&p.1:Abstract Morphology and distribution of the proboscissensilla in Vanessa carduihave been investigated in or-der to contribute to the understanding of flower-probingbehaviour in butterflies. The proboscis has a bend regionapproximately one-third of the length from the base. Ashort tip region is characterized by rows of intake slitsleading into the food canal. Along the dorsal, lateral andventral sides of the proboscis, sensilla trichodea, sensillabasiconica and sensilla styloconica are distributed invarying patterns depending on their distance from theb ase. The medial food canal bears one longitudinal rowof sensilla basiconica only. The bristle-shaped sensillatrichodea are longer in the proximal region of the pro-boscis and become gradually shorter towards the tip.They are most frequent in number near to the bend re-gion and near the beginning of the tip region. Sensillabasiconica arranged in longitudinal rows increase innumber the more distal they are on the proboscis. The tipregion is characterized by rows of sensilla styloconica onthe dorsal side whereas the sensilla trichodea are mostlyrestricted to the ventral side. The ultrastructure suggeststhat the aporous sensilla trichodea function as mechano-sensilla while the uniporous sensilla basiconica act ascontact chemosensilla. The sensilla styloconica are re-garded as bimodal contact chemo/mechanosensilla sincetheir sensory cones are equipped with a single terminalpore and a tubular body at the base. The mouthpart sen-silla appear to provide tactile cues on the positioning ofthe proboscis and on the degree of its insertion into a flo-ral tube. Furthermore, they receive chemical stimuli onthe availability of nectar and on the immersion status ofthe food canal.&bdy:

A. Introduction

Precise and rapid action of mouthparts is crucial to effi-cient foraging in flower-visiting insects. Apart from thedimensions of the feeding apparatus, the importance ofits sensory organs in localizing concealed floral rewardsmust be paramount. Although the general morphology ofmouthparts in Lepidoptera is well known (e.g. Easthamand Eassa 1955), the sensillary organs of the proboscishave not yet received much attention. The proboscis con-sists of the two greatly elongated galeae which originateon basal maxillary structures. The galeae interlock ontheir dorsal and ventral sides after eclosion (Krenn 1997)and thus form a functional unit; their concave medianwalls represent the central food canal. For nectar uptake,the uncoiled proboscis usually assumes a characteristicposture; a proximal region is held horizontally, a distalregion vertically and in between lies a distinct bend re-gion (“knee bend”; Eastham and Eassa 1955). The distalregion passes into a flexible tip region functionally char-acterized by intake slits (Paulus and Krenn 1996). Judg-ing by the external features of insect sensilla (cf. Schenk1903; Snodgrass 1926), the sensilla of the lepidopteranproboscis are usually classified into sensilla trichodea(sensilla chaetica), sensilla basiconica and sensilla styl-conica, the latter showing an amazing variety of sizesand shapes (Städler et al. 1974; Sellier 1975; Faucheux1991; Büttiker et al. 1996; Paulus and Krenn 1996). Sofar, the ultrastructure of the proboscis sensilla has onlybeen investigated in one species of Arctiidae where sen-silla styloconica occur in two subtypes, both equippedwith chemoreceptors and mechanoreceptors (Altner andAltner 1986). Since that particular species is known totake up pyrrolizidine alkaloids, it has been suggested thatthe proboscis sensilla might be involved in this special-ized feeding habit. It remains unclear whether these ul-trastructural results can also be applied to “common”nectar-feeding Lepidoptera such as Vanessa cardui, sinceno Papilionoidea has ever been investigated by transmis-sion electron microscopy. The aim of the present study isto describe the morphology of the various proboscis sen-

H.W. KrennInstitut für Zoologie, Universität Wien, Althanstrasse 14,A-1090 Vienna, AustriaFax: +43 1 31 336 778; e-mail: hwkrenn@zoo.univie.ac.at

Zoomorphology (1998) 118:23–30 © Springer-Verlag 1998

O R I G I N A L A RT I C L E

&roles:Harald W. Krenn

Proboscis sensilla in Vanessa cardui (Nymphalidae, Lepidoptera):functional morphology and significance in flower-probing

&misc:Accepted: 12 September 1997

silla in a representative of the Nymphalidae at light andelectron microscopic levels. The ultrastructural anatomyof the sensilla and their distribution patterns on the pro-boscis prompt discussions on their biological role in theflower-probing behaviour of Lepidoptera.

B. Materials and methods

Vanessa (Cynthia) cardui(Linné, 1758) was chosen for this inves-tigation because this species is an unspecialized nectar-feedingrepresentative of the Papilionoidea. Adult males and females wereobtained from laboratory cultures.

I. Light microscopy

For preparation of microscope slides, the proboscides of dried but-terflies were soaked in dilute lactic acid at 40–50° C for 1–2 days.After this procedure, the galeae were soft, loosely coiled, and flex-ible enough to separate them from one another. They were washedin aqua destillata, followed by 30% ethanol and were then embed-ded into polyvinyl lactophenol without dehydration. After addinga cover glass, the slides were dried at 50° C.

Semithin sections were prepared by fixing the heads in Du-boscq-Brasil solution (cf. Romeis 1989). The proboscides weredehydrated in ethanol and embedded in ERL-4206 epoxy resin un-der vacuum impregnation. Serial semithin sections (0.25–1 µm)were made using diamond knives. The sections were stained in amixture of 1% azure II and 1% methylene blue in an aqueous 1%borax solution at 80° C for approximately 1 min.

II. Scanning electron microscopy

Specimens were fixed in 70% ethanol or Duboscq-Brasil solution.They were dehydrated in ethanol and submerged into hexa-methyldisilazane prior to air drying (Bock 1987). They were thenmounted onto SEM viewing stubs using a graphite adhesive tape.The samples were sputter coated with gold and viewed in a JeolJSM-35 CF scanning electron microscope.

III. Transmission electron microscopy

After dissection, specimens were initially fixed in Karnovsky solu-tion at 4° C for 2–3 h and subsequently in 1% osmium tetroxide ina sodium cacodylate buffer pH 7.4 at 4° C for 2 h. Dehydrationand embedding were performed as described above. Ultrathin sec-tions were stained with uranyl acetate (50° C, 50 min) and lead ci-trate (20° C, 3–10 min) in an LKB 2168 Ultrastainer and exam-ined with a Zeiss EM 902.

IV. Measurements

Proboscis and tip lengths were measured in 25 individuals of ei-ther sex. Sensilla measurements and counting were done in 10 in-dividuals. All data were collected using a microscope with a draw-ing attachment. Plotted lengths were measured by means of a digi-tizing tablet and the Sigma Scan Scientific Measurement System3.9 (Jandel Scientific) installed on a personal computer. The num-ber of sensilla of the various types were counted in six areas locat-ed at 10, 30, 50, 70 and 90% of the total galeal length and in thetip region. To compensate for the gradually decreasing proboscisdiameter towards the tip, the examined areas had to be standard-ized to equal surface.

C. Results

I. Proboscis regions and distribution of sensilla

The components of the mouthparts in V. carduiare illus-trated in Fig. 1. The proximal end of the galea is hingedonto the stipes to form the basal proboscis joint. Elongat-ed bristle-shaped scales protrude from the pilifers (laterallobes of the labrum) and touch the dorsal galeal surfacenearby (Fig. 2). Serial sections through the labrum indi-cate that these bristles are innervated and can be regard-ed as mechanosensilla. No sensilla are discnerible on themaxillary palpus.

The galea of V. cardui is 11.5–15.5 mm long (mean13.32 mm, SD 0.79). When uncoiled, the distance fromthe base to the bend region comprises 37% of the totalproboscis length. The region distal from the bend com-prises about 59%; the bend region itself makes up 4% ofthe total length. The tip region, where fluid can actuallybe taken up into the food canal, encompasses the apical0.7–0.9 mm (mean 0.81 mm, SD 0.08) which corre-sponds to approximately 6% of the total proboscislength. Throughout the tip region, the dorsal galeal link-ing plates are not entirely interlocked. The single platesare elongated and twisted distally. Only their ends are in-terlocked with those of the opposite galea. Due to thismodified shape, a row of 50–60 intake slits into the foodcanal is produced between the linking structures on thedorsal side of each galea (Fig. 3). The apical end of theproboscis is completely closed off by anteriorly project-ing linking plates.

Various types of sensilla occur throughout the entireproboscis. Three morphological types can be found onthe exterior of the galea (Table 1, Figs. 3–8). Sensillatrichodea (Fig. 6) possess a bristle which greatly variesin length (8–48 µm; Table 1). Bristles are longest on theventral side of the proximal region (mean length 37.6µm, SD 7.7; Fig. 4), become gradually shorter distallyand are shortest in the tip region (mean length 10 µm, SD1.1), where they are restricted to the ventrolateral side(Fig. 3). With a mean total number of 224 per galea(range 187–266), sensilla trichodea are the most numer-ous type of sensilla on the proboscis (Table 1).

Sensilla basiconica of the exterior surface (classifiedtype 1) have a short blunt-tipped sensory cone whichbears a single terminal pore (Fig. 7). The sensory cone isabout 5 µm long and protrudes from a circular and some-times slightly elevated area of the wall. These sensillastand in longitudinal rows and are distributed throughoutthe dorsal and lateral sides of the galea. They average60.6 (range 49–73) sensilla basiconica per galea (Table1).

The most remarkable sensillum type of the proboscisis the sensillum styloconicum. They have a smooth,bulging stylus bearing a conical sensory cone (Fig. 8).The stylus is elliptical in cross-section and originatesfrom an area of thin cuticle surrounded by a circular cu-ticular elevation. Apically, the stylus bears six to eightsolid cuticular spines which surround the sensory cone

24

(Figs. 8, 15). The total lengths of the sensilla range from45 to 57 µm (mean 53.5 µm, SD 6.23); the length of thesensory cone is always about 8 µm. The styli of the mostproximal sensilla are shorter, but their sensory coneshave the same length as the other sensilla styloconica(Fig. 5). There are 26–33 sensilla styloconica (meannumber 28) arranged in two rows which lie close to eachother along the dorsal tip region (Table 1). The rows ofsensilla styloconica merge with rows of sensilla basicon-ica, the latter alternating with sensilla trichodea at thetransition of the tip region into the distal galea (Fig. 5).

Proboscis sensilla are arranged in characteristic pat-terns which gradually change from the proximal regionto the tip (Figs. 11, 12). Proximal to the bend region,sensilla trichodea are the most frequent sensilla occur-

ring on the dorsal, lateral and ventral sides of the galea(Fig. 4), whereas sensilla basiconica are scarce and ar-ranged in one longitudinal row running dorsolaterally.After the bend region, the number of sensilla basiconicaincreases to form a second, more lateral row. The com-parison of sensilla densities throughout the galea revealsthat sensilla trichodea have the highest densities at 30%of proboscis length (near the bend region) and in the dis-tal region near the beginning of the tip (Fig. 11). Thenumber of sensilla basiconica (type 1) rises steadily frombase to tip. The tip region is characterized by sensillastyloconica and an abundance of sensilla basiconicawhereas sensilla trichodea are getting scarcer towards thedistal end of the galeae (Figs. 11, 12).

Only one type of sensilla occurs on the food canalwalls (Figs. 9, 10). These sensilla basiconica (classifiedas type 2) show an elongated sensory cone with a lengthof 11–15 µm (mean 13 µm, SD 1.26) protruding into thefood canal (Fig. 13). They are arranged in one longitudi-nal row and number 25–31 per galea (mean 28.6; Table1). They are more numerous at the galeal base and in thetip region than in the middle of the galea.

II. Ultrastructure of proboscis sensilla

The TEM examination indicates that the four externallydistinguishable sensilla types only contain two types of

25

Fig. 1 Head and mouthparts of Vanessa cardui(Nymphalidae) inoblique frontal view, proboscis (pr) in recoiled resting position, coComplex eye, cl clypeus, lp labial palpus, lr labrum, s stipes&/fig.c:

Fig. 2 Basal proboscis joint (bgi). Each galea (ga) forms a jointwith the stipes (s); bristles of pilifer (pi) touch the dorsal side ofthe galea. mpMaxillary palpus&/fig.c:

Fig. 3 Tip region of proboscis. Dorsal galeal linking structures(dgl) are modified, leaving intake slits between them leading intothe food canal. Two rows of sensilla styloconica (ss) protrudeclose to each other on the dorsal side of the galea. Sensilla tricho-dea (st) mainly on the ventral side. sb1 Sensillum basiconicum(type 1)&/fig.c:

26

Fig. 4 Lateral view of proximal galea. The bristle-shaped sensillatrichodea (st) are the most frequent type of sensilla. sb1Sensillumbasiconicum (type 1)&/fig.c:

Fig. 5 All types of sensilla are found at the transition of distal re-gion into tip region. The styli of sensilla styloconica (ss) becomelonger towards the tip. dgl Dorsal galeal linking structures, sb1sensilla basiconica (type 1), st sensilla trichodea&/fig.c:

Fig. 6 Bristle-shaped sensillum trichodeum (st) on the ventralside of the galea&/fig.c:

Fig. 7 Blunt-tipped sensillum basiconicum (type 1) (sb1) bears aterminal pore (arrow) &/fig.c:

Fig. 8 Sensilla styloconica are composed of a large smooth stylus(sy) bearing apical cuticular spines which surround the sensorycone (sc). sb1Sensillum basiconicum (type 1)&/fig.c:

Fig. 9 Median side of the galea in the tip region. Food canal bearssensilla basiconica (type 2) (sb2). dgl Dorsal galeal linkingstructures, sssensilla styloconica, vgl ventral galeal linking struc-tures&/fig.c:

Fig. 10 Sensory cone of sensillum basiconicum (type 2) (sb2) iselongated. These sensilla are found only in the food canal&/fig.c:

receptor. The bristles of sensilla trichodea are porelessand consist of solid cuticle. A dendrite is fixed to thebase of each bristle. A large sensillar sinus below thejoint membrane is formed by tormogen cells. It is linedby microlamellae and microvilli of the inner wall of theouter sheath cell. Inner sheath cells enclose the dendrite.Based on the morphological evidence, these bristle-shaped sensilla can be regarded as mechanoreceptors.

Both types of sensilla basiconica (type 1 of the outergaleal wall and type 2 of the food canal) share a hollowsensory cone that bears one terminal pore. Two dendriteseach extend from the terminal pore to their sensory cellspositioned centrally within the galeal lumen. About 5 µmbelow the base of the sensory cone, a ciliary segment isdiscernible. No tubular body could be detected in any ofthese sensilla. The usual set of enveloping cells is pres-ent, forming an inner and an outer sensillar sinus as wellas a dendritic sheath. Therefore, they are regarded as uni-porous contact chemosensilla.

Sensilla styloconica house four sensory cells whoseperikarya are enveloped in basal sheath cells. The senso-

ry cells are assembled in a large protrusion of the epider-mis which projects into the centre of the galea. Theirdendrites extend into the stylus and the sensory cone(Fig. 13). Sheath cells form a small sensillar sinus belowthe stylus which is continuous with a sinus within thestylus. The stylus contains six to eight cells which dense-ly line the central sensillar sinus with microvilli and mi-crolamellae. The cells open up a cavity in the apical re-gion, below the sensory cone (Fig. 15). They have a bignucleus containing large electron-dense bodies. Rich inendoplasmatic reticulum, they contain many mitochon-dria and closely resemble the tormogen cells. The ciliary

27

Fig. 11 Numbers of the various sensilla in areas standardized toequal surface throughout the exterior proboscis of V. cardui(Nym-phalidae). Sensilla trichodea are more frequent near the bend re-gion and near the beginning of the tip region than in the tip. Num-ber of sensilla basiconica increases towards the tip; sensilla stylo-conica confined to tip region.&/fig.c:

Fig. 12a–d Distribution patterns of various proboscis sensillawhich are assumed to be responsible for food localization andcontrol of movements during flower probing (indicated by arrows)in V. cardui(Nymphalidae). a Bristle-shaped sensilla of pilifer (pi)detect flexion status of the entire proboscis at the basal proboscisjoint in relation to the head. b Proximal proboscis mainly bearsbristle-shaped sensilla trichodea (st) and only a few sensilla bas-iconica (type 1). c Distal proboscis bears short sensilla trichodeaand an increasing number of sensilla basiconica (type 1) (sb). dTip region characterized by sensilla styloconica (ss), sensilla bas-iconica (type 1) and few short sensilla trichodea&/fig.c:

Table 1 Numbers, arrangement and characters of the proboscis sensilla in Vanessa cardui(Nymphalidae) (N=10) (Ssensilla)&/tbl.c:&tbl.b:

Sensilla classified Range of Mean Mean length Location Charactersaccording to external numbers numbers (±SD; µm)morphology per galea (±SD)

S. trichodea 187–266 224(±30) 23.8(±14.86) Outside of galea, except Bristle without poresdorsal tip

S. basiconica 1 49–73 60.6(±8.9) <5 One or two irregular rows Sensory cone with one terminalon dorsolateral side pore

S. basiconica 2 24–31 28.6(±2.4) 13(±1.26) One row inside foodcanal Sensory cone with one terminalpore

S. styloconica 23–31 28(±3) 53.5(±6.23) Two rows on dorsal side Sensory cone with one terminalof tip pore and tubular body

&/tbl.b:

segments of the dendrites are found in the distal half ofthe stylus. The outer dendritic segment enters eccentri-cally into the hollow sensory cone (Fig. 13). One of thedendrites contains a tubular body at the base of the sen-sory cone (Fig. 16). It is attached to the wall and termi-

nates in the proximal region of the cone. The other threedendrites reach the tip without branching or forming la-mellae (Fig. 17). They terminate below a single terminalpore which has a diameter of approximately 0.2 µm (Fig.14). The wall of the sensory cone contains numerouselectron-dense filaments which extend longitudinallyfrom the tip to the joint membrane of the sensory cone(Fig. 17).

D. Discussion

In V. cardui, distribution patterns of the various sensillatypes on the proboscis gradually change from its proxi-mal region to the tip. Similar to other Rhopalocera, thebristle-shaped sensilla trichodea are the most frequenttype in the proximal region of the proboscis while sen-silla styloconica are strictly confined to the area wherefluid can be sucked in, i.e. the tip region (Sellier 1975;Paulus and Krenn 1996).

The general sensory function of many insect sensillacan be deduced from their ultrastructural features (for re-views see McIver 1975; Altner 1977; Zacharuk 1980).There is no doubt that the sensilla trichodea on the galeaof V. cardui can be considered to be mechanoreceptive

28

Fig. 13 Cross-section through tip region of proboscis in V. cardui(Nymphalidae) (composed of three photos from identical sectionseries). Perikarya (pe) of sensory cells of sensillum styloconicum(ss) localized near galeal nerve (ne) in lumen of galea. Dendrites(de) extend through stylus up to the tip of the sensory cone. Sen-sillum basiconicum (type 2) (sb2) projecting into the food canal(fc) houses two sensory cells; sensillum trichodeum (st) on ventralside. scSensory cone, systylus&/fig.c:

Fig. 14 Longitudinal section through sensory cone of a sensillumstyloconicum shows terminal pore&/fig.c:

Fig. 15 Apical region of sensillum styloconicum. Stylus (sy) con-tains several cells characterized by large nuclei (n). A sensory si-nus (si) is formed below the sensory cone (sc). Surrounded bydendritic sheath, the dendrites (de) extend into the sensory cone&/fig.c:

Fig. 16 One of the dendrites (de) forms a tubular body (tb) whichis attached to the base of the sensory cone (B) &/fig.c:

Fig. 17 Oblique section through the distal half of the sensorycone of a sensillum styloconicum. Three dendrites (de) extend tothe tip of the sensory cone. Arrows indicate electron-dense struc-tures in the cuticle&/fig.c:

bristles because a single dendrite is attached to the baseof the poreless bristle. Sensilla basiconica occur both onthe external galea and protruding into the food canal,with differences only in the lengths of their sensorycones. Both subtypes possess a single terminal porewhich suggests that both contain contact chemorecep-tors. In contrast to V. cardui, multiporous sensilla bas-iconica have been detected in Pyralidae, suggesting anolfactory function for sensilla basiconica (Faucheux1991). The sensilla styloconica are sensitive to varioussubstances, primarily to a variety of sugars (Frings andFrings 1956; Salama et al. 1984; Blaney and Simmonds1988). The only ultrastructural investigation of sensillastyloconica on the proboscis revealed two subtypes, onewith a uniporous sensory cone and another with wallpores in addition to the terminal pore. The multiporoussensilla styloconica in Rhodogastria bubo(Walker,1855) (Arctiidae) have been assumed to be involved inthe detection of pyrrolizidine alkaloids to meet the de-mands of this specialized feeding behaviour (Altner andAltner 1986). In V. cardui, all investigated sensilla stylo-conica showed only one terminal pore, so the secondtype of sensilla styloconica appears to be missing. Al-though in V. cardui, the stylus of a sensillum styloconi-cum is smooth in contrast to the ribbed ones in R. bubo,the ultrastructural anatomy of the uniporous subtype isidentical. Despite their different external morphology,the sensilla styloconica of V. carduiare likewise consid-ered to be bimodal chemo/mechanosensilla since theypossess a tubular body at the base of their uniporous sen-sory cone. As in the sensilla styloconica of the proboscis,chemosensilla on larval galea of Lepidoptera may con-tain an associated mechanoreceptor (Gaffal 1979; Albert1980; Baker et al. 1986). This equally holds true for thegustatory sensilla on the mouthparts of other flower-vis-iting insects, such as bees (Whitehead and Larsen 1976)or Diptera (for review see Morita and Shiraishi 1985).They are often associated with a mechanoreceptive unit,but do not have a stylus which indicates their simplermorphology compared with most adult Lepidoptera. Thestylus of the sensilla styloconica in V. cardui projectabout 50 µm beyond both sides of the proboscis andseem to provide a sensitive extension of the tip region.

Since there is no apical opening into the food canal,the intake slits of the tip region must be immersed intothe fluid prior to sucking. Their position on the dorsalside of the galea is the reason for the bent feeding pos-ture of the proboscis and for the flexing of the tip regionwhich can be primarily observed during fluid uptakefrom even surfaces (Krenn 1985, 1990). This feedingposture allows instant adjustment of the functional pro-boscis length to meet the changing demands during prob-ing. This enables butterflies to efficiently probe flowersof all shapes and sizes as well as to a take up extrafloralfluids. All Papilionoidea exhibit characteristic proboscismovements during flower probing (Krenn 1985, 1990;Krenn and Penz 1996; H.W. Krenn unpublished observa-tions in V. cardui). To-and-fro movements of the distalregion result in detecting the corolla tube entrance. Flex-

ion of the basal galeal joint pushes the proboscis deeperinto the corolla tube; subsequent extension of the jointwithdraws the proboscis. The bristle-shaped sensilla onthe labrum that touch the basal galeal joint might provideinformation on the position of the entire proboscis rela-tive to the head during up-and-down movements. Tactilestimuli perceived by mechanoreceptors of the tip sensillaare crucial in detecting the opening of corolla tubes. In-side the corolla, sensilla are believed to localize nectarvia chemical stimuli; the flexible tip wipes the inside of acorolla tube as has been observed in Sphingidae (Knoll1922) and also in Nymphalidae which were offered ex-perimentally opened flowers (H.W. Krenn unpublishedobservations). A gustatory function is implied for theuniporous chemoreceptors having the biological role offood detectors. The bristle-shaped sensilla trichodea inthe proximal region of the proboscis provide informationon the diameter of a corolla tube and on the insertiondepth of the proboscis. They become shorter towards thetip and can measure the dimensions of a tube withoutblocking the entrace for the proboscis. An additionalfunction of these tactile sensilla can be inferred fromSEM photos showing the posture of the recoiled probos-cis, since the coils are in close contact with each otheralong the entire length (Krenn 1990). Thereby, the bris-tles of the mechanosensilla touch the galeal surface ofthe following coil, possibly providing information on thecorrect resting posture of the proboscis. The uniporoussensilla of the food canal walls might provide informa-tion on flow rates via gustatory cues received from theimbibed fluid.

The present investigation on the sensory equipment inthe proboscis of the nectar-feeding V. cardui is intendedas a basis for comparison with other Nymphalidae thatfeature specialized feeding habits. Pollen-feeding speciesin Heliconiinae have been described as possessing spe-cialized tip regions (Gilbert 1972). However, preliminaryresults of a morphological comparison of pollen-feedingto non-pollen-feeding representatives indicate that spe-cialized features exist in the proximal region of the pro-boscis (Krenn and Penz 1996). Fruit-piercing species inNoctuidae possess highly derived sensilla which, in addi-tion to their sensory role, are modified to serve as tearinghooks (Bänziger 1970; Büttiker et al. 1996). An ongoingexamination of fruit-feeding representatives of the Nym-phalidae will provide evidence as to whether there areanalogous morphological modifications in correlationwith similar feeding habits in Rhopalocera.

&p.2:Acknowledgements I thank C. Exner and T. Gatschnegg fortechnical assistance, B.-A. Gereben-Krenn, H. Paulus and N.Szucsich for critically reading the manuscript, as well as T. Mi-cholitsch and B. Lorenz for linguistic help.

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