Studies on Plastron Respiration
Authors: Thorpe W. H. , Crisp D. J. 
Published: April 2021

In Part I the ability of the plastron mechanism of Aphelocheirus to retain its gas film against a pressure deficiency of several atmospheres was demonstrated, and preliminary experiments were carried out in order to establish that the plastron is in fact respiratory in function. In the present paper quantitative evidence is submitted in support of this view. A comparison is made between the efficiency of the cutaneous respiration of the fifth instar nymph and that of the plastron of the adult. The basic respiratory rate of nymphs and adults was determined by a metal micro-Barcroft respirometer (described in a separate paper, Crisp & Thorpe, 1947) and also by a micro-Winkler method. The basal metabolic rate when the insect is at rest is, for the adult, about 6 cu.mm./hr. O2 uptake and for the nymph about 3 cu.mm./hr. at 20 ° C. This corresponds in each case to 150 c.c./hr./kg. live weight. Insects when active in the apparatus increase their respiratory rate six- or sevenfold, and in a strongly swimming adult the increase is probably much more than this. In order to obtain a measure of respiratory efficiency which is independent of the momentary rate of metabolism the concept of ‘diffusion resistance’ R is employed, where Δp=fall in tension in atmospheres and q=the quantity flowing in units/sec. R= Δp/q. Another concept, the ‘respiratory index’, is defined as the reciprocal of the minimum oxygen tension difference between the animal and its surroundings required to maintain the basal respiratory rate. These quantities are determined approximately for Aphelocheirus nymphs and adults by measurement of oxygen uptake at low tensions of oxygen in nitrogen. The principles of this method are described. Similar measurements on adults whose spiracles were blocked with low melting-point wax made possible a comparison between the permeability of the cuticle of adult and nymph. The former was found to be 0·38 × 10−7 c.c. 02/sec./cm. thickness/sq.cm./atm. difference in tension, and the latter was considerably higher, being 1·6 × 10−7 unit. These values are compared with others found in the literature and are of the same order. The passage of oxygen through the plastron and tracheae of the adult is considered in a number of steps, and the fall in tension calculated at each step. It is shown that the total fall in tension for basal metabolism is about 0·03 atm. on a basis of simple diffusion, making certain necessary simplifying assumptions. This figure agrees with that computed experimentally, at least in order of magnitude, and substantiates the view that the plastron is an efficient respiratory organ over the whole of its surface and operates as a physical gill according to the established principles of diffusion. The chief resistances to diffusion appear to be at the water-air boundary of the plastron, and from the tracheal endings into the tissues. A comparison of adult and nymphal respiration is given in tabular form, and the necessity for a supplementary mode of respiration in the adult is shown. The development of the plastron in the fifth instar nymph prior to moulting is described, and gas is shown to appear there during the reabsorption of the moulting fluid without the animal having access to the surface. The gas is present before moulting takes place, and before the plastron becomes connected with the tracheal system. The waterproofing of the hairs is probably not a result of secretion of gas into the plastron, but is a necessary condition before gas can replace the moulting fluid there. Moulting takes place beneath the surface of the water. A general theory of the operation of the plastron and its relation to an air store is developed. Since the plastron is capable of withstanding external pressure it can be regarded functionally as a special case of the closed tracheal system. Thus not only the plastron but the whole of the tracheal system containing air must be capable of withstanding the excess pressure Δp. In practice some of this excess pressure may be taken up by the elasticity of the external cuticle, some by the elasticity of the tracheal walls themselves, but a single defect, allowing the volume of the air space to decrease continuously, will render the whole system ineffective. It is shown that the plastron in Aphelocheirus is unlikely to be rendered ineffective by any environmental condition which the animal is likely to meet, though should the animal descend to great depths the strain on the system would be considerable. The nature of the plastron is shown to be a major factor governing the ecology of the insect. As a result of the development of this method of respiration Aphelocheirus has become independent of contact with the air. This in turn has enabled it to inhabit swift-flowing streams without exposure to the danger of being swept away which visits to the surface would entail. The fact that it does not have to carry an air store has thus enabled it to become a true bottom-living predator heavier than water. It can thus occupy an ecological niche not available to any other comparable predatory insect in the adult stage. On the other hand, of course, its method of respiration necessarily restricts it to well-aerated, that is to turbulent, water. An appendix (Appendix II) is supplied in which certain general relations for diffusion in an absorbing medium are deduced, and these are applied to calculations of oxygen diffusion from the tracheal endings.