ABSTRACT
It is well known that the larvae of certain mosquitoes can thrive both in fresh water and in water of a high degree of salinity, while the larvae of other mosquitoes are quickly killed by salt water ; and that such differences may exist even between local races of the same species (Evans, 1931). It is evident that we have here to deal with a physiological adaptation, and it was the object of the present work to discover what the nature of this adaptation might be.
The physiological properties of larvae of the yellow-fever mosquito (Aedes (Stegomyia) argenteus Poir), which were accustomed to salt water, have been compared with those same properties in normal larvae. It so happens that this particular larva is unable to tolerate very salt water ; but it is probable that the relatively small degree of resistance which it can acquire is of the same kind as that occurring in more tolerant species, and therefore the mechanism of this resistance may throw light upon the general problem.
The artificial sea water used had the following composition: NaCl, 2·83 per cent.; KC1, 0·076 per cent. ; MgCl,, 0· 501 per cent. ; CaCl,, 0·122 per cent. This has an osmotic pressure approximately equal to 3·5 per cent, sodium chloride. No bicarbonate was added to this mixture which was therefore more add than natural sea water.
On the other hand, it is perhaps doubtful whether these “skeletal” elements in the cells can really resist the very great osmotic forces involved. Perhaps the more important changes in the cells are those affecting the cohesion of the colloids in the presence of salts, i.e. changes in the invisible elastic elements.
The fourth possibility, namely an extrarenal excretion of a concentrated salt solution such as occurs in marine bony fishes (Smith, 1932) seems highly improbable in the mosquito larva.
An artificial solution was used, in which the balance of metallic ions was approximately that of sea water. and (6) the mid-gut and caeca can absorb the salt fluid and so avoid the excessive distension which occurs in unadapted larvae.
