328
E. B. EDNEY
in water loss. We shall return to this discussion in the context of metabolic
water.
D. WATER LOSS IN EXCRETION AND DEFECATION
We turn now to the last of the main sources of water loss—the elimination of waste material. The products of nitrogen metabolism are eliminated
mainly as uric acid by insects, as in birds and saurian reptiles, so that
little or no liquid water need be involved. Arachnids mostly use the related
compound guanine, with the same effect. The disadvantage of excreting
nitrogen as ammonia (NH 3 ) is that the latter is highly toxic and usually
needs much water for its elimination. Furthermore it is more wasteful of
unoxidized H so that less oxidation water results. However, isopods, in
spite of their terrestrial habit, eliminate much of their nitrogen as NH 3 ,
not in solution in water, but directly as ammonia gas (W. C. Sloan, personal communication; Wieser and Schweizer, 1970), as do certain terrestrial snails (Speeg and Campbell, 1968).
The classical picture of excretion in insects was obtained by Wigglesworth (1931) for Rhodnius and is briefly as follows. Uric acid enters the
distal part of the Malpighian tubules as the soluble potassium salt. As it
moves down the tubule, C0 2 is added, the pH goes down from 7.2 to
6.3, and uric acid is precipitated out while KHC0 3 and H 2 0 are reabsorbed, either in the proximal tubule or, more usually, later in the rectum. More recent work of Berridge (1968, 1969) has shown that urine
formation depends upon the active transport of potassium and an anion
(phosphate or chloride), and Berridge (1967) and Berridge and Oschman
(1969) have provided an attractive model for the mechanism, based on
information concerning the microstructure of the tubules (see below). The
Malpighian tubule-rectal gland system in insects (see Fig. 7) functions in
a manner analogous to that of the vertebrate kidney nephron, achieving
both osmotic and ionic regulation by selective absorption of water and
of ions. Good evidence of this was found by Phillips (1964) who gave
desert locusts either tap water or strong saline to drink and observed the
effects on their hemolymph and rectal fluid. Ionic concentration of the
hemolymph rose in the saline-fed locusts, but did not nearly reach the
concentration in the water they drank. Rectal fluid showed very high concentrations in the saline-fed locusts and very low concentrations in the
controls (see Table IV). Work in this field has been reviewed by Stobbart
and Shaw (1964), by Phillips (1970), and by Riegel (1971). An admirably full account of the mechanisms involved, including hormonal control,
is provided by Maddrell (1971).
The rate at which urine is formed in the Malpighian tubule is strongly
affected by ionic concentration in the hemolymph, in particular K
+
, and
E. B. EDNEY
in water loss. We shall return to this discussion in the context of metabolic
water.
D. WATER LOSS IN EXCRETION AND DEFECATION
We turn now to the last of the main sources of water loss—the elimination of waste material. The products of nitrogen metabolism are eliminated
mainly as uric acid by insects, as in birds and saurian reptiles, so that
little or no liquid water need be involved. Arachnids mostly use the related
compound guanine, with the same effect. The disadvantage of excreting
nitrogen as ammonia (NH 3 ) is that the latter is highly toxic and usually
needs much water for its elimination. Furthermore it is more wasteful of
unoxidized H so that less oxidation water results. However, isopods, in
spite of their terrestrial habit, eliminate much of their nitrogen as NH 3 ,
not in solution in water, but directly as ammonia gas (W. C. Sloan, personal communication; Wieser and Schweizer, 1970), as do certain terrestrial snails (Speeg and Campbell, 1968).
The classical picture of excretion in insects was obtained by Wigglesworth (1931) for Rhodnius and is briefly as follows. Uric acid enters the
distal part of the Malpighian tubules as the soluble potassium salt. As it
moves down the tubule, C0 2 is added, the pH goes down from 7.2 to
6.3, and uric acid is precipitated out while KHC0 3 and H 2 0 are reabsorbed, either in the proximal tubule or, more usually, later in the rectum. More recent work of Berridge (1968, 1969) has shown that urine
formation depends upon the active transport of potassium and an anion
(phosphate or chloride), and Berridge (1967) and Berridge and Oschman
(1969) have provided an attractive model for the mechanism, based on
information concerning the microstructure of the tubules (see below). The
Malpighian tubule-rectal gland system in insects (see Fig. 7) functions in
a manner analogous to that of the vertebrate kidney nephron, achieving
both osmotic and ionic regulation by selective absorption of water and
of ions. Good evidence of this was found by Phillips (1964) who gave
desert locusts either tap water or strong saline to drink and observed the
effects on their hemolymph and rectal fluid. Ionic concentration of the
hemolymph rose in the saline-fed locusts, but did not nearly reach the
concentration in the water they drank. Rectal fluid showed very high concentrations in the saline-fed locusts and very low concentrations in the
controls (see Table IV). Work in this field has been reviewed by Stobbart
and Shaw (1964), by Phillips (1970), and by Riegel (1971). An admirably full account of the mechanisms involved, including hormonal control,
is provided by Maddrell (1971).
The rate at which urine is formed in the Malpighian tubule is strongly
affected by ionic concentration in the hemolymph, in particular K
+
, and