Table 2.

Morphology of the Labial Segments in the Nepomorphan Taxa.

Belostomatidae (Belostoma flumineum and Lethocerus uhleri). The first labial segment is much longer dorsally than ventrally. Its dorsal surface is indented, forming a very broad, open stylet groove, and the central, concave portion is separated from the lateral region of the segment by a narrow membrane. The stylet groove has no definite lips, the stylet bundle being held in place by the long, narrow labrum-epipharynx. The second segment is the longest of the four in the Belostoma, but it is shorter than the third segment in the Lethocerus. The intercalary sclerites between the third and fourth segments are larger than in the Gelastocoris, those of the Lethocerus being especially pronounced. Their length causes them to overlap the base of the fourth segment, to which they are attached with a long membrane. Both genera possess the apical plate.	Nepidae (Nepa apiculata and Ranatra fusca). The first segment is greatly shortened and is incomplete dorsally, being sclerotized only ventrally and laterally. The second labial segment is similar to that of the Belostoma. Proximally, it is constricted; it bears ventrolateral flanges and lacks a stylet groove; distally, it is wider, with a deep, closed stylet groove which is reinforced by the median process. In both genera, the third labial segment is the longest of the four. Intercalary, sclerites are present and are especially large in the Nepa. Both genera bear an apical plate in the stylet groove of the last segment.
Gelastocoridae (Gelastocoris oculatus). The first labial segment is quite long dorsally but much shorter ventrally, its latter portion being entirely concealed in the retracted labium, by the posterior plate of the cranium. In the first segment the sclerotized style groove is very deep and, unlike that of other Hydrocorisae, closed distally. The lips of the groove are separated only proximally, where they are overlapped by the short labrum-epipharynx. Ventrolaterally, the base of the second segment forms flanges articulating medially with the proximal segment, as in all other Hydrocorisae. The second labial segment is somewhat wider apically than basally. Its dorsal surface is not modified as that of the Notonecta and Ambrysus, and the lateral basal margins of the third segment do not project as far into its lumen as in those two genera. Two dorsolateral articulation points are present between these two segments. The third segment is the longest of the four, although proportionally shorter than that of the Notonecta. A pair of small intercalary sclerites (IS) flanks the stylet groove and is separated from the third and fourth segments by extensions of the intersegmental membrane. A similar arrangement is found in the Belostomatidae and Nepidae examined in this study, in the Ochterus (Ochteridae), and in Gerris. The nature of the sclerites from the remaining part of the labium indicates that they apparently increase the efficiency of the holdfast mechanism and allow it to move more independently. As for the fourth labial segment, proximally, its stylet groove is supported by a ventrally complete sclerotized ring (BS) which connects dorsally with the lips of the groove at the base of the segment.	Aphelocheiridae (Aphelocheirus aestivalis).  The first labial segment is the broadest of the four; it is quite short and its greatest length occurring dorsolaterally and at the stylet groove. Its thickened ventral wall bears a pronounced median internal process which articulates with a similar process on the second segment. The dorsal wall of the first segment is thin and nearly transparent except at the stylet groove. Laterally, the distal margin of the first segment is thickened, this thickening perhaps representing the region which has given rise to the ventrolateral intersegmental sclerites in many Naucoridae. The second labial segment articulates with the first by means of a single midventral condyle which allows the segment a wide range of movement. In the second segment, the floor and sides of the stylet groove are membranous proximally. The membrane of the groove is attached proximally to a median marginal thickening of the ventral wall of the oblique plate. The exposed dorsal surface of the second segment is divided, by a narrow membrane, into proximal and distal portions. The distal portion consists of a pair of flattened, triangular sclerites flanking the stylet groove and extending laterally as far as the articulations between the second and third segments. The greatly elongated segment is dilated basally, but its distal two-thirds are more slender and tapered. The stylet groove is closed dorsally by sclerotized lips. Proximally, the floor of the groove is membranously connected to the median process, as in the second segment. Between the third and fourth segments the stylet groove is flanked by a pair of subtriangular intercalary sclerites which lie within the intersegmental membrane and form the lips of the stylet groove between the two segments.
Naucoridae (Ambrysus magniceps). The first labial segments are relatively broader and shorter. The first segment is reduced and, when the labium is swung posteriorly, entirely concealed within the head. Its style groove is open, sclerotized, and marginally thickened; a short membranous region placed distally, continuous laterally with the intersegmental membrane, joins it to the dorsal wall of the oblique plate. Laterally, the intersegmental membrane between the first two segments contains three intersegmental sclerites which lie very close together and often appear to be a single sclerite. Ventrolaterally, the base of the second segment is turned inwards, forming flanges which articulate midventrally with a process from the first segment as in the Notonecta. The dorsal surface of the second segment is divided into a proximal, concave portion and a distal flattened, triangular portion in all Naucoridae. This separation extends as far laterally as the dorsolateral articulation between the second and third segments. The stylet groove of this segment is membranous and closed dorsally; its floor is reinforced by a stout median process which extends distally into the third segment and is separated proximally from the oblique plate by a short membrane.	Pleidae (Plea minutissima). In the second segment on the dorsal side, there is the triangular structure with the distal bead-like thickening [12]. Labium, short with a small apical labellum [4]. Helotrephidae (Helotrephes semiglobosus). The fourth segment of the labium is the longest in most species except for the Neotrephes. The second segment a representative of likewise shows an anterior ridge-like thickening. The second segment is otherwise very narrow [12]. Labium, short with a small apical labellum[4].  Notonectidae (Notonecta undulata). First labial segment: it is well developed. The stylet groove is open and covered dorsally by the flap-like labrum-epipharynx. Laterally, the distal margins of this first segment are slightly sclerotized. Second labial segment: on the dorsal side, there is a flattened triangular area on both sides of the closed stylet groove. The apices of the two triangles are somewhat raised and they project proximally. Together, they form a lid over the stylet bundle at the base of the second segment. Laterally, the segment is somewhat swollen in the dorsal part. There is a dicondylic joint between the second and third segments; the articulation points occur dorsolaterally, at the lateral edges of the flattened triangular areas of
The terminal portion of the labium resembles that of the Notonecta except for the fact that the fourth segment is relatively shorter and broader and the sclerotized basal process reinforcing its stylet groove encircles the groove ventrally. The intercalary sclerites are poorly developed in most naucorids (Ambrysus, Pelocoris, Limnocoris), and are absent in some genera (Cryphocricos and Heleocoris).	the second segment. Lateral extensions of the base of the third segment project into the second segment, just ventrolaterally with respect to the articulations. The third labial segment is the longest of the four; it tapers distally and the membranous stylet groove is closed.