M. C. Parsons and R. J. Heivson.
Plastral Respiratory Devices in Adult Cryphocricos (Naucoridae: Heteroptera).
Psyche 81:510-527, 1974.

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PLASTRAL RESPIRATORY DEVICES IN ADULT
CRYPHOCRICOS (NAUCORIDAE : HETEROPTERA)* Department of Zoology
University of Toronto
Toronto, Ontario M5S IAI
Adult Cryphocricos live in very rapid, well-aerated streams, under stones and gravel at depths of from 6 to 24 inches (Fritz Plaumann, personal communication; John Polhemus, ~ersonal communication; Randall Schuh, personal communication). Under these conditions it would be difficult for these insects to come to the water surface for atmospheric air, as do "slow-water" Naucoridae and most other aquatic Heteroptera, and neither Polhemus (personal communication) nor Schuh (personal communication) have observed them to do so. Even if it were possible for Cryphocricos to obtain atmospheric air, the insects lack a means of storing large amounts of it for underwater respiration. Aquatic Heteroptera with "air-bubble" respiration carry much atmospheric air under their long forewings and trapped by long hairs on the ventral surfaces of their bodies. Long-winged Crypho- cricos, however, are very rare. Most individuals lack hindwings and have forewings which reach only to the third abdominal segment. The reduced subalar space on the pterothorax is too small to hold any appreciable amount of air (Parsons 1974). Thorpe (1950) concluded, from examination of a dried specimen of Cryphocricos sp., that it is "a bubble-carrier rather than a true plastron insect" and has "a substantial air film". Polhemus, however, examined C. hungerfordi in the field and observed no such air layer except on the six pairs of small ventral abdominal sense organs (Polhemus, per- sonal communication). Parsons (1974) observed a very thin air layer on the ventral surfaces of the thorax and abdomen in formalin- preserved C. barwzzii, but this layer is visible only under the stereo- scopic microscope and is usually hidden by the debris which covers the insect.
Young's ( 1944) suggestion that Cheirochela (Naucoridae) sur- vives by cutaneous respiration appears highly dubious, and it is un- likely that Cryphocricos uses this method to any great extent. The *Manuscript received by the editor February 6, 1975 5 10




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19741 Parsons ‰â Hewson - Cryphocricos 511 exoskeleton is quite thick, and the relatively large size of the insect (C. hungerfordi is 8.5-9.5 mm long; C. barozzii is 10-13 mm long) produces an unfavorable ratio of volume to surface area. Among the aquatic Heteroptera, true plastral respiration has thus far been experimentally demonstrated only in Aphelocheirus (Thorpe and Crisp 1947a, 1g47b; Thorpe 1950). Whether this insect is a naucorid (Usinger 1956) or represents a separate family, the Aphelo- cheiridae (Parsons 1969) is debatable. Aphehcheirus possesses a very thin, permanent air layer, or "plastron", which is retained by short, densely-~acked hairs on most of the dorsal and ventral surfaces of the body. The plastron is exposed to the water and acts as a gill, obtaining enough dissolved oxygen from the water to make the in- sect independent of atmospheric oxygen (Thorpe and Crisp I 947a ; Thorpe 1950). Cryphocricos and Aphelocheirus share such morpho- logical similarities as reduced forewings, vestigial hindwings, lack of a large air store, and paired abdominal pressure receptors of a type not yet reported in any other Heteroptera (Aphelocheirus, Thorpe and Crisp I 9470, Lars& 1955 ; Cryphocricos, Thorpe I 950, Parsons 1974). These similarities suggest that Cryphocricos, like Aphelo- cheirus, has a plastral type of respiration. Hinton (1g6ga, p. 198) has remarked that "Por ejemplo, entre insectos adultos hay por lo menos cuatro origenes independientes de respiration de plastron den- tro de la familia Naucoridae: Aphelocheirus, Cryphocricos, Heleo- coris, e Idbcarus". He included a photomicrograph of thoracic plastral hairs in Heleocoris but did not further elaborate on his statement.
Although the presence of plastral respiration would be best dem- onstrated by both morphological and physiological studies, live speci- mens of Cryphocricos were not available for experimentation. Our investigation, like Hinton's (1g6gb) study on aquatic beetles, was therefore confined to morphology. Its purposes were (I) to discover whether the exposed surfaces of the body possess hydrofuge devices which might retain a thin plastral air layer, and (2) briefly to com- pare any such structures with those of other insects in which plastral respiration is believed to occur.
MATERIALS AND METHODS
Species used :
Cryphocricos barozzii Signoret (preserved in formalin; collected in Brazil by F. Plaumann; two specimens) C. hungerfordi Usinger (preserved in alcohol; collected in Mexico by J. T. Polhemus; one specimen)




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5 12 Psyche [September-December
Aphelochevus aestivalis Fabricius (preserved in Bouin's fluid ; collected in England by D. T. Crisp; two specimens) The specimens were first dissected in 80% alcohol under a stereo- scopic microscope. They were cut midsagittally with a razor blade and the ventral and dorsal surfaces of one or both halves were sep- arated along the lateral edge. On one specimen of Aphelocheirus a small section of the metathoracic episternum was removed so that the cut edge could be examined. A major obstacle was the presence of large amounts of silt and other debris on the external surfaces of all three species. Treatment with weak solutions of either potassium hydroxide or Sparkleen detergent failed to remove it. Somewhat better results were obtained by scraping small areas with watch- maker's forceps.
The specimens were then mounted with double-sided tape on aluminum stubs, freeze-dried, and coated, in an Edwards vacuum evaporator, Model 4, with a layer of gold which was approximately 200-400 A thick. They were examined in a Cambridge Stereoscan, Model 2A, and photographed with a Singer camera, using Ilford 120 roll film.
Ventral Surface
( I ) Plmtral Hairs
In both species of Cryphocricos most of the ventral surface is covered with densely-packed hairs (Fig. 3). They occur on the pro- Fig. 1. Cryfhocricos barozzii; ventral view, legs removed. Stippling on fourth through sixth abdominal sternites indicates area of microtrichia (Fig. 11) ; stippling on ventral abdominal paratergites, left side of figure, indicates area of short leaf-like setae (Figs. 6-8). Longer leaf-like setae occur on the six pairs of abdominal sense organs. Most of the (ventral surface is covered with plastral hairs (Figs. 3, 4). Abbreviations: EM, epimeron ; ES, episternum ; FW, forewing ; LS, longitudinal sulcus; MCI, metacoxal indentation on abdomen; PT, ventral abdominal paratergite; SO, abdominal sense organ; SP, abdominal spiracle; ST, sternite. Arabic numerals indicate abdominal segments; Roman nu- merals indicate thoracic segments.
Fig. 2. Cryphocricos barossii; dorsal view, right forewing removed, revealing smooth subalar surfaces on right side (even stippling). Other surfaces are roughened. For distribution of plastral hairs, see text. Abbreviations: TI, TIl, Till, thoracic terga; Tl, T2, T3, abdominal terga.




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514 Psyche [September-December
thoracic epimeron and sternum (Fig. I, EM I, ST I), the meso- thoracic epimeron, episternum, and sternum (EM I1 ES 11, ST 11)) the metathoracic episternum (ES 111)) and all seven ventral ab- dominal paratergites (PT 2-PT 8). They also cover all of the third and seventh abdominal sternites and all but the anteromedial por- tions of the fourth through sixth sternites (ST 4-ST 6). The close spacing of the hairs makes it difficult to see their bases and to measure their length. Their visible portions are at least 6 pm long and from 0.25 to 0.45 pm in diameter on the thorax and para- tergites (Fig. 4). They are considerably shorter and wider on the abdominal sternites; on a sagittal cut through the fifth sternite, where the entire length of a few hairs was visible, they were only 2.7 pm long but 0.65 pm in diameter.
Comparison of the thoracic and paratergal hairs with the plastral hairs of Aphelocheirus (Fig. 5) shows similarities which strongly suggest that those of Cryphocricos (Fig. 4) are also plastral devices. The most striking similarities are (I ) the presence of small nodules on the basal portions of the hairs, (2) the densely-packed arrange- ment of these hairs, and (3) the posterior bending of their apical portions, which lack nodules. Using the light microscope and trans- mission electron microscope, Thorpe and Crisp (1947a) observed the bending of the hairs in Aphelocheirus but did not see the nodules. The bending appears to be more pronounced in both specimens of Aphelocheirus than in the three specimens of Cryphocrioos. In both genera the hairs tend to adhere to each other in clumps, making the direction of the bending difficult to detect in surface view (Fig. 3) ; the clumping is probably an artifact.
Fig. 3.
Cryphocrlcos barozzii; surface view of apical parts of plastral hairs on metathoracic episternum. More basal parts of hairs, with nodules, visible at lower left. Clumping of tips is probably an artifact. Left Ì pos- terior, right = anterior. Scale line = 2 /~i. Fig. 4.
C. barozzli; clump of plastral hairs on fifth ventral abdominal paratergite. Leaf-like setae, which concealed these hairs, were scraped off during preparation of specimen. Bases of hairs were concealed by tips of surrounding hairs. Note posterior (to right) bending of apices and pres- ence of nodules on proximal parts of hairs. Scale line= 1 pm. Fig. 5. Aphelocheim aesticaiis; cut edge of metathoracic episternum, showing full length of plastral hairs. Note posterior (to right) bending of apices and presence of basal nodules. Scale line = 1 pm.



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516 Psyche [September-December
The thoracic plastral hairs of Afihelocheirus are approximateI~7 5-6 pm in length, as Thorp (1950) also noted. They are thus somewhat shol-ter than those of Cry~hocricos, but appear to be com- parable to the latter in diameter. Their bases are somewhat thicker (approximately 0.4 ,um) than their tips (approximately 0.3-0.35 ,urn). In both genca adiacent hairs seem to 1:e very clcse to each other. The nodules m2y help to hold them apart? allowing room for air to cixulate betwe~n them. Also, because of the thickness of the gold coating (o.oz-~~o~t pm) , the hairs probablj? appear, under the scan- ning electron microscope, somewhat thicker, and the intervening spaces smaller, than thev actually are. By means of the light and transn~ission electron microscopes, Thorpe and Crisp (19~57a) esti- mated the space betwecn acliace~~t hairs on the abdominal sternites of Afihlochrir-us to be app+oximateIy twice as great (0.4 ,urn) as the
diameter of the tip of a shgle hair (0.18-0.25 pm). The discrepancy between their obsel-v~tions 2nd ours may be due, in part, to the dif- ferent techniques employed.
It is also possible that the haiys are less densely packed on the abdominal sternites than on the thorax of Aphelocheirus.
The hairs of both Cryphocricos and Apheloch!~irus a--e ~nuch shoi-ter) narrower) and more densely packed than those whic3 cmer the ventral surface of the abdomen of Pelocoris femoratus, a "slow- water" naucorid with air-bubble respiration (Parsons 1974). A brief observation of one specimen of PeZocoris revealed that the hairs on the second abdominal paratei-gite lack basal nodules and are 30-40 ,urn long, 2-3 ,urn wide at the base) and approximately 10 ,urn apart. (2) Leaf-lihe Setue
Leaf-like setae occur on the ventral paratergites of the abdomen (Fig, r ) light stippling lateral to LS) . They extend 211 the way to the lateral edgc of the body on the second abdominal paratergite) and nearlv to the lateral edge on the more posterior paratergites. On the second paratergite (PT 2) a narrow anteromedial strip adjoining the mrtathoracic episternum (ES 111) and the metacoxal indentation (MCI) lacks leaf-like setae (Fig. 6).
This "nonf01iated" region is
larger in Cryphocricos hungerfordi than in C. barozzii. The second abdominal spiracle (Fig. I, SP 2) and sense organ (SO 2), like those of the other abdominal segments, lie in the "foliated" region. In C. hurozzii the flattened setae are widest (8-20 pm) near their bases and are 20-40 pm long. In C. hungerfordi, a smaller insect, they are approximately 75% this size. They appear to be socketed (Fig. 8) and are arranged somewhat like shingles, in closely packed,



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518 Psych6 [September-December
somewhat irregular rows, each row overlapping the one posterior to it (Figs. 6) 7). The tapered tip of each seta points posteriorly and somewhat laterally on the body except in the spisacular regions, where those immediately lateral to the spiracles point posteromedially. On the eighth paratergite the setae are fewer and farther apart and do not overlap.
Under the stereoscopic microscope the ventral paratergites) unlike the "nonfoliated" regions of the body, have a faintly golden sheen. The sheen is visible when specimens are immersed in alcohol, which would remove any adherent air layer. It is pro~bably the result of light diffraction by the fine longitudinal ridges on the ventral surfaces of the flattened setae (Figs. 7) 8).
In their shape and overlapping arrangement the setae resemble the plastral devices of Phytobius ( Coleoptera ; Thorpe and Crisp 1949). Unlike the latter) however, they do not bear plastral hairs on their external surfaces. The plastral hairs arise from the exoskeletal sur- face beneath the leaf-like setae and are largely concealed by them (Fig. 7) but were clearly visible in regions where the setae had been scraped off with watchmaker's forceps (Fig. 4). Although the setae do not completely cover the ventral abdominal spiracles they ov,erlap so much of their margins that could not observe spiracular struc- ture or determine whether or not they possess radiating "rosettxs)' similar to those of the second through seventh abdominal spiracles of Aphelocheirus (Thorpe and Crisp I 947a). The second through seventh abdominal sense organs (Fig. I, SO 2- SO 7) are con~posed of leaf-like setae which are two to three times longer than those covering the rest of each paratergite (Fig. 9). Most of these elongated setae project more laterally than the shorter ones, and have blunter tips and more pronounced longitudinal ridges Fig. 8. Cryphocricos hungerfordi; detail of one leaf-like seta on fifth ventral abdominal paratergite.
Note longitudinal ridges and tapering tip) which points posterolaterally (lower left). Surrounding setae w,:re rubbed off, revealing tips of underlying plastral hairs and apparently socketed base of seta. Scale line = 2 ,urn.
Fig. 9. C. baromii; sense organ on third ventral abdominal paratergite Sense organ is composed of elongated leaf-like setae. Tips of shorter leaf-like setae) covering rest of paratergite, are visible at bottom and upper left of figure. Right = lateral) bomom = posterior. Scale line = 30 pm.
Fig. 10. C. barozzii; detail of Fig. 9) showing longitudinal ridges on elongated leaf-like setae. Scale line = 2 ,urn.



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520 Psyche [September-December
(Fig. 10). A considerable quantity of air is trapped among these overlapping, elongated setae. On living specimens the sense organs are the only parts of the body on which air can be seen with the naked eye (Polhemus, personal communication), and the air on these regions is easily seen with the stereoscopic microscope. In Aphelocheirus similar sense organs, which are probably pressure receptors, occur only on the second abdominal segment (Thorpe and Crisp 19470). Like those of Cryphocricos they are composed of flattened, elongated setae, approximately 75 pm long, which point posterolaterally and overlap each other. The setae appear to be longi- tudinally ridged, although this could not be determined with certainty because of the large amount of debris which covered them. Leaf-like setae were not observed on any other region of the body of Aphelo- cheirus.
On the anteriormost portions of the fourth through sixth abdomi- nal sternites (Fig. I, ST 4-ST 6) plastral hairs occur only laterally; near the midline they are lacking. The only structures observed in this hairless region are short, blunt microtrichia, approximately I p 1 wide at the bases and 2 pm long, which project posteriorly. They are arranged in widely-spaced groups of from two to six. Each group of microtrichia arises from the slightly raised, curved, posterior edge of a roughly sen~icircular region of the cuticle. The microtrichia lack sockets and lie nearly parallel to the surface of the cuticle. More posteriorly on the fourth through sixth sternites, but not as far as the posteriormost edge, the microtrichia are more numerous per group, longer (up to 3 ,urn), and occur in more closely-spaced groups (Fig. I I). Each group consists of 5-15 microtrichia and these, along with the region of smooth cuticle to which they attach, form a fringed, semicircular unit. The units are arranged like shin- gles. On this central part of the sternite, unlike the more anterior region, hairs appear to be present between adjacent rows of micro- trichia. Since only the tips of these hairs are visible, we could not determine whether they are similar to the short plastral hairs which cover the lateral and posterior regions of the fourth through sixth sternites; these regions lack microtrichia (Fig. I, unstippled portions of ST 4-ST 6).
(4) Fringed Setae (Fig. 12)
Fringed setae may occur either singly or in large numbers. They arise from sockets in the cuticle and project posterolaterally. A trans-



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19741 Parsons &? Hewson - Cry fthocricos 52 1 verse section through a fringed seta near its base would appear as a nearly complete circle, with the open end facing away from the cuti- cle. Towards the tips the setae are wider, more flattened, and would appear as more widely open arcs in transverse section. Their outer surfaces bear longitudinal grooves corresponding to pronounced ridges on their inner surfaces. The apical portion of each of these ridges subdivides into several short projections which give the seta a fringed appearance. The sizes of the fringed setae vary considerably; on the lateral edges of the ventral paratergites their length ranges from 12 to 38 pm, and their diameter from 6 to 15 ,am. Single fringed setae occur on nearly all parts of the ventral sur- face, but appear to be less common on the prothorax and the medial portions of the abdominal paratergites than on other regions. Large concentrations of them occur along the lateral edges of the third through eighth ventral pal-atergites. On the second paratergite the leaf-like setae extend all the way to the lateral edge, and there are few fringed setae. On the most posterior segments, however, the ex- treme lateral portions of the paratergites bear fringed rather than leaf- like setae. They occur in largest numbers on the edges of the fifth, sixth, and seventh abdominal segments and become increasingly less numerous on each of the more anterior ones. The eighth paratergite
bears fewer of them than the seventh.
In Cryphocricos hungerfordi a narrow strip bearing only plastral hairs separates the leaf-like and fringed setae, and the latter, which occur in a single row, are fewer and farther apart than in C. baro-zzii. The two types of setae are less clearly separated in C. barozzii, whose fringed setae are more numerous, closer together, and in several poorly-defined rows (Fig. 12). Their dense distribution on the lateral edges of the fifth through seventh paratergites in this species sug- gested, at first, that they might be hydrofuge devices. On the para- tergites of C. hungerfordi, however, they appear to be too far apart to function in this way. Their sporadic occurrence on nearly all the exposed surfaces of the body suggests that they have some other func- tion, probably a sensory one.
Dorsal Surface
The dorsal surface of the body was studied only briefly in one specimen of each species of Cryphocricos. In both specimens this surface is covered with considerably more silt and debris than the ventral one. The detritus clings to all the parts of the dorsum which are exposed to the water (Fig. 2, left side) ; these regions have a roughened appearance under the stereoscopic microscope.



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522 Psyche [September-December
The surfaces which are concealed by the shortened forewings, how- ever, are relatively free of debris. Except for the third abdominal tergite (Fig. 2, T3) these regions appear smooth under the stereo- scopic microscope (Fig. 2, light stippling on right side). The smooth terga and the inner (ventral) surfaces of the forewings lack fringed setae and are covered with densely-packed hydrofuge hairs. Those on the metatergum (T 111) and second abdominal tergite (T 2). which were examined in more detail, possess nodules basally, are at least 5 pm long, and have diameters similar to those of the plastral hairs on the ventral surface of the body. The hairs on the inner surface of the forewing are considerably shorter (approximately 1.5-2 pm in length) than those on the second abdominal tergite and metatergum. Basal nodules were observed on these alar hairs only in the region of the forewing which is externally visible in ventral view (Fig. I FW) ; here the hairs are longer, but since their tips appear to coalesce their height could not be estimated. The surfaces which are exposed to the water are covered with small, wart-like, cuticular protuberances which give them a roughened texture. The raised surfaces of these protuberances, which are ap- proximately 20-35 pm in diameter and one to two times this distance apart, lack hairs. Each "wart" bears a small central indentation con- taining a curious fringed papilla, possibly a sensory device. The large spaces between the protuberances were, unfortunately, covered with debris on all of the exposed abdominal tergites. The anterior third of the third abdominal tergite, however, is overlapped by the posterior edge of the forewing, which largely protects it from debris. In this region, densely-packed hairs, at least 5 pm long and 0.3 to 0.4 urn wide at the tips, cover the areas between the cuticular "warts". Similar hairs occur between the protuberances on the exposed portion of the mesothoracic tergum (Fig. 2, T 11). Quite probably hydro- fuge hairs are present between the protuberances on the prothoracic and fourth through seventh abdominal terga, but are hidden beneath the debris. Occasional fringed setae occur on all of the exposed dorsal surfaces of the body, and are especially numerous along the lateral edges of the abdominal terga and in the abdominal intersegmental sulci.
Although in most areas the fringed setae appear to be too widely spaced to retain an air layer, at least two of the other three types of fine structures on the cuticular surface of Cryphocricos appear to be



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19741 Parsons OS? Hewson - Cryphocricos 523 plastral devices. The densely-packed hairs on the ventral surfaces of the thorax and abdomen strongly resemble those of Aphelocheirus, which has plastral respiration (Thorpe and Crisp 1947a) and differ from those of Pelocoris, which has "air-bubble" respiration (Parsons 1974). The microtrichia in the central portions of the fourth through sixth abdominal sternites have a shingle-like arrangement which strongly resembles that of the plastral devices on the sternites of Torridincola (Coleoptera, Hinton 1969b) ; these microtrichia, along with the hairs between them, probably also retain a thin layer of air.
The function of the leaf-like setae on the pal-atergites is more problemmatic. They may serve merely to protect the underlying plastral hairs from becoming too clogged with debris to allow the adherent air layer to carry on gas exchange with the water. Although they may, indeed, do this, it is unlikely that it is their sole function, since they are absent in other regions with plastral hairs. In Aphelo- cheirus, which is also covered with debris, leaf-like setae occur only on the second abdominal sense organs, but the experiments of Thorpe and Crisp (1g47a, 1947b) have shown that the insect is able to maintain efficient plastral respiration. The presence of large amounts of air among the elongated leaf- like setae on the sense organs of Cryphocricos suggests that the shorter ones on the rest of the paratergites are similarly capable of retaining air, although in smaller and less visible amounts. They may well act in conjunction with the underlying plastral hairs, in much the same way that long and short hydrofuge hairs hold a "macroplastron" external to a "microplastron" in some aquatic Cole- optera such as Hydrophilus piceus (Thorpe and Crisp 1949). Like the long hairs which retain the "macroplastron" in the latter, the leaf-like setae may, under increased hydrostatic pressure, help to protect the thin layer of air, which is trapped in the underlying short hairs, against water entry. When the insect dives to regions of in- creased pressure the posterolateral slope of the setae may increase, so that the setae become pressed together and form a barrier against