Anterior displacement of the metathoracic spiracle and lateral intersegmental boundary in the pterothorax of Hydrocorisae (Aquatic Heteroptera)

Abstract

1. In the lateral pterothoracic region of the Hydrocorisae, the features which vary most are the position of the metathoracic spiracle, the structure and position of the intersegmental boundary, and the size and distribution of 1. the concealed subalar and intersegmental air spaces and 2. the exposed ventral thoracic air layer. Since not all of the observed variations appear to reflect phylogenetic relationships they must be at least partly due to functional differences. 2. The metathoracic spiracle lies in the posterior part of the mesothoracic epimeron and may occupy any one of three positions. In representatives of the Belostomatidae, Nepidae, and Gelastocoridae, and in some Naucoridae, it faces the lateral intersegmental air space (Position 1). In Cryphocricos and Limnocoris (Naucoridae) it faces the small ventral intersegmental air space (Position 2), while in representatives of the Notonectidae and Corixidae it faces the subalar air space (Position 3). The subalar or lateral intersegmental space onto which the spiracle faces is often enlarged by the invagination of the circumspiracular part of the epimeron into the body. 3. The lateral intersegmental boundary projects sharply anteriorly, into the mesothoracie cavity, in most of the Hydrocorisae examined. Among the Naucoridae this characteristic varies both intergenerically and intragenerically. In Gelastocoris (Gelastocoridae) and in at least some species of Cryphocricos and Limnocoris the boundary appears to shift anteriorly during postecdysial development. A possible reason for the anterior displacement of the boundary is that it provides a greater surface of origin for one of the large extrinsic muscles of the hindleg. This may also explain the unusually ventromedial position of the metathoracic spiracle in Cryphocricos and Limnocoris. 4. In Belostoma (Belostomatidae) and Ranatra (Nepidae) the metathoracic spiracle is not highly porous and the concealed thoracic air spaces do not appear to serve as oxygen sources for underwater respiration. They probably protect the spiracle against entry of water, however, and play a role in hydrostatic balance and pressure reception. 5. The respiratory significance of the pterothoracic variations in other Hydrocorisae is more difficult to assess without more information on such subjects as spiracular fine structure and function. It is here suggested, however, that the size of the concealed air space onto which the metathoracic spiracle faces may be related to the extent to which an aquatic bug is dependent upon atmospheric oxygen. Most Hydrocorisae are buoyant and their concealed air spaces are large. “Slow-water” Naucoridae appear to be of this type. In Aphelocheirus (Naucoridae), however, the concealed air spaces are small and the insect is heavier than water. A thin, exposed air layer, covering much of the body, obtains enough dissolved oxygen to make Aphelocheirus independent of atmospheric oxygen. Some other “fast-water” Naucoridae, such as Cryphocricos, which cannot easily reach the surface of the water, show a similar reduction of the concealed air spaces and a consequent decrease in buoyancy. This suggests that they, like Aphelocheirus, rely less upon atmospheric oxygen than upon dissolved oxygen obtained by their exposed air layers.