334
E. B. EDNEY
Possibly rectal water absorption by locusts may work in a similar way,
as Stobbart (1968) suggests, and Irvine and Phillips (1971) agree. Certainly the cockroach rectal pads have a microstructure that strongly suggests a function analogous to that in the blowfly, and Sauer et al. (1970),
using dinitrophenol to inhibit active transport, have shown that the extent
of résorption of water in vitro, at least, from the cockroach lumen is
adapted to the state of the insect's water reserves. These authors also propose a mechanism for the control of water and solute transport across
the wall, also based essentially on the model of Diamond and Tormey
(1966).
Finally, some interesting information has recently been obtained from
the adult mealworm beetle, Tenebrio molitor, bearing on the same problem. This insect and several others, notably those where water may be
in short supply (Wigglesworth, 1965), possess a so-called cryptonephridial
system in which the distal ends of the Malpighian tubules are closely applied to the rectal wall and held there by a firm perirectal membrane (Fig.
10). It has been thought for some time that this system might help in some
way to extract water from the rectal contents, and some very illuminating
work by Ramsay (1964) and by Grimstone et al. (1968) goes a long
way toward explaining just how this may occur.
The rectum of Tenebrio is covered by the convoluted ends of six Malpighian tubules bound down by a double membrane which is impermeable
except at a number of thin windows or leptophragmata. When the insect
is in water shortage (but not otherwise) the rectal complex is set in action,
perhaps by the relatively high osmotic pressure of the hemolymph
(Δ 1.4°C compared with the normal Δ 0.7°C).
The tubules then actively absorb ions, but not water, from the hemolymph through the leptophragmata, thus raising the osmotic concentration
in their lumina. Sodium and potassium are also taken up from the perirectal fluid. Water then moves passively from the perirectal fluid inwards to
the tubules, and the osmolarity of the former rises, not as a result of an
increase in electrolytes, but rather by the concentration of large molecules
and of nonelectrolytes. Such a high osmolarity on the inner side of the
rectal cells reduces the gradient against which they have to work in absorbing water from the rectal contents. Finally the tubular fluid which consists
of water and electrolytes, and possibly the perirectal fluid, passes out of
the rectal complex and becomes generally available.
The osmotic pressure in the posterior region of the tubules (where it
is greatest) is as high as Δ 8°C (which is equivalent to about 2 M KC1,
or a theoretical 90 atm). The efficiency of the system can be judged by
the fact that the fecal pellets of Tenebrio may be produced with a water
content which is in equilibrium with air at 90% relative humidity or lower;
E. B. EDNEY
Possibly rectal water absorption by locusts may work in a similar way,
as Stobbart (1968) suggests, and Irvine and Phillips (1971) agree. Certainly the cockroach rectal pads have a microstructure that strongly suggests a function analogous to that in the blowfly, and Sauer et al. (1970),
using dinitrophenol to inhibit active transport, have shown that the extent
of résorption of water in vitro, at least, from the cockroach lumen is
adapted to the state of the insect's water reserves. These authors also propose a mechanism for the control of water and solute transport across
the wall, also based essentially on the model of Diamond and Tormey
(1966).
Finally, some interesting information has recently been obtained from
the adult mealworm beetle, Tenebrio molitor, bearing on the same problem. This insect and several others, notably those where water may be
in short supply (Wigglesworth, 1965), possess a so-called cryptonephridial
system in which the distal ends of the Malpighian tubules are closely applied to the rectal wall and held there by a firm perirectal membrane (Fig.
10). It has been thought for some time that this system might help in some
way to extract water from the rectal contents, and some very illuminating
work by Ramsay (1964) and by Grimstone et al. (1968) goes a long
way toward explaining just how this may occur.
The rectum of Tenebrio is covered by the convoluted ends of six Malpighian tubules bound down by a double membrane which is impermeable
except at a number of thin windows or leptophragmata. When the insect
is in water shortage (but not otherwise) the rectal complex is set in action,
perhaps by the relatively high osmotic pressure of the hemolymph
(Δ 1.4°C compared with the normal Δ 0.7°C).
The tubules then actively absorb ions, but not water, from the hemolymph through the leptophragmata, thus raising the osmotic concentration
in their lumina. Sodium and potassium are also taken up from the perirectal fluid. Water then moves passively from the perirectal fluid inwards to
the tubules, and the osmolarity of the former rises, not as a result of an
increase in electrolytes, but rather by the concentration of large molecules
and of nonelectrolytes. Such a high osmolarity on the inner side of the
rectal cells reduces the gradient against which they have to work in absorbing water from the rectal contents. Finally the tubular fluid which consists
of water and electrolytes, and possibly the perirectal fluid, passes out of
the rectal complex and becomes generally available.
The osmotic pressure in the posterior region of the tubules (where it
is greatest) is as high as Δ 8°C (which is equivalent to about 2 M KC1,
or a theoretical 90 atm). The efficiency of the system can be judged by
the fact that the fecal pellets of Tenebrio may be produced with a water
content which is in equilibrium with air at 90% relative humidity or lower;