Respiratory adaptations of the Pupae of beetles of the family Psephenidae

Abstract

Plastron-bearing spiracular gills have been independently evolved in two groups of the Psephenidae, the Psephenoidinae and one genus of the Eubriinae. The spiracular gills of the pupae are exclusively spiracular structures. The plastron is on the spiracle rather than on the body wall adjacent to the spiracle, as in the pupae of flies. In some species the spiracular gills are borne at the end of projections from the body wall. In one genus of Eubriinae, epidermal cells that remain in good condition are isolated in the projections from the body wall in such a way that they are completely separated by a thick wall of cuticle from the remaining tissues of the body in both the pupal stage and in the pharate adult stage. The origin of plastron respiration in the Psephenidae is discussed. Non-aquatic pupae are found near the edges of streams where they are apt to be flooded by rises in stream level. The water/air interface of normal spiracles is too small (400 to 1100 μm 2 /mg) to satisfy oxygen demands by extracting oxygen from the ambient water when they are flooded. The water/air interface of the least well-developed plastrons in insects is equivalent to about 15000 μm 2 /mg of body weight. It is suggested that every increase in the length of the spiracles has a selective advantage in that it enables the pupa to utilize atmospheric oxygen when covered by correspondingly thicker layers of water. At some stage in this process, plastron respiration through the spiracles becomes significant in satisfying oxygen demands. When this stage is reached, selective pressures begin to operate directly to increase the water/air interface of the spiracles. It is shown that if all spiracles of some forms, such as Metaeopsephenus , were like its longest spiracles, the linear dimensions of the spiracles would only have to be increased by a factor of 2*2 for these to have a water/air interface per mg of body weight equivalent to that of some insects with plastrons. Spiracles that do not function in gas exchanges between the insect and the ambient environment nevertheless persist because they subserve two other functions: ( a ) when they are first formed their chambers or ecdysial tubes provide a lumen through which the old tracheae of the previous instar may be withdrawn, and ( b ) after the appearance of the new instar their chambers, now collapsed, are the means by which the tracheae of the previous instar are anchored to the cuticle that is to be shed. Spiracles that do not function in gas exchanges and have their orifices closed are known as non-functional spiracles. Once a spiracle becomes non-functional in a particular instar it remains non-functional in that instar despite the fact that it is temporarily open between the moult and the ecdysis. The loss of functional spiracles is irreversible irrespective of changes in the habits or environment of the group. Examples of irreversible losses of functional spiracles are cited that concern more than one million cases. In some Psephenidae the spiracles of the first abdominal segment are non-functional. The spiracular atrium and the regulatory apparatus of such spiracles may nevertheless persist and be more or less identical in structure to those of functional spiracles. The evidence suggests that in the subfamily Eubriinae such non-functional structures have persisted since at least the Eocene. Plastron-bearing spiracular gills are polyphyletic in origin. They have been independently evolved at least nine times in the Diptera and twice in the Goleoptera. In the Diptera spiracular gills are modifications of the body wall adjacent to the spiracle (e.g. Tanyderidae, Deuterophlebiidae, and Simuliidae) or of both the body wall and the spiracle (e.g. Tipulidae). In the Coleoptera they are modifications of the spiracle only although the spiracle may be borne on a long projection from the body wall (e.g. Psephenoides volatilis Champ). Because in each group of insects the spiracular gills are independently evolved, a phylogenetic classification of these gills is excluded, but a classification of convenience is proposed.