The Active Transport and Passive Movement of Water in Insects

Abstract

Insects transfer water into their blood (R.H. equivalent 99 %) from air of relative humidity as low as 70%. Water can also be taken up through the rectum against large concentration gradients. In both instances, there is an active transport of water without the transport of other materials. The uptake of water from the tracheole and from solutions in contact with the cuticle requires a supply of energy. The cuticle can be perfectly semipermeable. The facility to transport water actively may be a common property of insects; the particular examples that are discussed in this chapter are free from the objection on grounds that complementary processes could have promoted the movement of water passively. The absorption of liquid water into the cuticle is accompanied by temporary reorganization of lipid molecules. A careful distinction is drawn in the chapter between an absorption process—which stops when saturated—and a continuous-flow transporting mechanism. The cuticular protein is under an active control by the epidermis, and there is specific evidence that it can modify the hydration of the cuticle. Changes in the permeability of cuticular lipids and especially of monolayers are brought about by thermal agitation. Monolayers can be inverted in natural circumstances and by the application of electrical forces. Natural inversion may account for the production of hydrofuge areas in insect respiratory systems. The evidence for the asymmetric permeability of insect cuticle is discussed and new experiments are described in the chapter. Some of the wider implications of the cuticle model upon the general field of cellular transport and electrical phenomena are also discussed in the chapter.