Quarterly Journal of Microscopical Science, Vol s3-94, 507-522, Copyright © 1953 by Company of Biologists

Surface Forces in the Tracheal System of Insects

¹ Department of Zoology, University of Cambridge

The mechanism by which the tracheal system becomes filled with air is reviewed. It is concluded that the fluid contents are actively absorbed by the walls of the system. The air often enters from the atmosphere through the spiracles. In some insects the system fills while the insect is submerged in water or while the spiracles are closed. It is shown by means of simple models how the tanning of the lining of the larger tracheae or the secretion of wax over the walls will bring about the liberation of gas from solution when the fluid is subjected to a very slight negative pressure.

The movements of fluid in the tracheole endings of insects are also reviewed. The removal of fluid which takes place during activity, particularly under conditions of deficient oxygen supply, is not caused by secretory activity but by the physical forces produced by the products of metabolism.

The evidence supports the view that the fluid in the tracheoles is a cell sap whose passage up the tracheole under the action of capillarity is opposed by the elasticity or swelling pressure of the cytoplasmic sheath of the tracheoles. It is shown by means of a simple gelatin model how osmotic changes in the surrounding fluid, acting by way of a cytoplasmic sheath, will bring about the absorption of such a cell sap.

A more exact model, illustrating the greater permeability of the inner wall of the tracheole which the proposed mechanism requires, is provided by the anal papillae of mosquito larvae. These structures show the same adaptation to a saline medium as is postulated in the tracheoles of mosquito larvae from salt water.

Alternative mechanisms are discussed. It is suggested that the capillary forces in the tracheoles are probably small. The effects of probable contamination of the tracheole fluid by an oily film (Beament) and the effects of changes in hydrogen-ion concentration on interfacial tensions are illustrated by reference to further models.