ALBERT

Notes: Summary 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.

Notes: The bodies of adult and fifth instar Notonecta possess external air stores which are periodically renewed at the surface of the water. Both nymphs and adults have large ventral air stores on the thorax and abdomen and obtain atmospheric air at the posterior end of the latter; the adult also has dorsal subalar and supra-alar air stores on both these regions. Ten pairs of spiracles open onto the air stores. Although the seven small, ventrally placed abdominal spiracles are probably both exhalant and inhalant in nymphs and adults, the three large anterior spiracles (mesothoracic, metathoracic, and first abdominal), which play a more important respiratory role, appear to function differently in mature and immature Notonecta. In the nymph they are probably both inhalant and exhalant, and communicate broadly with each other and with the ventral air stores. In the adult, however, they open onto separate, air-filled chambers, each of which communicates differently with various parts of the air stores. Although all three probably function in exhalation, only the first abdominal spiracle, whose spiracular chamber is widely continuous with the dorsal and ventral air stores, appears to be well suited for inhalation.Several morphological features, most notably the development of long prothoracic lobes, separate spiracular chambers, and long, movable forewings, allow the adult a greater variety of respiratory modes than are available to the nymph. Some of the respiratory advantages of the adult are: (1) a larger amount of stored air; (2) a longer subalar air store, which can serve as an alternate pathway between the air stores and the atmosphere; (3) a greater capacity to utilize dissolved as well as atmospheric oxygen; (4) greater separation and functional specialization of the three anterior spiracles, thus allowing more separation of exhaled air from oxygen-rich air on the external surface of the thorax; (5) the probable ability to regulate the continuity between various parts of the air stores, thus utilizing alternate pathways of air circulation and/or changing the functions of the three anterior spiracles; and (6) better protection of the latter against the entry of water during prolonged submergence.

Notes: The cibarial food pump of heteropteran insects conveys fluid food from the piercing stylets to the pharynx. In aquatic Heteroptera the pump also grinds and filters particulate matter in the food stream. The pump's sclerotized triturating devices differ from one family to the next and are often quite elaborate: because of their small size they are best studied by means of the scanning electron microscope. In Notonecta the main triturating devices occur on the transverse plates of the epipharyngeal roof of the pump. They consist of a complex anterior zone with raised nodes and bifurcating longitudinal ridges, and a simpler posterior zone with small nodules. Additional triturating surfaces occur on the hypopharyngeal floor of the pump. The oblique folds of the epipharynx, which lie anterior to the transverse plates, play only an accessory role. The fine structure of the grinding surfaces on the transverse plates of Gelastocoris (Gelastocoridae), Ambrysus (Naucoridae), and Aphelocheirus (Aphelocheiridae) is here briefly described and compared with that of Notonecta.

Notes: The heteropteran cibarium forms a sucking pump which conveys fluid foods into the pharynx. The food pumps of Hydrocorisae have the additional function of grinding or filtering particulate matter; they contain ridges, hairs, and sclerotized processes which have probably evolved at least twice among the hydrocorisine families. Aphelocheirus, like the Naucoridae, possesses a modified anteclypeus and a tripartite type of food pump. The main sucking action occurs in the pump's anterior and posterior regions, while the middle region is specialized for grinding and filtering. The anteclypeus has broadened and fused with other parts of the cranium, and is thus braced against the pull of the powerful cibarial dilator muscles. In the Naucoridae the three regions of the pump have the same functions as those of Aphelocheirus. The pumps of the five naucorid genera here studied are structurally very similar to each other but differ considerably from that of Aphelocheirus. Cibarial morphology, as well as respiratory differences, thus supports the contention that Aphelocheirus is not a member of the Naucoridae but should be placed in a separate family.

Notes: The cibarial food pumps of aquatic Heteroptera contain specialized epipharyngeal triturating devices. In the Naucoridae, striated bands and transverse plates triturate particles against the underlying hypopharynx. Anterior to them lie a pair of oblique folds which play an accessory role. The gross morphology of these devices is very similar in representatives of five genera of typical Naucoridae (Ambrysus, Pelocoris, Limnocoris, Cataractocoris, Cryphocricos) and differs from that of the atypical genus Aphelocheirus.The scanning electron microscope reveals additional differences between Aphelocheirus and the typical genera as well as variations, among the latter, which are not visible with the stereoscopic microscope. The oblique folds of the typical Naucoridae are well developed and contain processes for trapping particles; in three genera the region posterior to the folds is also modified. In Aphelocheirus only the latter region appears to trap particles, and the oblique folds are smooth and weakly developed. The striated bands of all genera bear ventral ridges arranged into transverse zones with precise patterns. The fourzoned bands of Aphelocheirus have a very different pattern than the two-zoned bands of the other genera. Among the latter, Cryphocricos has a simpler pattern of ridges than the other typical Naucoridae. The ventral surfaces of the transverse plates are highly modified in Aphelocheirus and less so in the other genera; those of Cryphocricos differ from those of the other Naucoridae.The fine structure of the cibarial epipharynx supports the views of some systematists that (1) Aphelocheirus should be placed in the monogeneric Family Aphelocheiridae rather than in the Naucoridae, (2) Cryphocricos represents a different subfamily than the other four typical Naucoridae, and (3) Cataractocoris belongs in the same subfamily as Ambrysus rather than with Cfyphocricos.