Some salts will be gained from food, but in addition to this, some aquatic larvae are able to take up salts from very dilute solutions in the water they live in. Larvae of the dipteran genera Aedes , Culex and Chironomus take up salts though specialized structures called anal papillae, also known as anal gills. This uptake requires energy expenditure. The enery is exerted in pumping sodium, potassium, chloride and phosphate ions against gradients from dilute fresh water to hyperosmotic extracellular compartments. The anal papillae hypertrophy as media become more dilute, presumably giving greater surface area for transport of salts.
Some insects are able to extract ions from very dilute solutions.
Aedes aegypti are able to maintain steady osmotic states in media containing only 1 X 10-6 moles per liter of sodium. Other aquatic larvae, such as the neuropteran Sialis , have no specialized surface for salt uptake. In addition to their ability to resorb salts in rectums, these insects are able to compensate for osmotic imbalance by maintaining higher levels of hemolymph amino acids. These amino acids serve as osmolytes and thereby help maintain the crucial balance between the major fluid compartments. This emphasizes the point that osmotic compositions of the two major fluid compartments can be quite different while osmotic concentrations can be quite similar.