Modification of the Intersegmental Region in the Pterothorax of Cryphocricos (Heteroptera: Naucoridae).
Psyche 81(1):42-50, 1974.
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MODIFICATION OF THE
INTERSEGMENTAL REGION IN THE
PTEROTHORAX OF CRYPHOCRICOS
( HETEROPTERA : NAUCORIDAE) *
BY MARGARET C. PARSONS
Department of Zoology, University of Toronto Toronto, Ontario M5S IAI
In most Hydrocorisae (aquatic Heteroptera) the hindlegs are used for swimming, and their extrinsic muscles, which originate in the metathorax, are well developed. The tergal and pleural depressors of the metathoracic trochanter (Muscles 70 and 7 I, respectively, of Lars&, 1945) are especially large in many Hydrocorisae, including the Naucoridae (Lars&, 1945) and the closely related family Aphelocheiridae ( Parsons, 1969 ) . These muscles slant anterolaterally in the metathorax, their anteriormost fibers originating near its boundary with the mesothorax.
In at least three genera of Naucoridae (Limnocoris, Ambrysus, and Cryphocricos) the anterior part of the metathoracic episternum, on which Muscle 71 originates, usually projects into the cavity of the preceding segment. This makes it appear, at first glance, that the anteriormost fibers of the muscle attach on the mesothorax. The metapleural projection, which will be described in a later publication, appears to be produced by post-ecdysial growth of the skeleton, since it is absent in newly-moulted adults and is present only in older ones. Another example of post-ecdysial skeletal change occurs in the tergum of adult Cryphocricos barozzii and makes the anteriormost fibers of Muscle 70 appear to originate on the mesothorax rather than on the metathorax. This tergal modification seems to occur only in micropterous specimens, with reduced forewings. I have not ob- served it in macropterous Cryphocricos or in dmbrysus, Limnocoris, or Pelocoris, all of which possess forewings of normal length. Cryphocricos barozzii Signoret, collected in Nova Teutdnia, Brazil, and preserved in 70% alcohol, were dissected in 80% alcohol under *Manuscript received by the editor January 20, 1974.
19741 Parsons - Pterothlorax of Cryphocricos 43 a stereoscopic microscope. The attachment of Muscle 70 was studied by cutting the tergum sagittally, with a razor blade; the cut was made half way between the ventral phragmal process and the base of the mesothoracic wing. A few Pelocoris fernratus Palisot-Beauvois, preserved in Bouin's fluid, were also examined for comparison. Most of the Cryphocricos were micropterous, lacking hindwings and indirect flight muscles and possessing forewings which reached only as far as the third abdominal segment. Only two of the rare macropterous forms, with forewings and hindwings extending nearly to the tip of the abdomen, were available for the investigation. Al- though they were not extensively dissected, their indirect flight mus- cles appeared to be either degenerate or absent. Newly-moulted Cryphocricos could be distinguished from older individuals by the thinness of their exoskeletons and by the distinct- ness and relative thickness of the underlying epithelial layer. In older specimens the epithelium was thinner, less distinct, and easily torn, and the thickened exoskeleton had a layered appearance, prob- ably owing to the deposition of successive internal layers of endo- cuticle after ecdysis, as Neville (1970) has observed in several in- sects. These layers could be peeled away from each other in an older specimen of Cryphocricos which had been immersed for 24 hours in a concentrated solution of potassium hydroxide. OBSERVATIONS
(figs. 1, 5)
In most macropterous naucorids, such as Pelomris, the posterior margin of the mesothoracic notum (fig. I ; N 11) is evaginated, form- ing a double-walled scutellar lobe (SL,). The opening into the lobe (OSL) extends anterolaterally to the base of the forewing (WB). Immediately ventral to this opening lies the well developed meso- thoracic postnotum ( PN ) , which bears the second phragma (PH) . The postnotum separates the metathoracic notum (N 111) from the scutellar lobe and is mostly concealed by the latter externally. Only its most lateral part, which joins the mesothoracic epimeron (EM), forming a postalar bridge, is externally visible. The postalar bridge bears a large sensory membrane (SO), part of the mesothoracic scolopophorous organ ( Larsln, I 957).
The concealed medial portion of the postnotum forms a two- walled, vertical invagination, the second phragma (PH) . The
NOI SL Nil 1
ES SO EM
Figures 1-4, Diagrammatic internal views of dorsolateral intersegmental region of pterothorax in various adult Naucoridae. Left side of body has
been cut parasagittally through terga (cut edge), between ventral phragmal process (not shown) and wingbase (WB). Fig. 1. Typical macropterous Naucoridae (based on Pelocoris fernoratus). Fig. 2. Macropterous Crypho- cricos barozzii Fig. 3. Newly-moulted micropterous C. barozzii. Fig. 4. Older micropterous C. barozzii (dotted line indicates approximate position of opening into scutellar lobe, which is no longer visible).
19741 Parsons - Pterothlorax of Cry$ hocricos 45 phragma is especially pronounced on either side of the midline, where it bears a pair of large ventral processes (fig. 5; VP) upon which two of the mesothoracic indirect flight muscles attach (Muscles 30 and 31 of Lars&, 1945 ; present but degenerate in most Pelocoris). According to Snodgrass (1935)
the phragma represents the pri-
mary boundary (antecosta) between the metathorax and mesothorax, and the mesothoracic postnotum is thus an intersegmental plate which is only secondarily associated with the mesothorax. This interpreta-
tion, which has been challenged by Matsuda (1g70), is supported by the fact that the posterior wall of the phragrna is continuous with the metathoracic episternum (fig. I ; ES) . Muscle 70 (fig. I ; M. 70) originates posterior to the phragma and postnoturn, its fibers attaching only on the metathoracic notum. The latter forms a sharp angle with the posterior wall of the phragma. The angle is sclerotized medially (fig. I) but contains a narrow membrane laterally (fig. 5; MEM). The membrane lies immediately anterior to the origin of Muscle 70. Macropterous Cryphocricos
Most of the typical structural relationships described above are also present in macropterous Cryphocricos. In the latter, however, the postnotum forms a definite, double-walled second phragma only near the midline, at and between the ventral processes (position same as in micropterous Cryphocricos, figs. 6, 7). Lateral to the ventral processes the postnoturn does not seem to be invaginated, and appears merely as a transverse thickening (fig. 2, PN) just posteroventral to the opening into the scutellar lobe (OSL). Neither of the two
available macropterous specimens were newly-moulted. It is thus possible that a low, double-walled phragrnal invagination is present in this region immediately after ecdysis. As new layers of endocuticle are subsequently laid down, the structure of the phragma could become obscured, giving it the appearance of an uninvaginated thick- ening in older specimens.
Laterally the anterior portion of the postnotal thickening forms a well-defined postalar bridge with the mesothoracic epimeron (EM). The bridge contains a large sensory membrane (SO) similar to that of typical naucorids. The posterior portion of the postnotal thicken- ing is continuous laterally with the metathoracic episternum (ES) and medially with the metanotum (N 111). Its boundary with the
. . . . .
. . NIH
19741 Parsons - Pteroth~orax of Cryphocricos 47 latter is marked dorsolaterally by a narrow, unpigmented band of sclerotization,
the homologue of the membrane (fig. 5; MEM) which is present in typical Naucoridae. Muscle 70 originates only on the metanotum, posterior to this band,
as in other macropterous
naucorids, and its fibers do not extend onto the postnotum (fig. 2). Micropterous Cryp hocricos
(figs. 3, 4, 6, 7)
In short-winged Cryphocricos, as in macropterous ones, a well- defined second phragma
(figs. 6, 7; PH) is present only at and
between the ventral phragmal processes (VP), In addition, the mesothoracic postnotum appears to be incomplete in micropterous Cryphocricos. These skeletal modifications are more easily observed in newly-moulted adults (figs. 3, 6) than in older ones in which parts of the thickened exoskebon have coalesced (figs. 4, 7). In newly-moulted specimens the portion of the postnotum which usually lies immediately lateral to the ventral phragmal processes appears to have disappeared. The scutellar lobe (figs. 3, 6; SL) is shorter than in macropterous Cryp hocricos, and its ventral wall is directly continuous medially with the metanotum (N 111), upon which Muscle 70 originates. The junction between these two regions forms a fold (F) which projects anteriorly and is continuous with the phragmal process (fig. 6). Short, sclerotized struts connect its edge secondarily with the mesonotum (N 11). The anteromedial fibers of Muscle 70 originate on the edge of this fold, immediately beneath the opening into the scutellar lobe (fig. 3 ; OSL). They thus attach farther anteriorly than in macropterous Cryphocricos (fig. 2)) in which they originate well posterior to the opening. The more anterolateral fibers of Muscle 70 do not attach to the edge of the fold but are separated from it by a roughly triangular area which represents the lateral portion of the incomplete postnotum (fig. 3 ; PN). There is no membrane or unpigmented band marking its posterior boundary with the metanotum. It is ventrally continu- --
Figures 5-7, Diagrammatic internal views of intersegmental part of pterothoracic terga in various adult Naucoridae. Terga have been cut parasagittally, medial to wingbases, and removed from body; only left side, between ventral phragmal process (VP) and cut edge, is shown. Arrow indicates level at which anteriormost fibers of Muscle 7'0 originate. Fig. 5. Pelocoris fernoratus (ventral phragmal process cut off at base). Fig. 6. Newly-moulted micropterous Cryphocricos barozzii. Fig. 7. Older micropterous C. baroszii.
48 Psyche [March
ous with the mesothoracic epimeron (EM), and the postalar bridge contains a very reduced scolopophorous organ (SO). In older micropterous Cryphocricos skeletal boundaries are much more difficult to interpret, owing to the thickening of the cuticle, the differential growth of some regions, and the obliteration of folds, making them appear as solid thickenings rather than hollow invagina- tions or evaginations. Comparison of older specimens (fig. 4) with newly-moulted ones (fig. 3) indicates that during post-ecdysial de- velopment several changes have occurred in the lateral and dorso- lateral intersegmental region of the pterothorax. The scolopophorous sense organ is no longer visible on the internal surface of the postalar bridge. The cavity of the scutellar lobe (fig. 4; SL) has been reduced, and the opening into the lobe (fig. 4; dotted line) is no longer visible. The ventral wall of the lobe merges indistinguishably with the fold which borders on it ventrally. In newly-moulted Cryphocricos this well-defined fold (fig. 3; I?) marks the boundary of the scutellar lobe with the metanotum medially (fig. 3; cut edge) and with the postnotum laterally (PN). In older adults these boundaries have become obscured. The fold and scutellar lobe appear as three separate layers only posteriorly; more anteriorly they appear as a single, somewhat thickened layer (fig. 4). In addition, the antesomedial fibers of Muscle 70, which origi- nate on the metanotum, attach considerably more anteriorly in older Cryphocricos than in younger ones (figs. 3, 4; M. 70; figs. 6, 7; arrows). This suggests that the fold (figs. 3, 6; F) between the scutellar lobe and the metanotum grows anteriorly as new layers of endocuticle are added to the internal surface of the skeleton. One older specimen, in which the muscle attached at the level indicated by the arrow in Figure 7, was treated in potassium hydroxide so that the more internal layers of endocuticle could be peeled away from the outermost ones. Although the inner layers lacked a fold, the more external ones showed a definite one which lay posterior to the level at which the muscle fibers had originated. In these descriptions I have taken the view that Muscle 70 origi- nates on the metanotum in all four types of Naucoridae. In the two macropterous forms (figs. I, 2) its origin is separated from the meso- scutellar lobe by the postnotum. In micropterous Cryphocricos (figs. 3, 4), however, a portion of the postnotum, immediately lateral to the
19741 Parsons - Pteroth~orax of Cryp hocricos 49 ventral phragmal process, is absent. In this region, consequently, the part of the metanotum on which Muscle 70 originates borders directly on the scutellar lobe. This atypical intersegmental boundary appears to grow anteriorly during post-ecdysial development. Figures 3 and 4 could be interpreted in two other ways, either ( I) that there are no post-ecdysial changes in skeletal boundaries and that in Figure 4 the anteriormost fibers of Muscle 70 have simply shifted anteriorly, onto the mesonotum, or (2) that a post- notum is present lateral to the phragmal process and is represented by the fold (fig. 3 ; I?) upon which the anteromedial fibers of Mus- cle 70 attach. According to this second view the origin of the muscle has shifted from the metanotum onto the homologue of the thickened postnotum of macropterous Cryphocricos (fig. 2 ; PN) . Conse- quently, it is the boundary between the mesothoracic scutellum and postnotum (secondary intersegmental boundary of Snodgrass, 1935) which appears to grow anteriorly after ecdysis, rather than a primary intersegmental boundary. Neither of these two interpretations has been adopted here because in Cryphocricos and at least two other genera of Naucoridae post-ecdysial skeletal growth takes place in the pterothoracic pleuron (see p. 46) and almost certainly occurs along the primary intersegmental boundary between the metathoracic episternum and the mesothoracic 'epimeron. Degeneration or absence of the three mesothoracic indirect flight muscles, and consequent loss of the ability to fly, is a common oc- currence among Hydrocorisae (La.rsin, I 950). Extreme alary di- morphism such as that of Cryphocricos is much less common. Larsin ( I 950) studied Aphelocheirus (Aphelocheiridae) which has both micropterous and macropterous forms, and found that the size and shape of the ventral phragmal processes, upon which two of the indirect flight muscles attach, differ in the two types. Similar dif- ferences in the phragmal processes occur in flying and flightless Ilyocoris (Naucoridae ; Larsin, I 970). Both forms of Ilyocoris possess forewings and hindwings of normal length, but most in- dividuals lose their indirect flight muscles soon after the final moult. Lars& did not, however, observe post-ecdysial skeletal changes, ab- sence of the lateral part of the second phragma, or loss of a portion of the mesothoracic postnotum in either Aphelocheirus or Ilyocoris. All these features appear to occur in micropterous Cryphocricos. I wish to thank Fritz Plaumann (Nova Teutonia, Brazil) for
50 Psyche [March
collecting the Cryp hocricos, John Polhemus ( Englewood, Colorado ) for identifying them, my husband, Thomas Parsons, for examining the manuscript, and Donald Chant and the Department of Zoology, University of Toronto, for making available the laboratory facilities for this research. The investigation was made possible by a grant from the National Research Council of Canada. LARSEN, 0.
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1970. The flight organs of Ilyocoris cimicoides L. (Hem., Naucoridae). Ent. Scand. 1: 227-235.
1970. Morphology and evolution of the insect thorax. Mem. Amer. Ent. Inst. 76: 1-431.
NEVILLE, A. C.
1970. Cuticle ultrastructure in relation to the whole insect. In: Insect Ultrastructure, A. C. Neville (dl.), Sympos. Roy. Ent. Soc. Lond. 5 : 17-40.
PARSONS, M. C.
1969. Skeletomusculature of the pterothorax and first abdominal seg- ment in microptero'us Aphelocheirus aestivalis F. (Heteroptera : Naucoridae). Trans. Roy. Ent. Soc. Lond. 121: 1-39. SNODGRASS, R. E.
1935. Principles of insect morphology. New York: McGraw-Hill.
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