Perfect Compromise Research Articles

SUMMARY The closed tracheal system of the aquatic larvae of insects, together with the typical tracheal gill, can only function if the tracheal tubes themselves have sufficient resistance to compression. The work of Ege showed conclusively that the ‘air stores’ of many aquatic insects can, if they are in communication with the spiracles, function as gills for a limited time. Such bubbles will, however, since they have no resistance to compression, gradually dissolve in the water unless there is opportunity for regular renewal at the surface. The significance of the concept of invasion coefficient is considered in relation to gas‐bubble respiration. The term plastron is restricted to a ‘gas store’ communicating with the tracheal system and usually in the form of a thin film of constant and negligible volume and large surface area, retained in position by a system of hydrofuge hairs or scales in such a manner that it is not subject to the ‘Ege effect’ under the normal conditions of its environment, and is therefore not liable to loss by diffusion. Provided there is adequate oxygen in solution in the medium, such a plastron can enable the insect to remain below indefinitely, obtaining all the oxygen it requires from the surrounding water. The structure and biology of Aphelocheirus as a representative plastron insect is described. The plastron hair‐pile appears to be so perfect that an additional pressure of 4–5 atm. is required before the insect is in danger of losing its plastron and becoming wet. The spiracular adaptations which secure communication between the plastron and the tracheal system are described. The theory of plastron respiration as established by a study of Aphelocheirus is outlined. It is shown that in Aphelocheirus it is fully efficient as a respiratory structure and that this method of respiration has here nearly, if not quite, attained theoretical perfection. The resistance of the plastron hair‐pile to wetting is also discussed, and it is shown that this insect appears to have attained a very nearly perfect compromise between the conflicting requirements of a large area of gas‐water interface for diffusion and a minute scale hair‐pile for efficient resistance to water penetration. The development of the plastron hair‐pile in the individual and the first appearance of gas on its surface are described. A survey of plastron respiration in the Coleoptera is given. There are here three main groups in which the method has been elaborated, and while probably none of them presents as efficient an equipment for the purpose as Aphelocheirus there are several which can be regarded as having reached virtual perfection for the particular environments in which they live. It is shown that the plastron insects can be divided conveniently into three groups on the basis of the scale and arrangement of the hair‐pile and the consequent efficiency of the resistance to wetting. A number of the Coleoptera carry, over and above the plastron, a thicker gas layer, the macro‐plastron, held either by longer hairs or by means of a ‘fluffing out’ of the plastron hairs. Such a macroplastron is unstable and can only be maintained by the grooming or special renewal activities of the insects. It is subject to the ‘Ege effect’ and thus resembles in respiratory action a gas bubble rather than a plastron. It may, however, constitute an important first line of defence against wetting, and insects which have a macroplastron provide a number of transitional forms between gas‐bubble respiration and true plastron respiration. The distinction is made between ‘water‐proofing’ and ‘rain‐proofing’. ‘Waterproofing’ implies resistance to water under pressure and this demands a fine‐scale structure of some rigidity under a maximum of solid‐liquid contact. ‘Rain‐proofing’, on the other hand, implies no pressure of water and requires a more rigid structure of larger scale, the hairs being resistant to matting by lateral surface forces. For such rain‐proofing larger hairs are desirable as giving a minimum solid‐liquid contact. It is possible to trace the development of both types of structure among aquatic and semi‐aquatic insects. Some of the ecological implications of plastron respiration are described. A number of other instances of water‐resistant hair‐piles and felts are considered in relation to the evolution of plastron respiration. In the Hemiptera there appear to be no other forms approaching Aphelocheirus in plastron efficiency. In the Trichoptera and Lepidoptera there are one or two semi‐plastron forms and one moth, Acentropus niveus, which is perhaps a true plastron insect. The parasitic Hymenoptera provide a number of interesting problems in relation to the wetting and water protection of the cuticle. The larva of one aquatic ichneumonid Agriotypus spins a ribbon‐like appendage to the cocoon, which contains air and appears to act as a plastron. The existence of water‐resistant hair‐piles in the remaining groups of the insects and arachnids is briefly surveyed.

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