Studies on Plastron Respiration

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ABSTRACT Aphelocheirus lives in swiftly flowing streams, an environment in which, chiefly because of its method of plastron respiration, it must be able to maintain itself in a particular stretch of river and must especially have some safeguard against sudden and uncontrolled descent into regions of low oxygen tension. These necessary powers of orientation are provided : (a) by the ordinary type of tactile trichoid sensillum, (b) by light-sensitive organs, and (c) by specialized pressure receptors. When stationary or when climbing on weeds and stones the insect should have no difficulty in appreciating direction and rate of flow of the water by means of its antennae and trichoid sensilla. When swimming the dorsal light response, light compass orientation and probably also rudimentary form sight enable the animal to maintain position except when in deep smoothly flowing water under conditions of very low illumination. A certain gross sensitivity to pressure changes probably resides in the proprioceptors of the cuticle, but, in addition, there are specialized pressure receptors which consist of a pair of oval depressions on the sterna of the second abdominal segment closely associated with the spiracles of that segment and bearing a pile of long recumbent hydrofuge hairs, approximately 60,000 per sq.mm., holding a film of gas. Special tactile sensilla are interspersed among the hydrofuge hairs. The hairs move with the movements of the air-water interface which result from changes in pressure. Experiments have shown that these two organs are exceedingly sensitive both to uniform and differential pressure changes, and that by their means the animal is enabled to direct its swimming upward or downward and also to maintain an even keel irrespective of directional light stimulation. The adult bug responds immediately by jerky upward swims when exposed to sudden increase of pressure, and the same response is also elicited independently by oxygen lack. But the insect cannot fully distinguish between the two stimuli and is unable to distinguish loss of gas by the plastron due to unsaturation of the medium from an actual pressure increase. Thus bugs placed in water deficient in nitrogen but containing 20% saturation in oxygen give persistent pressure-increment responses. The possibility is considered that stimulation of the pressure receptors caused by changes in the gas pressure in the tracheal system, due to muscle or body movements or to changes in the gas tension of the medium, might interfere with their efficiency as organs of balance. It is suggested that the small, very compressible, concertina-like air sacs associated with each sense organ serve to damp out such fluctuations and so avoid this difficultv. Aphelocheirus nymphs lack the specific pressure receptors. They are accordingly more dependent on directional light for orientation, and in the dark behave as do adults with the pressure receptors extirpated. Aquatic insects which carry substantial air bubbles with them and so are lighter than water have a ready means of perceiving pressure changes by volume changes of the bubble. Aphelocheirus being heavier than water and carrying no bubble therefore needs the special pressure receptors. Similarly, Nepa, which is also heavier than water, has specialized pressure receptors. The organs of the latter are redescribed and their mode of action compared with those of Aphelocheirus. The action of the Nepa organs is not finely graduated as in Aphelocheirus but is of the ‘all-or-none’ type. The three pairs of organs in Nepa are co-ordinated into a single differential manometer system giving no response to changes in absolute pressure but conferring high sensitivity to relative changes as between different parts of the body and correspondingly accurate orientation in the vertical axis. It is shown how the two types of sense organ are exactly suited to the peculiar contingencies to which the insects are exposed by their particular environments. The experiments of Baunacke on Nepa are repeated and extended and his conclusions confirmed.