PLASTRAL RESPIRATORY DEVICES IN ADULT CRYPHOCRICOS NAUCORIDAE HETEROPTERA

Adult Cryphocrico live in very rapid, well-aerated streams, under stones and gravel at depths o rom 6 to 24 inches (Fritz Plaumann, personal communication; John Polhemus, personal communication; Randall Schuh, personal communication). Under these conditions it would be di(ficult or these insects to come to the water surface or 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 ii: it were possible t:or Cryphocricos to obtain atmospheric air, the insects lack a means o storing large amounts o it or underwater respiration. Aquatic Heteroptera with "air-bubble" respiration carry much atmospheric air under their long orewings and trapped by long hairs on the ventral sur(aces o( their bodies. Long-winged Cryphocricos, however, are very rare. Most individuals lack hindwings and have orewings which reach only to the third abdominal segment. The reduced subalar space on the pterothorax is too small to hold any appreciable amount o( air (Parsons 974). Thorpe (95o) concluded, rom examination o( a dried specimen o Cryphocricos sp., that it is "a bubble-carrier rather than a true plastron insect" and has "a substantial air film". Polhemus, however, examined C. hunger[ordi in the field and observed no such air layer except on the six pairs o small ventral abdominal sense organs (Polhemus, personal communication). Parsons (I974) observed a very thin air layer on the ventral surfaces o the thorax and abdomen in ormalinpreserved C. bat,ozzii, but this layer is visible only under the stereoscopic microscope and is usually hidden by the debris which covers the insect. Young’s (944) suggestion that Cheirochela (Naucoridae) survives by cutaneous respiration appears highly dubious, and it is unlikely that Cryphocricos uses this method to any great extent. The

Parsons & Hewson--Cryphocricos 5II exoskeleton is quite thick, and the reIatively large size of the insect (C.hunger[ordi is 8.5-9.5 mm long; C. barozzii is to-t3 mm long) produces an unfavorable ratio o volume to surface area.
Among the aquatic Heteroptera, true plastral respiration has thus i:ar been experimentally demonstrated only in Aphelocheirus (Thorpe  and Crisp t947a, t947b; Thorpe t95o).Whether this insect is a naucorid (Usinger t956) or represents a separate amily, the Aphelo- cheiridae (Parsons 969) is debatable, dphelocheirus possesses a very thin, permanent air layer, or "plastron", which is retained by short, densely-packed hairs on most o the dorsal and ventral sunCaces ot: the body.The plastron is exposed to the water and acts as a gill, obtaining enough dissolved oxygen rom the water to make the in- sect independent o( atmospheric oxygen (Thorpe and Crisp 947a; Thorpe t95o).Cryhocricos and dphelocheirus share such morpho- logical similarities as reduced forewings, vestigial hindwings, lack o a large air store, and paired abdominal pressure receptors ot: a type not yet reported in any other Heteroptera (dhelocheirus, Thorpe an[ Crisp t947c, Larsn 955; Crykhocricos, Thorpe 95o, Parsons 974).These similarities suggest that Crykhocricos, like dkhelocheirus, has a pIastral type o respiration.Hinton (969a,p. t98) has remarked that "Por ejemplo, entre insectos adultos hay por lo menos cuatro orlgenes independientes de respiraci6n de plastr6n den- tro de la familia Naucoridae" dphelocheirus, Cryhocricos, Heleo- coris, e Idiocarus".He included a photomicrograph of thoracic plastral hairs in Heleocoris but did not t:urther elaborate on his statement.
Although the presence of plastral respiration would be best dem- onstrated by both morphological and physiological studies, live speci- mens o Cryphocricos were not available or experimentation.Our investigation, like Hinton's (t969b) study on aquatic beetles, was therefore confined to morphology.Its purposes were (I) to discover whether the exposed surfaces o( the body possess hydroi:uge 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.

Species used"
Cryphocrico barozzii Signoret (preserved in ormalin; collected in Brazil by F. Plaumann; two specimens) C. hunger[ordi Usinger (preserved in alcohol; collected in Mexico by J. T. Polhemus; one specimen) [September-December A#helocheirus aestivalis Fabricius (preserved in Bouin's fluid; collected in England by D. T. Crisp; two specimens) The specimens were irst 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 /llhelocheirus 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 watchmaker'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 _h thick.They were examined in a Cambridge Stereoscan, Model 2A, and photographed with a Singer camera, using Ilford 12o roll film.

Plastral Hairs
In both species o Crylhocricos most o the ventral surface is covered with densely-packed hairs (Fig. 3).They occur on the pro- Fig. 1.Crylhocricos barozii; 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).

Psyche
[September-December thoracic epimeron and sternum (Fig. I, EM I, ST I), the meso- thoracic epimeron, episternum, and sternum (EM II, ES II, ST II), the metathoracic episternum (ES III), 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 long and from 0.25 to o.45 /,m 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 >m long but o.65 /,m in diameter.
Comparison o.f the thoracic and paratergal hairs with the plastral hairs of A15helocheirus (Fig. 5) shows similarities which strongly suggest that those o.fCryt)hocricos (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 (947a) observed the bending of the hairs in dphelocheirus but did not see the nodules.The bending appears to be more pronounced in both specimens of dphelocheirus 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.The thoracic plastral hairs of d phelocheirus are approximately 5-6 /m in length, as Thorpe (195o) also noted.They are thus somewhat shoz'ter than those of Cryphocricos, but appear to be com- parable to the latter in diameter.Their bases are somewhat thicker (approximately 0.4/m) than their tips (approximately 0.3-0.35In both genera adiacent hairs seem to l!e very close to each other. The nodules may help to hold them apart, allowing room fo air to circulate between them.Also, because o the thickness of the gold coating (o.o2-o.o4/zm), the hairs probably appear, under the scan- ning electron microscope, somewhat thicker, and the intervening spaces smaller, lan they actually are.By means of the light and transmission electron microscopes, Thorpe and Crisp (I947a) esti- mated the space between adiacent hairs on the abdominal sternites o( d lhelocheirus to be approximately twice as great (o.4 /zm) as the diameter of the tip o( a single hair (o.I8-.25 /m).The discrepancy between their observations and ours may be due, in part, to the dif- ferent techniques employed.It is alo possible that the hairs are les densely packed on the abdominal sternites than on the thorax o Aphelocheirus.
The hairs of both Cryphocricos and A thelocheirus az'e much shorter, narrower, and more densely packed than those which cover the ventral surface of the abdomen o Pelocoris femoratus, a "slow- water" naucorid with air-bubble respiration (Parsons I974).A brief observation of one specimen of Pelocoris revealed that the hairs on the second abdominal paratergite lack basal nodules and are 3o-4o /zm long, 2-3 tm wide at the base, and approximately IO /m apart.
(2) Leaf-like 8etae Leaf-like setae occur on the ventral paratergites of the abdomen (Fig. 1, light stippling lateral to LS).They extend all the way to the lateral edge of the body on the second abdominal paratergite, and nearly to the lateral edge on the more posterior paratergites.On the second paratergite (PT ) a narrow anteromedial strip adjoining the metathoracic episternum (ES III) and the metacoxal indentation (MCI) lacks leaf-like setae (Fig. 6).This "nonfoliated" region is larger in Cryl)hocricos hungerfordi than in C. barozzii.The second abdominal spiracle (Fig. , SP 2) and sense organ (SO :), like those of the other abdominal segments, lie in the "foliated" region.
In C. barozzii the flattened setae are widest (8-o m) near their bases and are o-4o /m 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,  , 7).The tapered tip o each seta points posteriorly and somewhat laterally on the body except in the spiracular regions, where those immediately lateral to the spiracles point posteromedially.On the eighth paratergite the seta.e are ewer and arther apart and do not overlap.
Under the stereoscopic microscope the ventraJ paratergites, unlike the "nonoliated" regions o the body, have a aintly 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 o light diffraction by the qne longitudinal ridges on the ventral sura.ceso the flattened setae (Figs.. 7, 8).
In their shape and overlapping arrangement the setae resemble the plastral devices o Phytobius (Coleoptera; Thorpe and Crisp I949).
Unlike the latter, however, they do not bear plastral hairs on their external surfaces.The plastral hairs arise rom the exoskeletal sur- face beneath the lea-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 orceps (Fig. 4).Although the setae do not completely cover the ventral abdominal spiracles they overlap so much o their margins that we could not observe spiracular struc- ture or determine whether or not they possess radiating "rosettes" similar to those o the second through seventh abdominal spiracles o Aphelocheirus (Thorpe and Crisp I947a).
The second through seventh abdominal sense organs (Fig. I, SO 2- SO 7) are composed o lea-like setae which are two to three times longer than those covering the rest o each paratergite (Fig. 9).
Most o 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 were rubbed off, revealing tips of underlying plastral hairs and apparently socketed base o.f seta.Scale line--2 #m.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 I947c).Like those of Cryphocricos they are composed flattened, elongated setae, approximately 75 /m long, which point posterolaterally and overlap each o.ther.The sete 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 A phelo- cheirus.
(3) Microtrichia 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 wide at the bases and 2/m long, which project posteriorly.They are arranged in widely-spaced groups of from two to six.Each group microtrichia arises from the slightly raised, curved, posterior edge a roughly semicircular region of the cuticle.The microtrichia lack sockets and lie nearly parallel to the surface of the cuticle.
More posteriorly on the ourth through sixth sternites, but not as far as the posteriormost edge, the microtrichia are more numerous per group, longer (up to 3 /m), and occur in more closely-spaced groups (Fig. II).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 regio.nslack microtrichia (Fig. (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-

1974.]
Parsons Hewson Crylhocricos 5:I 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 /xn, and their diameter from 6 to 15 Single ringed setae occur on nearly all parts of the ventral sur- tace, 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 o.f the third through eighth ventral paratergites.On the second paratergite the leaf-like setae extend all the way to the lateral edge, and there are few tringed 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 abdo.minalsegments 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. barozzii.The two types of setae are less clearly separated in C. barozzii, whose ringed 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 to.ofar 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.

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 III) and second abdominal tergite (T 2), which were examined in more detail, possess nodules basally, are at least 5 /zm 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 .5-2/m 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. , 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 /m 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 /zm long and 0.3 to 0.4 /m 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 II).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 intersegrnental sulci.

DISCUSSION
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 Crylhocricos appear to be     1974]   Parsons & Hevason--Cryphocricos 523 plastral devices.The densely-packed hairs on the ventral surfaces of the thorax and abdomen strongly resemble those of dphelocheirus, which has plastral respiration (Thorpe and Crisp I947a) and differ from those of Pelocoris, which has "air-bubble" respiration (Parsons 1974).The microtrichia in the central portions of the ourth through sixth abdominal sternites have a shingle-like arrangement which strongly resembles that of the plastral devices on the sternites of Torridincola (Coleoptera, Hinton I969b) 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 paratergites 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 A1)helo- cheirus, which is also covered with debris, leaf-like setae occur only on the second abdominal sense organs, but the experinaents of Thorpe and Crisp (I947a, I947b) 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 H,drophilus 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 water penetration.Their presence only on the paratergites, and their absence in other regions, may be connected with the need to protect the second through eighth abdominal spiracles, which lie on the sur- faces of the paratergites, against flooding.The three more anterior pairs of spiracles (mesothoracic, metathoracic, and first abdominal) are concealed within invaginations of the exoskeleton and are thus more protected than the posterior ones against the entry of water.

Psyche
[September-December Thorpe and Crisp (947c) have described how the elongated setae on the abdominal sense organs of dl)helocheirus become com- pressed under increased pressure, stimulating shorter sensory hairs which are interspersed among them, and thus warning the insect against diving to dangerous depths.The elongated lea-like setae on the six pairs o abdominal sense organs of Cryt)hocricos probably have a similar function, and the shorter lea(-like setae on the rest of each paratergite may have evolved frorn these longer ones. Fhe efficiency of the plastron in Cryphocr!cos cannot be deter- mined without experiments similar to those which Thorpe and Crisp (I947a, I947b) performed on dbhelocheirus.Nearly all o the ventral surface of Cryphocricos, and at least a part o the exposed dorsal surface, bear apparent hydrouge devices.I these device3 do, indeed, retain a thin, permanent, plastral air layer, a considerable suriace area of air is exposed to the water and is capable of gas ex- change with the latter.Dissolved oxygen could pass directly to the mesothoracic, metathoracic, and second through eighth abdominal spiracles, all of which face portions o the ventral plastron.Although the dorsal subalar space is not exposed to the water, it communicates with the ventral air layer by means o the narrow gap between the ventrally exposed portion o the forewing (Fig. , FW) and the ventrolateral edge of the body.The hydrofuge hairs lining this gap would permit a small amount ot oxygen to pass rom the ventral plastroa to the first abdominal spiracle, which iaces the subalar space (Parsons 974).The extent to which the spiracles, especially the first abdominal ones, are capable o.f inhalation is not known, however, and will require experimentation and further fine-structural studies.
l'here is a strong possibility that well-developed p!astral respira- tion has evolved in other Naucoridae, as Hinton (969a) claims.
Further investigation will probably reveal it not only in other mem- bers o: this family but quite possibly in other families o( aquatic Heteroptera as well.

ACKNOWLEDGMENTS
Acknowledgment is made for the use of the scanning electron microscope in the Royal Ontario Museum, established through a grant from the National Research Council of Canada to the Department of Zoology, University of Toronto.We wish to thank John T.
Polhemus, Randall T. Schuh, Fritz Plaumann, and D. T. Crisp, who provided specimens and/or data on habitats.We are also grate- ful to Eric Lin and Kian Chua for their technical help, to W. H. Thorpe and Thomas S. Parsons for their advice and encouragement, and to Donald A. Chant and the Department of Zoology, University of Toronto, who made available the laboratory facilities for this study.The investigation was made possible by a grant in aid of research to the senior author from the National Research Council of Canada.

SUMMARY
The scanning electron microscope shows that in Cryphocricos barozzii and C. hungerfordi most of the ventral surface of the body and at least a part of the dorsal surface are covered with fine struc- tures which appear to be hydrofuge devices.Both surfaces bear short, fine hairs which are very similar to those of Aphelocheirus aestivalis, the only aquatic heteropteran in which plastral respiration has thus far been demonstrated.These hairs are much smaller and more densely-packed than those of Pelocoris femoratus, which carries a large air bubble and relies upon atmospheric oxygen.On the ventral abdominal paratergites of Cryphocricos the plastral hairs are covered by curious, leaf-like, ridged setae which overlap each other and per- haps retain a "macroplastron".Very elongated leaf-like setae occur on the six pairs of abdominal pressure receptors of Cryphocricos.
Aphelocheirus possesses leaf-like s.etaeonly on its single pairs of ab- dominal sense organs.The fourth through sixth abdominal sternites of Cryphocricos bear groups of microtrichia with a shingle-like ar- rangement; they resemble the plastral devices of some aquatic Coleoptera.These fine structural ob.servations strongly suggest that Cry- phocricos has plastral respiration and utilizes dissolved rather than atmospheric oxygen.

Fig. 3 .
Fig. 3. Cryphocricos barozzii; surface view of apical parts of plastral hairs on metathoraeie episternum.More basal parts of hairs, with nodules, visible at lower left.Clumping of tips is probably an artifact.Left pos-

Fig. 4 .
Fig.4.C. barozzii; clump of plastral hairs on fifth ventral abdominal paratergite.Leaf-like setae, which eoneealecl these hairs, were scraped off during preparation o.f 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 #m.

Parsons
somewhat irregular rows, each row overlapping the one posterior to it (Figs.6

Fig. 9 .
Fig. 9. C. barozzii; 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, bottom

Fig. 11 .
Fig. 11. C. barozzii; shingle-like arrangement of several groups of microtrichia on median portion of fourth abdominal sternite, approximately midway between its anterior (towards .top)and posterior (towards bottom) boundaries.Arrow indicates where tips of hairs, mostly concealed by debris, appear to be interspersed among groups of microtrichia.Scale line