ABSTRACT
The larvae of the mosquito Aedes detritus have been reported only from definitely saline waters. They have been found in water of salinity equivalent to c. 10 % NaCl.
In the laboratory they were acclimatized with ease to distilled water, sea water (7 % NaCl), 3 ·5 % NaCl, and glycerol (3 ·5 % NaCl). They also show considerable resistance to N/20 NaOH, but less to N/20 KOH and N/50 HC1. They are unable to live permanently in solutions of the chlorides of potassium, calcium and magnesium of osmotic pressure equivalent to 3 ·5 % NaCl.
In sea water of varying salinity they can regulate both the total osmotic pressure and chloride content of the haemolymph. A rise from nil to 6 ·0 % NaCl in the osmotic pressure of the medium is reflected in an increase of from c. 0 ·8 % to 1 ·4% NaCl in that of the haemolymph.
In hypotonic solutions and distilled water much chloride is lost, but this is compensated by an increase in the non-chloride fraction. In hypertonic sea water the rise in osmotic pressure is due to increase in the chloride fraction, the nonchloride fraction remaining constant.
From this and from experiments with non-electrolytes it is concluded that the larva is permeable to salts and to molecules as large as glycerol, and that the regulatory mechanism in hypertonic saline is concerned with compensation rather for penetration of salts than for loss of water by osmosis.
Ligature experiments suggest that this mechanism is the excretion of salt by the Malpighian tubes, but further proof is required.
Salt exchange with the environment takes place via the gut, the body surface being impermeable to salts and water.
The larvae are able to concentrate chloride from hypotonic solutions but not as effectively as fresh-water species and only when the chloride content of the medium is a little below that of the haemolymph.
The anal gills, as in all salt-water species, are very small and appear to be impermeable to salts and water. It is therefore concluded that they are not the seat of the chloride-absorbing mechanism.
The osmotic pressure of the haemolymph is trebled by treatment with glycerol (3 ·5 % NaCl), which must be mainly the result of penetration of glycerol. The larva will however live normally in this, and an important factor in the resistance to abnormal media is therefore the adaptability of the tissues to changes in the concentration and composition of the haemolymph.
The increase in the osmotic pressure of the haemolymph induced by hypertonic sea water and glycerol does not alter the amount of fluid in the tracheoles. This is discussed in relation to the possible mechanism for the absorption of the trecheóle fluid.
The expedition was supported by grants from the Government Grants Committee of the Royal Society, from the British Association, and from the British Museum of Natural History.
Camboumac (1937) records A. detritus and A. caspius from waters of chloride content ranging from 7 to 10 % NaCl in salt marshes on the Portuguese coast. Their recorded range in the Hayling Island salt marshes is from slightly saline water to 133 % sea water or 4 ·7 % NaCl (private communication-from Mr J. F. Marshall).
I am indebted to Dr G. Senevet, Faculté de Médecine, Alger, for the identification of these species.
A total analysis of this and some other waters in which these larvae were found (to be published later) show that they are moderately well balanced from a physiological point of view, but the percentage concentration of Na is usually lower and that of Ca, Mg, and SO4 higher than in sea water.
