Margaret C. Parsons.
Respiratory Significance of the Thoracic and Abdominal Morphology of Three Corixidae, Diaprepocoris, Micronecta, and Hesperocorixa (Hemiptera; Heteroptera: Hydrocorisae).
Psyche 83:132-179, 1976.

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RESPIRATORY SIGNIFICANCE OF THE THORACIC AND ABDOMINAL MORPHOLOGY OF THREE CORIXIDAE, DIAPREPOCORIS, MICRONECTA, AND HESPEROCORIXA (HEMIPTERA: HETEROPTERA: HYDROCORISAE)*
BY MARGARET C. PARSONS
Department of Zoology, University of Toronto Toronto, Ontario M5S 1A1, Canada
A. INTRODUCTION
The Corixidae are an extremely successful and abundant family of Hydrocorisae (aquatic Heteroptera). The more than 500 living species of corixids were grouped by Hungerford (1948) into six subfamilies; more recently Popov (1971) has recognized only three subfamilies, the Diaprepocorinae, Micronectinae, and Corixinae. The Corixinae, which include four of Hungerford's subfamilies, have been extensively investigated; they consist of approximately 30 genera, occur throughout the world, and range up to 16 mm. in length. Morphological and physiological studies are almost totally lacking, however, for representatives of the other two sub- families. The Micronectinae are nearly worldwide in distribution but members of their three genera are very small (1.5-6.0 mm.) and difficult to dissect. The Diaprepocorinae consist of a single rare genus, Diaprepocoris, which occurs only in Australia, Tasmania, and New Zealand.
The mode of respiration in the Corixinae has been studied by many workers, including Hagemann (19 lo), Ege (1 91 8), Popham (1960) and Parsons (1970). Like most Hvdrocorisae, the Corixinae use "air-bubble" respiration and depend upon atmospheric air, which they carry in air stores on their bodies. The parts of the air stores which are exposed to the water can extract dissolved oxygen from the water by acting as "physical gills" (Thorpe 1950). The stored air is not a true plastron (sensu Thorpe 1950) because it is of considerable volume and must be periodically renewed at the surface of the water.
The air-bubble mode of respiration is reflected in the gross morphology of Hesperocorixa (Corixinae; Parsons 1970, 1974). In the present study the respiratory morphology of this corixid is *Manuscript received by the editor June 15, 1976. 132




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Parsons - Morphology of Corixidae
compared with that of representatives of the other two subfamilies, whose respiration has never been investigated. The fine structure of the hydrofuge hairs and of the metathoracic and first abdominal spiracles, which has not yet been studied in any of the Corixidae, is also compared in Hesperocorixa, Micronecta (Micronectinae), and Diaprepocoris (Diaprepocorinae).
The Corixinae differ markedly from other Hydrocorisae in many morphological features. Their unusual characteristics include the very complex mesothoracic scolopophorous organ ("Hagemann's organ" of many authors), the modified mesothoracic and meta- thoracic epimera, the elongation of the pronotum and the posterior margin of the head, the metathoracic "air trough", and the unusual position of the metathoracic and first abdominal spiracles (Parsons 1970, 1974). These characteristics are compared in representatives of all three subfamilies, and their phylogenetic implications are discussed, below.
B. MATERIALS AND METHODS
Observations were made on Hesperocorixa interrupta (Say) from Massachusetts and Ontario, Diaprepocoris zealandiae Hale, from New Zealand, and Micronectasedula Horvhth, from Fukuoka, Japan. The insects, preserved in ethanol or Bouin's fluid, were dissected in 80% ethanol under a stereoscopic microscope. For examination under the scanning electron microscope, speci- mens were transferred, through decreasing concentrations of etha- nol, to distilled water. They were then mounted on aluminum stubs with silver Electrodag and kept briefly in distilled water until they were freeze-dried in an Edwards Speedivac-Pearse Tissue Drier, Model 1. They were coated with 20-40 nm of gold in an Edwards Vacuum Evaporator, examined under Cambridge Stereoscan, Model 2A, and photographed with Kodak Verichrome Pan 120 roll film.
I. Hydrofuge hairs
The distribution of the air stores in living specimens of Hespero- corixa was described in an earlier study (Parsons 1970). There are, to my knowledge, no similar descriptions of the air stores of Dia- prepocoris, and Leong (1961) mentions those of Micronecta only briefly. Although only preserved specimens of all three corixids



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were available for the present study, the location of their air stores can be deduced from their morphology, particularly the distribu- tion of hydrofuge hairs on the body. These hairs, which are clearly visible under the scanning electron microscope, can conveniently be divided into two categories according to their length. The long hairs (Figs. 1-3) range from 20 to 70 pm in length and are shorter in Micronecta (20-40 pm) than in the other two corixids. They are from one to five pm in diameter basally, and taper termi- nally. In Diaprepocoris and Hesperocorixa most, if not all, of the long hairs bear spiral grooves (Figs. 1 and 2); although those of Micronecta may have similar grooves they were not visible in any of the photomicrographs. When closely-packed, as on the abdo- men of Hesperocorixa (Fig. I), the long hairs are capable of holding a relatively thick air layer or "macroplastron" (term of Thorpe and Crisp 1949).
The short hairs (Figs. 2-4), unlike the long ones, are visible only under the scanning electron microscope and are much more densely packed. They range from two to ten prn in length and from 0.35 to 0.55 pm in diameter, and their tips are strongly curved. In many photomicrographs of Micronecta, and in occasional ones of Hes- perocorixa, the short hairs have the appearance shown in Fig. 3; they resemble low arches, with several branch-like ribs, rather than separate hairs. It is not clear whether these arches are artifacts caused by the clumping of the tips of short hairs during preparation for the scanning electron microscope, or whether they represent a different type of hydrofuge structure. 11. Ventral Air Store
The ventral surfaces of the thorax and abdomen are covered with hydrofuge hairs in all three corixids. The ventral abdominal surfaces bear closely-packed long hairs which are approximately 9 to 23 pm apart in Micronecta and 13-18 pm apart in Diapre- pocoris and Hesperocorixa. A conspicuous, thick ventral abdom- Figs. 1-4.
Hydrofuge hairs; tops of figures are anterior. Fig. 1. Hesperocorixa interrupta; long hairs on ventral surface of third abdominal segment in region of left third abdominal spiracle (right side of figure). Scale line = lOpm. Fig. 2. Diaprepo- coris zealandiae; short hairs and parts of three spirally-grooved long hairs on external surface of corium of forewing. Scale line = 5 pm. Fig. 3. Micronecta sedula, short hairs and parts of three long hairs on ventral surface of right prothoracic epimeron. Note arched appearance of most short hairs. Scale line = 5 p. Fig. 4. H. inter- rupta; short hairs on ventral surface of right prothoracic epimeron. Scale line= 2 pm.



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19761 Parsons - Morphology of Corixidae 135



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136 Psyche [June
inal air layer is visible in living specimens of the latter (Parsons 1970). The third abdominal spiracles (Figs. 5-7, S3) and the spira- cles posterior to them are located among the hairs in all three in- sects. In Diaprepocoris (Fig. 5) a longitudinal sulcus lies medial to these spiracles on the third through seventh abdominal segments. Micronecta, unlike the other two insects, possesses short hairs as well as long ones on the ventral abdominal surface. The short hairs are more sparsely distributed medially than laterally, and on the fifth and more posterior abdominal segments they end ap- proximately 10 jm~ from the lateral edge of the body, which is devoid of hairs. Leong (1961) mentioned the presence of "fine hairs" and a silvery ventral air layer on the abdomen of Micro- necta sp.
Fig. 5.
D. zealandiae, ventral view. Head and legs removed; coxae and long setae shown only on right side of figure. Mesothoracic spiracle (S 11) concealed by prothorax.




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19761 Parsons - Morphology of Corixidae 137 The anteroventral portion of the abdomen is concave on each side of the midline, forming a pair of metacoxal indentations (Figs. 5-7, MI) which accomodate the metacoxae and contain the second abdominal spiracles (S2).The metacoxae of living Hesperocorixa are surrounded by an air layer which extends into the metacoxal indentations (Parsons 1970). In all three corixids the ventral and posterodorsal surfaces of the metacoxae bear closely-packed long hairs, but these hairs are lacking on the anterodorsal coxal surfaces and on the metacoxal indentations in which they lie. The air in the indentations of Hesperocorixa, and presumably in those of the other two insects, must be retained by the close apposition between the coxae and the abdomen rather than by hydrofuge hairs. 0.5 mrn.
-w
Fig. 6. M. sedula, ventral view, preparedas in Fig. 5. Second abdominal spiracle (S2) concealed by lobe of metathoracic episternum. Efferent channel (EC) of meta- thoracic scent gland, indicated by dotted lines, lies beneath exoskeleton. Fig. 7.
H. interrupts, ventral view, prepared as in Fig. 5.



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138 Psyche [June
In all three corixids the exposed ventral surfaces of the prothorax and pteroihorax are covered with densely distributed short hairs. Although long hairs are also present on the thorax, they are usually farther apart than on the abdomen and are entirely absent in some areas. Long hairs are rarer on the ventral thorax in Diaprepocoris than in the other two insects, and they are shorter than those on the abdomen. In Corixa (Corixinae; Thorpe 1950) and Hespero- corixa, however, the thoracic long hairs are longer than the ab- dominal ones. The ventral thoracic air layer is easily visible in living Hesperocorixa (Parsons 1970); the presence of hydrofuge hairs on the ventral thorax of the other two corixids indicates that they possess a similar, although perhaps thinner, air store in this region.
111. Supra-alar Air Store
In Diaprepocoris almost all of the exposed surface of the fore- wing bears hydrofuge hairs (Fig. 8, stippling on left). The clavus (CA), corium (CO) and membrane (MB) are covered with densely- packed short hairs which extend nearly to the lateral edge of the wing. The clavus and corium also bear long hairs which are 20 to 30 pm apart (Fig. 2). The extreme lateral edge of the corium lacks hairs, the bare strip being widest (approximately 0.2 mm) posterior to the nodal furrow (Fig. 8, F). The embolium (E), unlike that of the other two corixids, is not sharply differentiated from the rest of the corium and is indented only anteriorly, approximately half way to the nodal furrow.
In Micronecta (Fig. 9) and Hesperocorixa (Fig. 10) the em- bolium is .more sharply differentiated and most of the exposed surface of the forewing lacks hydrofuge hairs. Living specimens of Hesperocorixa have an air layer (Fig. 10, stippling on left) on the strongly-indented embolium, on the anterior part of the claval suture (CS) and on the portion of the clavus which is overlapped by the pronotum (N I) (Parsons 1970). All these regions have a pruinose texture and are covered with short hairs and occasional long ones in Hesperocorixa. The hairs on the embolium extend well posterior to the nodal furrow. Anterior to the furrow they reach to the lateral edge of the wing, but posterior to the furrow they end a short distance medial to the lateral edge. Micronecta has an even less extensive supra-alar air store (Fig. 9, stippling on left). The corium and clavus bear thick setae, up to



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19761 Parsons - Morphology of Corixidae 139 Figs. 8-10.
Dorsal views; extent of exposed dorsal air stores indicated by stip- pling on left half. Fig. 8. D. zealandiae; exposed air stores on mesonotum (N 11) and most of forewing. Fig. 9. M. sedula; exposed air store confined to anterior portion of embolium (E) and anteriormost part of claval suture (CS). Air on mesonotum is concealed by pronotum (N I). Fig. 10. H. interrupta; exposed air store on forewing more extensive than in Fig. 9 but much less extensive than in Fig. 8. Air on meso- notum mostly concealed by pronotum.
15 pm in length, which are spaced too far apart (500-600 pm) to retain air. Only the embolium (E), the anteriormost part of the claval suture (CS), and the part of the wing beneath the pronotum (NI) have short hydrofuge hairs. On the embolium, which is less indented than in Hesperocorixa, they extend only as far posteriorly as the nodal furrow. The extreme lateral edge of the embolium bears short hairs only at the level of the mesothorax. Posterior to this level the lateral edge lacks hairs; at the nodal furrow they end approximately 80 pm from the lateral edge. IV. Cervical Air Store
In the Corixidae, unlike other Hydrocorisae, the posterior and lateral parts of the head are flattened and form flanges which over- lap the prothorax (Figs. 11-13). The cervical space is filled with air which is directly continuous with the ventral air store and is exposed to the water along the posterior edges of the head (Parsons 1970).




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140 Ps~che [June
Figs. 11-13.
Lateral views. Long setae on lateral abdomen omitted; legs removed distal to coxae. Arrow indicates gap through which ventral air store communicates with subalar air store, which contains 1110~ aouuminal spiracle (broken circle). Fig 1 1. D. zealandiae. Fig. 12. M. sedula. Fig. 13. H. interrupts. In all three corixids studied here the part of the pronoturn which is overlapped by the head bears hydrofuge hairs which help to retain the air store. In Hesperocorixa this area, unlike the exposed part of the sclerite (Figs. 10 and 13, N I), has a pruinose texture similar to that of the embolium of the wing. The scanning electron microscope shows that in Diaprepocoris and Micronecta the exposed posterior parts of the pronota (Figs. 8, 9, 11, and 12,



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19761 Parsons - Morphology of Corixidae 141 N I) lack hydrofuge hairs while the anterior parts, which lie beneath the head, are covered with closely-packed short hairs. Long hairs are also present in the posteromedial part of the overlapped region in both insects. A curious feature of this region is that in both Diaprepocoris and Micronecta the localized spots at which muscle fibers attach on the pronotum are devoid of hydrofuge hairs ex- ternally.
V. 1-11 Air Store
Although all Hydrocorisae carry some air between the prothorax and mesothorax, this air space is greatly enlarged, in Hespero- corixa, by the very long pronotum (Parsons 1970). The I-II air space contains the mesothoracic spiracle (Figs. 5-7, S 11). In all three corixids the spiracle is somewhat smaller than the meta- thoracic one and lies ventrolaterally in the intersegmental mem- brane, just ventromedial to the prealar bridge of the mesothorax. In Hesperocorixa, in which the spiracle is large enough to be ex- amined under the stereoscopic microscope, it is covered by a "sieve- plate" similar to that of the metathoracic spiracle (see p. 161). The I-II air store is directly continuous with tKe ventral thoracic and supra-alar stores. Its dorsal portion differs in the three insects. In Hesperocorixa (Fig. 10) the long pronotum (N I) conceals almost all of the mesonotum (N 11). The latter has a pruinose texture like that of the hydrofuge parts of the forewings. The air on the meso- notum is largely concealed, although it is exposed along the pos- terior margins of the pronotum.
In Diaprepocoris (Fig. 8) the pronotum is quite short and covers only the anteriormost part of the mesonotum. The exposed part of the latter (N 11), however, has long and short hydrofuge hairs like those on the dorsal surface of the forewings. It probably bears an extensive air layer which is fully exposed to the water, rather than only partly exposed as in Hesperocorixa. The pronotum of Micronecta (Fig. 9) is longer than that of Dia- prepocoris but shorter than that of Hesperocorixa, and does not cover the posteromedial tip of the mesonotum. The scanning electron microscope shows that the exposed portion of the meso- notum, unlike that of Diaprepocoris, lacks hydrofuge hairs. The overlapped anterior part, however, is covered with short hairs except laterally and posterolaterally, in the groove which holds the anteromedial margins of the forewings.



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142 Psyche [June
VI. Subalar Air Store
In all three insects the forewings are fully developed and extend to the tip of the abdomen (Figs. 11-13). All the Hesperocorixa examined had fully developed hindwings (Fig. 13, HW) and flight muscles. All specimens of the other two corixids, however, had vestigial indirect flight muscles and short hindwings. The latter extended to the level of the fourth and fifth abdominal tergites in Diaprepocoris and Micronecta respectively. i-fesperocorixa, like most Hydrocorisae, carries air beneath the forewings. The large subalar air store is exposed along the edges of the forewings posterior to the level of the mesothoracic epimeron, and the amount of exposure is increased when Hesperocorixa bends the abdomen ventrally on the thorax (Parsons 1970). Leong (1961) reported the presence of a subalar air store in Micro- necta sp., and Diaprepocoris almost certainly possesses one. The scanning electron microscope shows that in both insects the ventral surfaces of the clavus and corium bear short, densely-packed hydro- fuge hairs. In these two insects the ventral margin of the forewing . (Figs. 5 and 6, FW) projects laterally, beyond the edge of the body, as far posteriorly as the anterior abdominal segments. The short hydrofuge hairs extend onto the exposed, ledge-like ventral edge of the wing in both insects. In Micronecta they extend all the way to the lateral edge, while in Diaprepocoris they cover only the medial half of the exposed portion.
In Hesperocorixa the ventral edge of the wing does not project as far laterally as in the other two corixids. At the level of the meta- thorax the extreme dorsolateral edge of the embolium bears a patch of very long, curved setae (Figs. 10 and 13, WS), and immedi- ately ventral to the posteriormost setae there is an extensive gap between the edge of the wing and the edge of the body (Fig. 13, arrow). In living specimens the subalar, supra-alar, and ventral air stores communicate with each other along this gap and for some distance posterior to it (Parsons 1970). The subalar and ventral abdominal air stores also appear to communicate with each other along the posterolateral margins of the abdomen, which bear long setae (Fig. 7, right side of figure).
The amount of communication between the subalar and supra- alar stores cannot be determined in the other two corixids without living specimens. Like Hesperocorixa, however, both Diapre- pocoris and Micronecta possess a large space between the fore-



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19761 Parsons - Morphology of Corixidae 143 wing, the metathoracic epimeron, and the anterolateral part of the abdomen (Figs. 11 and 12, arrow) which allows the subalar air store to communicate with the ventral one. The two air stores probably also communicate along the posterior part of the abdomen, which bears lateral setae (Figs. 5 and 6, right sides of figures). I. Typical Hydrocorisae
The pterothorax of typical Hydrocorisae has been described in previous publications (Parsons 1970, 1974) and will be only briefly summarized here.
The mesothoracic (Fig. 14A, EM 11) and metathoracic episternum (ES 111) form flat evaginated lobes laterally, posteriorly, and pos- Fig. 14A and B.
Diagrammatic lateral views. (A) Pterothorax and anterior abdomen of typical Hydrocorisae. Mesothorax cut transversely, wings removed posterior to transverse cut. Heavy broken line indicates position of lateral inter- segmental boundary, concealed by external wall of mesothoracic epimeron (EM 11); heavy stippling indicates intersegmental air store, which contains metathoracic spiracle (concealed in this view). (B) Anterior mesothorax, cut as in Fig. 14A, show- ing how lateral epimeral lobe (LL) is modified in M. sedula and H. interrupts.



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1 44 Psyche [June
teromedially. The lateral lobes (LL) subdivide each pleurite into dorsal and ventral portions. The dorsal portions (D) of the pleurites are vertical and concealed by the forewings; the ventral portions (V) are more horizontal and are exposed to the water. The pos- terior lobes (LP) of both pleurites are continuous with the lateral ones. The posterior lobe of the mesothoracic epimeron overlaps the metathoracic episternum, whose posterior lobe overlaps the metacoxa (CX).
The posterolateral corner of the mesothoracic epimeron bears a knob-like process (Fig. 14A, K) which fits into a depression on the costal margin of the forewing, anchoring it against the edge of the lateral epimeral lobe. The subalar air space between the fore- wing and the body is enlarged lateral to the vertical, dorsal portions of the mesothoracic epimeron and metathoracic episternum, which are indented on the body.
The mesothoracic postalar bridge projects laterally into the subalar space forming a collar-like postalar projection (Fig. 14A, PP 11). A double-walled lateral ridge (LR) of the mesothoracic epimeron runs from the postalar projection to the wing-anchoring knob. The postalar projection encircles the membrane-of a meso- thoracic scolopophorous organ (SO 11) either partially, as in Fig. 14A, or completely. The function of this paired sense organ, and of its homologues on the metathorax (SO 111) and first abdominal segment (SO 1) has been debated. Larsh (1957) believed them to be equilibrium perceptors. More recently, however, experi- mental evidence has indicated that the mesothoracic scolopo- phorous organs of Notonecta obliqua (Notonectidae; Arntz 1972) and Corixapunctata (Corixinae; Prager 1973) and the mesothoracic and metathoracic organs of Nepa cinerea (Nepidae; Arntz 1975) are sensitive to sound.
The boundary between the mesothorax and metathorax curves sharply anteriorly into the mesothoracic region in the lateral part of the body (Fig. 14A, heavy broken line). In some Hydrocorisae this lateral intersegmental boundary forms a prominent, flap-like invagination which grows anteriorly during postecdysial develop- ment (Parsons 1974). A lateral intersegmental air space (heavy stippling), which is posteriorly continuous with the subalar air space, is formed between the invaginated boundary and the lateral ridge of the mesothoracic epimeron. It is ventromedially con- tinuous with a much smaller ventral intersegmental air space which lies dorsal to the posterior epimeral lobe.



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19761 Parsons - Morphology of Corixidae 145 The metathoracic spiracle (concealed in Fig. 14A) is located anterior to the intersegmental boundary in all Hydrocorisae which have been examined thus far (Parsons 1974). It lies on the meso- thoracic epimeron, and usually opens onto the lateral interseg- mental or ventral intersegmental air space (Positions 1 and 2, respectively, of Parsons 1974). It is thus concealed externally by the lateral ridge or posterior lobe of the mesothoracic epimeron. In Hesperocorixa and Notonecta, however, the spiracle lies im- mediately adjacent to the mesothoracic scolopophorous organ, on the dorsal, vertical part of the epimeron, and opens directly onto the subalar air space (Position 3 of Parsons 1974). 11. Diaprepocoris
Of the three corixids examined, Diaprepocoris shows the least modification of the typical plan diagrammed in Fig. 14A. The lateral lobe of the mesothoracic epimeron (Fig. 15, LL) is relatively narrow and projects laterally and somewhat dorsally, as in typical Hydrocorisae, rather than curving sharply dorsally, as in Micro- necta and Hesperocorixa (Fig. 14B). Thus the costal margin of the wing lies farther ventrally on the body than in the latter two insects.
Posterior to the wing-anchoring knob of the mesothoracic epi- meron, the margin of the forewing fits into a horizontal groove (Fig. 15, W) on the metathoracic episternum. The homologies of this groove are uncertain. Unlike the wing-holding devices on the metathoracic episterna of Micronecta (Fig. 16, W) and Hespero- corixa (Fig. 17, W), it lies far ventral to, rather than at the level of, the metathoracic coxal process (Fig. 18, PR), and appears ex- ternally as a groove rather than as a ridge. In addition, it does not form the ventral boundary of a well-developed "air trough" such as that of Hesperocorixa (Fig. 17, AT). In the latter, both the epi- meron and the episternum of the metathorax are strongly indented in the region of the pleural sulcus (PL 111). In Diaprepocoris, as