces of the plasma membranes, where they could well supply energy for
the active transport of ions (Berridge and Gupta, 1968).
A similar mechanism, also based on the more general model proposed
by Diamond and Tormey (1966), has been suggested for the transport
of water and ions across the tubule wall in isosmotic proportions during
the formation of urine (Berridge and Oschman, 1969).