ABSTRACT
As is well known from the work of Ege and others many aquatic insects (e.g. Dytiscidae, Hydrophilidae, Corixidae, Notonectidae) which carry air stores in the form of bubbles or films attached to their bodies or in special subelytral spaces can, if the water is well aerated, obtain a proportion of their total oxygen requirement by diffusion across the air-water interface of the exposed bubble surface just as if it was a gill. These insects, however, have no means of preventing decrease in the volume of their bubbles; consequently if they remain submerged for too long the tension difference which is the basis for the inward diffusion of oxygen will also bring about a slower outward diffusion of nitrogen, with the resulting danger of loss of bubble and final waterlogging. Nevertheless, in contrast to the vast majority of adult aquatic insects which carry substantial ‘air-stores’ with them in the form of bubbles or thick films or in special subelytral chambers, there are certain insects which have evolved a technique of holding an extremely thin film of gas, of negligible volume, on the surface of their bodies by means of the surface forces provided by a system of minute hydrofuge hairs. This type of air film we call the ‘plastron’. It differs fundamentally from any kind of ‘air store’ in that the surface forces involved are of sufficient magnitude to hold the interface in position, effectively resisting any changes in pressure and surface tension of the medium that the insects are likely to meet, and so maintaining a constant volume of gas. Such a plastron acts solely as a gill, and insects which possess it can stay permanently below the surface and, as long as the medium is sufficiently well aerated, thus become virtually independent of contact with atmospheric air.
For various reasons the only British insect which is in practice a suitable experimental animal for the study of plastron respiration is the Hemiptera Aphelo-cheirus aestivalis. It is mainly an active predatory insect, inhabiting rapidly flowing streams and rivers. It appears to have its main habitat in the large rivers of eastern Europe and Scandinavia, and to be very scarce in other parts of Europe. In Britain it has probably been largely overlooked and may in fact be commoner than has been supposed. The wingless form is the only one known in this country. An outline of its life history and ecology is given. The nymph has the tracheal system closed and lacks the plastron, gas exchange being entirely cutaneous.
The greater part of the body surface of the adult Aphelocheirus is covered with an extremely fine plastron held in position by an epicuticular hair pile having approximately 2,000,000 hairs per sq.mm. The structure and dimensions of the plastron hairs have been investigated by means of ultra-violet photography and by the electron microscope. The tracheal system is described in detail; its chief characteristic consists in the greatly modified abdominal spiracles which take the form of ‘rosettes ‘of branching tubes in the exocuticle, filled with plastron hairs and opening by numerous minute pores into the plastron of the ventral surface. This modification of the spiracles gives a highly efficient protection against the entry of water.
Experiments have shown that, provided the water is kept well aerated, no contact with the atmosphere is necessary; nor, indeed, are visits to the surface of much use since there is no provision for carrying any air store. It is shown that the gaseous plastron is retained intact and the hairs unwetted even when the animal is submerged in oxygen-saturated water (cf. the ‘Ege experiment’) or in gas-free water. The respiratory function of the different regions of the plastron has been studied by experiments in which a given part or the whole of the gaseous plastron has been removed by treatment with a suitable wetting agent. The behaviour of such animals is then observed when they are kept in waters of different known tensions of oxygen. The oxygen deficiency tolerance of nymphs was also investigated. It appears that in the nymph, in which the flattened lateral margins of the abdominal segments are specialized to some degree as tracheal gills, the body surface/volume ratio is such that simple diffusion of oxygen through the cuticle is adequate to supply all the needs of the animal. When, however, the insect reaches the size characteristic of the late fifth instar, a critical point is reached at which the diffusion across the cuticle is no longer sufficient, and a new method of respiration, the plastron, becomes a necessity. This subject will be considered in detail in a succeeding paper.
The plastron hairs are epicuticular, but when sections are examined by ultraviolet photomicrography it is seen that the hairs possess root-like structures which pass through several layers of exocuticle, layers which are indistinguishable by other methods. Pore canals can be seen in the endocuticle but not certainly in the exocuticle.
The resistance of the hair pile to wetting by surface forces was determined by treatment with graded concentrations of pure isobutyl alcohol, the replacement of sheen by a dull black colour being the indication that wetting had taken place. Wetting proceeds slowly in 10% butyl alcohol and rapidly at 12% (contact angle 65°, surface tension 26 dynes/cm.). The sheen cannot be restored by immersion in water supersaturated with air at atmospheric pressure, but if the cuticle is thoroughly washed and then dried in air, resistance to wetting is again practically normal.
The resistance to wetting by pure water under increased hydrostatic pressúre was studied. It was found that the sheen disappeared at excess pressures between 3-5 and 5-0 atm. In this case the blackening is not as complete as on wetting with butyl alcohol and a considerable recovery takes place when pressure is released—evidence that the darkening in this case is due not to wetting but to a collapse of the hair pile itself. In discussing the rigidity of the hair pile, it is shown that a reasonable agreement between theory and observation exists if it is assumed that the material of which the plastron hairs are composed has a Young’s modulus of 0·5− 1· 0×1011 dynes/sq.cm. This figure is considered to be a reasonable one in view of the available information on the chemical nature of the hairs and the Young’s modulus of a variety of ‘plastic’ and other substances. Wetting by a graded series of butyl alcohol solutions of less than 10% under increased pressure was also studied.
A mechanical theory is formulated for the resistance of a system of hydrofuge hairs arranged in various ways. It is shown that the structure, dimensions and packing of the plastron hairs are such as to give what must be nearly the most favourable compromise between the conflicting requirements of high resistance to wetting and to mechanical collapse on the one hand and of a large water-gas interface for respiratory-exchange on the other. The system of larger recumbent hydrofuge hairs found only on the organs of pressure sense, while even more resistant than the general plastron to wetting, is shown to suffer from serious theoretical disadvantages as a respiratory structure.
The idea of an invasion coefficient is spurious, but since the gas must diffuse through a thin stationary layer of the medium at the interface, the fiction is a useful one in defining the permeability of this layer. The coefficients of diffusion of oxygen and nitrogen in water will be practically identical. The permeability of water to oxygen is about twice as great as that to nitrogen owing to the greater solubility. Ege’s figure for the invasion coefficient of oxygen of three times that of nitrogen seems doubtful, but since the invasion coefficient is wholly dependent on velocity gradients at the interface, experimental differences in convection during determinations may account for this.
‘Nujol’ is a purified paraffin oil.
