Larvae of Aedes argenteus reared in fresh water are killed by 1.1 per cent. NaCl or by "sea water"1 isotonic with 1.3-1.4 per cent. NaCl. Newly hatched larvae are killed by 1.1 per cent. NaCl or "sea water" equivalent to 1.3 per cent. NaCl.

By gradually increasing the concentration, larvae can be made resistant to 1.1 per cent. NaCl and to "sea water" equivalent to 1.75 percent. NaCl (50 per cent. sea water).

The nature of the physiological adaptation in these larvae has been studied and the following conclusions reached:

1. The elastic strands in the cells of the gills become exaggerated, and these cells resist swelling in hypertonic salt solutions.

2. There are changes in the epithelium of the mid-gut so that: (a) the cells are no longer caused to swell up and become detached from the basement membrane; and (b) the mid-gut and caeca can absorb the salt fluid and so avoid the excessive distension which occurs in unadapted larvae.

3. It is possible that the Malpighian tubes excrete a more concentrated urine and that the reabsorptive activity of the rectum is increased.

The mosquito larva appears to be homoiosmotic in both fresh water and in hypertonic salt water.

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