ABSTRACT
The only important examples of plastron respiration outside the Hemiptera (Aphelocheirus) are all to be found in the Coleoptera where it has been known or suspected in three groups: (I) the Dryopoid family Elmidae; (II) the Donaciine (Chrysomelid) genus Haemonia (Macroplea); and (III) Phytobius and possibly certain other genera of the Curculionidae (Rhynchophora). It is the object of the present paper to give as comprehensive an account as possible of the extremely interesting series of examples which the order Coleoptera thus provides.
The Dryopoidea
Habits. The two standard species used for the basic experimental work on this group were Elmis maugei Bedel and Riolus cupreus (Mull.). These beetles are found crawling about on submerged stones in swiftly flowing streams and rivers, or other well-oxygenated waters, browsing upon algal growth. They are quite unable to swim and it was established that provided the water is well aerated they need never come to the surface. Elmids may often be seen grooming their main plastron areas by means of plastron ‘brushes’ situated on the inner faces of the femora. They may also be seen to capture small oxygen or air bubbles adhering to aquatic plants with their mouthparts and to replenish the gas on their plastron surfaces by pushing and smearing these bubbles over themselves with these same brushes. These two types of behaviour are known as ‘plastron replacement activities ‘. The importance of grooming in keeping the fléxible hairs regularly spaced is emphasized.
Plastron. The plastron area of Elmis and Riolus is described and figured., The basic respiratory rate of Elmis maugei is 1 · 17 × 10−7 c.c. O2/sec. There are eight pairs of open spiracles (Th 2+Ab 6) each with its closing apparatus. They open into the sub-elytral space which communicates with the plastron area via the lateral groove of articulation of the elytra. This groove is very effectively protected by hydrofuge hairs and constitutes a satisfactory watertight junction. The tracheal system has no air sacs.
Hydrostatic control. Elmis cannot swim actively but possesses an elaborate method for hydrostatic control which is described and figured in detail.. It.depends for its efficacy on proper functioning of the plastron. Both Elmis. and Riolus can control buoyancy so successfully that they can float or sink, at will) rising and falling in the water in rapid succession if necessary.
Respiratory efficiency of the Elmid plastron. Whereas-in Aphelocheirus the volume of the plastron is extremely constant owing to the erect and rigid plastron hairs, in Elmids, after a bubble has been captured and smeared over the plastron as described above, the hairs (owing to their flexibility and degree of overlap) become slightly more erect and fluffed out. The plastron-area is then correspondingly thicker and the sheen more brilliant. This enhanced or thickened layer of gas we distinguish from the more tenaciously held and duller sheen (plastron) by the term ‘macroplastron’. This thicker layer of gas is unstable and is actively maintained by the ‘plastron replacement activities’ of the insect. It thus constitutes; a first line of defence against unfavourable environmental-conditions. When such conditions supervene and opportunities for plastron replacement from bubbles no longer occur, the macroplastron is lost, the hairs pack down more closely leaving the plastron proper which is held more tenaciously and does not require replacement under ordinary conditions. The plastron of Elmis is in every respect inferior to that of Aphelocheirus. Elmiss therefore, although a true plastron insect, has nothing like the latter insect’s margin of safety against wetting under increased pressure and in fact is only able to withstand a, pressure difference of slightly less than 1 atm. Once the macroplastron is lost the more closely packed hairs tend to occlude the interface obstructing the diffusion paths with a resulting decrease in respiratory efficiency.
The Dryopoidea can be divided into three fairly distinct groups on the basis of hair-pile dimensions and waterproofing efficiency.
Haemonia (Macropled)
As far as is known this is the sole Donaciine genus which carries a plastron and is thus independent of visits to the surface.
The Plastron in Haemonia covers almost the whole of the ventral surface as well as the whole of the long antennae. It is very uniform and even and the hairs are very stiff with a beautifully adjusted bend of about 130° at the tip giving an extremely smooth plastron interface without the excessive overlapping and consequent tendency to pack and occlude the interface which is characteristic of Elmis and Riolus.
Resistance to wetting. The plastron is remarkably efficient as a water-protecting mechanism owing to the evenness, rigidity and absence of packing of the hairs. Thus there is need for neither macroplastron, buoyancy control nor plastron replacement activities and all these are in fact absent.
Respiration. Haemonia shows little immediate response to oxygen lack though it tends to climb upwards when the aquarium is not well aerated and may occasionally be seen with its antennae floating on the surface of the water when they probably have a respiratory function. Haemonia can survive severe oxygen lack for periods of several hours.
The tracheal system presents no unusual features and the spiracles are normal save for a highly efficient water-protecting mechanism.
Evolution of plastron respiration in the Donaciinae. The probable course of evolution of the plastron mechanism of Haemonia has been considered by comparison between this genus and typical members of the genus Donada, where the probable function of the hair pile is to protect from wetting during accidental and temporary submergence. Reduction in size and increase in regularity and density of the hairs together with some change in hair shape would be the main steps required to equip Donada for plastron respiration.
Of a number of aquatic and semi-aquatic weevils studied Phytobius velatus is the only one fully adapted as a plastron insect. This species swims actively and apparently need never come to the surface unless under conditions of very prolonged and severe oxygen deficiency. It carries no air store though there is a small sub-elytral space and the insect is able to fly. Phytobius possesses a complete and highly efficient plastron of minute hairs at a density of I-8-2-O × io8 per cm.2 borne on an almost complete armour or vestiture of touching or overlapping flattened scales. The hairs are parallel to the long axis of the scale and are gently curved at the tip so as to lie along the surface—a plastron arrangement not far from the ideal. In its resistance to wetting the insect shows the same high order of efficiency as Aphelocheirus. Average oxygen. consumption was found to be 1-24 mm.3/hr. There are no plastron replacement activities and the ability to control buoyancy is very slight. The ability to endure temporary oxygen lack is very high.
General conclusions. Three important factors in the ability of a hair pile to resist water penetration are discussed. These are: (1) arrangement and regularity; (2) rigidity; (3) scale of hairs.
Aquatic insects which have hydrofuge hairs may be conveniently grouped in four categories the first three of which correspond to those already described in connexion with the Dryopoid beetles.
Series I. Members of this group have a density of io8 hairs per cm.2 and can probably all withstand a pressure of 2 atm. without wetting. The plastron is very thin and does not afford any reserve of oxygen but functions as a gill. This group comprises Group I of the Dryopoidea, Phytobius and Aphelocheirus. The plastron of this group is virtually ‘perfect’ both structurally and functionally and does not require replacement.
Series II. Comprises the second group of the Dryopoidea and the genus Haemonia. These insects have a density of 106 − 108 hairs per cm.2 and can withstand a pressure of 0 · 5 − 2 · 0 atm. The plastron is thicker than in the previous group but is not sufficient to encumber the insect on account of its buoyancy nor is it sufficient to offer more than a small reserve of oxygen, even when, as in the Elmidae, it is expanded into a ‘macroplastron’ and actively maintained. The insects in both of these groups have a functionally perfect plastron in that they do not need to come to the surface, but that of the Elmids in Group II is not structurally perfect and needs maintenance by the activities of the insect.
Series III. This includes the Dryopoid Group III and the Hydrophilids, Hydrophilus, Hydraena and Berosus spinosus. The penetration pressure is less than 0· 5 atm. and there are only 105 − 106 hairs per cm.2. The plastron in these insects is of considerable volume and acts as much as an air store as it does a gill. It needs periodical renewal at the surface and its volume is sufficient to increase buoyancy to the point at which the insects cannot remain submerged unless either actively swimming or holding on to stones or vegetation. Many of the forms in this group have a double hair pile; the outer set of hairs being readily pressed down on the inner giving them a more complete water protection. The macroplastron formed is thus a definite structure and not a mere increase in the amount of gas in the plastron as it is in the Elmids. It is of course unstable and will be lost by the ‘Ege effect’ if the insect remains submerged. This series contains insects ranging from almost completely aquatic forms to those which only occasionally enter the water. Periodical grooming behaviour of Hydrophilus is described in detail.
Series IV. These insects are without a regular plastron. The hairs only offer protection against accidental or temporary wetting. Many insects living in the proximity of water might be placed in this group with Donada, Stenopelmus and Tanysphyrus. The hair pile in these forms is more akin to ‘rain proofing’ than to protection from penetration. The distinction between these two types of protection is discussed.
In all the coleopterous groups investigated it appears that the fully adapted plastron-bearing insect can have been evolved from a riparian form with a hair pile the sole function of which was to enable the insect to enter the water for oviposition or to safeguard it against accidental immersion.
We have since confirmed this in Limnius troglodytes.
For a description see Beier (1948).
M. Beier (1948), in a paper published just as this is going to press, suggests that the ‘massage movements’, as he calls them, are not plastron replacement activities but aeration activities, the plastron being first distended by increased pressure under the elytra. This distension causes the appearance of the more brilliant silver-gilt sheen and often an actual ‘bulging out’ of the gaseous plastron which is then kneaded and so ventilated mechanically. This suggestion seems unlikely in view of our experiments, since all Beier’s observations seem to accord equally well with our view of the need for constant grooming and fluffing out of the plastron hairs. Moreover, from our present knowledge of the respiratory efficiency of plastrons in general and the Elmid plastron in particular, it seems very improbable that such mechanical aeration would be required whereas the need for plastron grooming and replacement activities seems obvious.
* We have also found recently that Hydrophilus regularly grooms itself, approximately every 7 or 10 days. This operation is not done underwater, as in Elmids, nor was it ever observed when we ourselves kept Hydrophilus in a small aquarium with stones projecting, but our attention was drawn to it by Dr M. G. M. Pryor who noticed Hydrophilus climbing the stems of growing reeds in a large aquarium, which presumably corresponded more closely to its natural habitat. The beetle hangs upside down by its hind legs several inches above the water surface, combing and grooming itself for an hour or more. The metathoracic plastron is combed by the spurs on the tibiae of the 2nd pair of legs, the mesothoracic with the tibiae of the front legs, the antennae between the tibia and femur of the front legs, and the eye region, side of head, and antennal groove are brushed by the tarsi of the front legs. The region between the eyes is cleaned by the proximal segments of the maxillary palp, and both pairs of palps are waved about during the process. The prothorax and head show great mobility and the front legs are occasionally rubbed together as if to remove debris. Dr Pryor reports that sometimes the abdominal sterna, though lacking plastron, are groomed by the hind legs, the insect clinging by its middle legs. They will on occasion groom in almost any attitude, even when on the back lying in the mud. Perhaps there is a risk in Hydrophilus with its very coarse hair pile, that water invasion may occur, and the periodic grooming out of the water will overcome this. There is no evidence of any hydrofuge secretion being applied in the process. It is interesting that Wesenberg-Lund (1943, p. 340) reports his inability to keep this species alive through the winter, all individuals becoming waterlogged and dying in February or March. Perhaps this is due to lack of reed stems or other vegetation in the aquarium suitable for climbing out of the water.
