3J3 the Chorionic Plastron and Its Role in the Eggs of the Muscinae

3J3 The Chorionic Plastron and its Role in the Eggs of the Muscinae (Diptera) By H. E. HINTON (From the Department of Zoology, University of Bristol) With one plate (fig. 6) SUMMARY In flies of the subfamily Muscinae the egg-shell has both an outer and an inner meshwork layer, each of which holds a continuous film of air. Between these two meshwork layers there is a more or less thick middle layer to which the shell chiefly owes its mechanical strength. Holes or aeropyles through the middle layer effect the con- tinuity of the outer and inner films of air. Both meshwork layers consist of struts that arise perpendicularly from the middle layer. In both layers the struts are branched at their apices in a plane normal to their long axes. These horizontal branches form a fine and open hydrofuge network that provides a large water-air interface when the egg is immersed. When it rains or when the egg is otherwise immersed in water, the film of air held in the outer meshwork layer of the shell functions as a plastron. To be an efficient respiratory structure a plastron must resist wetting by both the hydrostatic pressures and the surface active materials to which it is normally exposed. The plastrons of all the Muscinae tested resist wetting in clean water by pressures far in excess of any they are likely to encounter in nature. The resistance of a plastron to hydrostatic pressures varies directly as the surface tension of the water, and the surface tension of water in contact with the decomposing materials in which the Muscinae lay their eggs is much lowered by surface active materials. These considerations seem to provide an explanation for the great resistance of the plastron of the Muscinae to wetting by excess pressures and for the paradox that the plastrons of these terrestrial eggs are more resistant to high pressures than are the plastrons of some aquatic insects that live in clean water. INTRODUCTION REOCCUPATION with the aquatic adaptations of aquatic insects has Presulted in an almost total neglect of the aquatic adaptations of terrestrial insects. But whenever it rains heavily a very large number of terrestrial insects are submerged beneath a layer of water. The immobile stages of these insects, the eggs and pupae, are normally glued or otherwise fastened to the substrate and necessarily remain submerged until it has stopped raining and the water has evaporated or flowed away. Thus in most climates many of the terrestrial insects are alternately dry and flooded. To be submerged in water for several hours or even days, a period that may, for instance, exceed the duration of the egg stage, is no rare and isolated event but is a normal hazard of their environment. It therefore seems likely that many terrestrial insects are adapted for respiration in water in a manner no less complex than are many aquatic insects. [Quarterly Journal of Microscopical Science, Vol. 101, part 3, pp. 313-32, Sept. I960.] 314 Hinton—Chorionic Plastron in Muscinae When the terrestrial environment is seen as one that is alternately dry and flooded, and that when flooded the water is usually well aerated, it is possible to predict the kind of respiratory adaptations that might be expected to be found amongst terrestrial insects. For instance, one of the most characteristic respiratory adaptations of insects that live in aquatic environments that at one moment are flooded by well aerated water and that at another may be dry is the physical gill called a plastron. The great advantage of a plastron in such environments is that when the insect is submerged it provides a relatively enormous water-air interface for the extraction of oxygen from the ambient water, but when the insect is exposed above water the plastron does not in- volve water-loss over an enormous surface area because the connexion between the plastron and the internal tissues is restricted. The great disadvantage of a plastron is that it becomes an efficient means of extracting oxygen from the tissues should the oxygen pressure of the environment fall below that of the tissues; and it is no accident that the aquatic insects with plastrons are restricted to environments in which the oxygen pressure is maintained at a high level such as streams, the littoral of large lakes, and intertidal areas. The problems of respiration in aquatic environments liable to sudden drying and in many terrestrial environments are so similar as to suggest that the respiratory adaptation characteristic of the former will be found in the latter. And this is indeed so. A plastron was first reported from terrestrial insect eggs in 1959 (Hinton, 1959), and from more recent work (Hinton, 1960a, 19606) it is taking no great risk to predict that examples of plastron respiration amongst terrestrial insects will be found to be much more numerous than amongst aquatic insects. The eggs of all oviparous species of Muscinae examined have a plastron. The principles of plastron respiration have been summarized by Thorpe (1950). The hydrostatic pressures that have to be applied in order to wet the plastron of the Muscinae are considerably greater than any to which they are at all likely to be exposed in nature. In this respect they resemble the plastrons of other dipterous eggs that are also laid in decaying organic matter. Not only do the plastrons of such eggs resist wetting by hydrostatic pressures far in excess of any they are likely to encounter, but those of some species are even more resistant to excess pressures than those of many aquatic insects. For instance, the plastron of the terrestrial egg of Drosophila funebris F. resists wetting by an excess pressure of 1-3 atm (Hinton, 1960a), whereas that of the aquatic pupa of Taphrophila vitripennis Meig. only resists about 0-3 atm (Hinton, 1957). Eggs laid in decaying organic matter are probably often exposed to con- centrations of surface active substances that rarely if ever occur in streams. For instance, the surface tension of the temporary pools of rain water on cow pats is reduced to about 50 dyn/cm, and under comparable conditions the surface tension of water on the surface of decomposing flesh is reduced to about 40 dyn/cm (Hinton, 1960a). To be an efficient respiratory structure a plastron must resist wetting by surface active substances. Any change in the Hinton—Chorionic Plastron in Muscinae 315 geometry or the nature of the surface of the plastron meshwork that increases its resistance to wetting by surface active substances also increases its resistance to wetting in clean water by excess pressures, since wetting by excess pressures always occurs before there is a mechanical breakdown of the plastron meshwork. Therefore selection for greater resistance to wetting by surface active substances inevitably results in greater resistance to hydrostatic pressures; and this fact provides an explanation of the paradox that the plastrons of some terrestrial insects resist higher hydrostatic pressures than those of some aquatic insects. A comparative account of the respiratory structures of the egg-shell of the 10 genera of the subfamily Muscinae that occur in Britain is given here. Details of the respiratory system of the shell have previously been given for only one species in the subfamily, Musca (Eumusca) autumnalis Deg. (Hinton, 1960a). To reduce description of morphological detail, the structure of the shell and the respiratory system of what is conceived to be the primitive egg of the Muscinae is given; and under the headings of the different species only the way in which these structures are modified is noted. The oviposition habits of each species are briefly summarized; they assist in understanding the adaptive significance of differences between the species in the structure of the shell and respiratory system. MATERIALS AND METHODS Most of the eggs used were collected in the field or bred in the laboratory. The identity of eggs collected in the field was established by comparison with eggs laid by isolated females in the laboratory. The eggs of PyreUia. Stomoxys, and Lyperosia were dissected from dried museum specimens, and those of M. domestica were obtained from cultures kept in the Pest Infestation Labora- tory at Slough. The structure of the respiratory system was examined by means of whole mounts and serial sections. In most species the branches of the network of both the outer and inner surfaces of the shell are only resolved with the light microscope by the most careful adjustment and the use of a blue filter: most of the branches that are normal to the vertical struts exceed the wavelength of visible light in only one dimension. Electron micrographs of the respiratory system of the egg of the fly Dryomyza flaveola F. (Hinton, 1960a) revealed the fact that all essential features could be resolved with the light microscope. A column of water was used to study the resistance of the plastron to a hydrostatic pressure of 7 cm Hg. Its resistance to higher pressures was tested in a chamber connected to a mercury reservoir by a long piece of rubber tubing. The pressure required was obtained by raising the mercury reservoir the appropriate distance above the chamber. In all experiments live eggs were used. The figures for the resistance of the plastron to excess pressures are therefore minimum figures. Any oxygen uptake by the egg must produce a pressure gradient in the plastron and therefore a fall in the back pressure of 316 Hinton—Chorionic Plastron in Muscinae the system. The maximum possible fall in the back pressure of the system due to the respiration of the egg would be equivalent to a rise in the hydrostatic pressure of about 16 cm Hg.

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