Transport Across Insect Excretory Epithelia

Abstract

One of the most remarkable features of insects is their ability to flourish in environments characterized by extreme osmoregulatory stress. They can live in fresh water, in deserts, in extremely hypersaline fluids or in waters of very unbalanced ionic content. They can do all this in spite of their very small size, which accentuates the problem by increasing the ratio of their surface area to volume. Part of this ability is due to the low permeability of their external cuticle and part to the relatively large volume of their extracellular fluids, which reduces the effects of ion and water movements into or out of the animal. The excretory system, however, is of paramount importance in that its activity ultimately controls the composition of the internal environment of the animal. It should not surprise us, therefore, to find that it can do some remarkable things. For example, in blood-sucking insects it can eliminate fluid at a rate equivalent to removing 5–10 percent of the extracellular fluid in one minute; while at the other extreme it can take up water vapour from air of a relative humidity of only 45 percent. Other cases are known where, for example, parts of the system secrete fluid containing 3 mol 1⁻¹ KCl or 2 mol 1⁻¹ MgSO₄ or 1.0 mol 1⁻¹ NH₄C1 or 150 mmol 1⁻¹ nicotine.