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Zoomorphology (2008) 127:37–47 DOI 10.1007/s00435-007-0049-x

ORIGINAL PAPER

Comparative morphological assessment and phylogenetic signiWcance of the wing base articulation in Psylloidea (Insecta, Hemiptera, Sternorrhyncha) David Ouvrard · Daniel Burckhardt · Adeline Soulier-Perkins · Thierry Bourgoin

Received: 1 October 2007 / Accepted: 7 October 2007 / Published online: 30 October 2007 © Springer-Verlag 2007

Abstract The wing articulation sclerites, as well as wing base environment, of phylogenetically distant Psylloidea taxa were examined by optical and electron microscopy in order to estimate the phylogenetic signiWcance of observed morphological patterns. The basiradial bridge is strongly developed and links the fused humeral plate, basisubcostale, basiradiale and second axillary sclerite to the fused veins R + M + Cu. The proximal median plate has a vertical orientation, which may have a role in moving the wing forward and backward. The weak sclerotization posteriad of the second axillary sclerite and anteriad to the third axillary sclerite facilitates the backward movement of the wing. The horizontal hinge (= basal hinge), the vertical hinge and the torsional hinge are the most important fold- and Xexionlines for the mobility of the wing, whereas humeral folds and the anterior axillary fold-line play a minor role. The basalare presents two horns or processes that are autapomorphic traits for the superfamily Psylloidea. The monophyly of Psylloidea is also supported by the absence of the subalare, of the median notal wing process and of the anterior arm of the third axillary sclerite (lacking articulation with second axillary sclerite). Major interspeciWc variations are observed in tegula, Wrst axillary sclerite and basalare shape and size. The second distal median plate is absent in

D. Ouvrard (&) · A. Soulier-Perkins · T. Bourgoin Département Systématique & Evolution, Muséum national d’Histoire naturelle, USM601 MNHN/UMR5202 CNRS, C.P. 50, (Bât. Entomologie) 45 rue BuVon, 75005 Paris, France e-mail: [email protected] D. Ouvrard · D. Burckhardt Naturhistorisches Museum, Augustinergasse 2, 4001 Basel, Switzerland

Homotoma Wcus (Homotomidae) and Glycaspis brimblecombei (Spondyliaspidinae), whereas it is present in Calophya schini (Calophyidae) and Psylla buxi (Psyllinae/ Arytaininae); the presence of this sclerite could be a synapomorphy linking Calophyidae and the “psyllid assemblage”. Keywords Jumping plant-lice · Psylloidea · Axillary sclerites · Phylogeny · Forewing

Introduction Jumping plant-lice or Psylloidea are small sap-sucking insects. Most of the some 3,500 described species are associated with dicotyledonous plants and have very narrow host plant-ranges (Hodkinson 1974; Hollis 2004; Burckhardt 2005). Often related psyllid species develop on related host plants. For this reason they may constitute an excellent model group for studying coevolutionary patterns and processes. However, the base for this kind of studies, i.e. a well corroborated phylogeny, is currently not available for psyllids. In particular, the phylogenetic relationships at family or subfamily-level are still poorly understood, as in all major Sternorrhyncha lineages (Gullan and Koztarab 1997). White and Hodkinson (1985) published a detailed psyllid phylogeny based on morphological characters, although not using parsimony-based methodology. Ouvrard (2002) investigated the basal psyllid lineages using both molecular and morphological characters. However, due to the small taxon sampling, the results were diYcult to interpret. With increasing taxon sampling the number of characters should increase as well for getting resolution. While sequencing genes is now a straightforward procedure, hypothesizing new homologous morphological

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structures implies that prior morphological descriptions of still unused features (for phylogenetic purpose) must be done carefully. In that sense, Yoshizawa and Saigusa (2001) renew the phylogenetic interest in wing base sclerites for comparative study of Paraneoptera, which includes groups with small-sized species as aphids or psyllids. They conclude for Sternorrhyncha that some modiWcations observed within the superfamilies Psylloidea and Aphidoidea are probably useful for phylogenetic purpose at the family or genus level. In their study at ordinal level, these variations are considered as synapomorphies inside Sternorrhyncha and are neglected. Here, we investigate from a morphological point of view the wing base articulation in order to select new characters and evaluate their phylogenetic signiWcance in inferring relationships at the family or subfamily level in psyllids. First, all the structures of the axillary apparatus are described precisely, including sclerites and fold- and Xexion-lines. Second, variation is estimated between homologous structures in evolutionary, widely divergent jumping plant-lice taxa. This particular area of the thorax, which links the wing to the body, has been very poorly studied in insects. The most important contributions treat aspects of functional morphology and Xight “mechanics” (Pringle 1957; Brodsky 1994) or evolution (Kukalová-Peck 1991). These studies put little emphasis on the description of the axillary apparatus and focus on the wing itself including folds and veins. Betts (1986) is an exception in associating, in Heteroptera, functionality with comparative morphology. The wing base has the particular advantage of being similarly circumscribed in all pterygotes by the notal and pleural sclerites on one side, and by the wing membrane on the other side. However, its relevancy in systematics is often limited by the reduced size of the axillary sclerites that compose this region. It explains why this area has been only used in systematic studies of large insects such as scarabaeid beetles (Browne et al. 1993; Browne and Scholtz 1995, 1998) or large Neuropterida (Hörnschemeyer 1998). Here, we describe the wing articulation in Psylloidea, based on these previous works and on the innovative work of Yoshizawa and Saigusa (2001), and assess its interspeciWc variability.

Zoomorphology (2008) 127:37–47

Taxonomic sampling Following psylloid material from the collections of the Museum of Natural History, London (BMNH) and the Muséum national d’Histoire naturelle, Paris (MNHN) was examined with SEM and/or optical microscopy: Calophyidae Apsylla cistellata (Buckton, 1896), India, C. Thakur (BMNH, dry). Calophya schini Tuthill, 1959, Mexico, Edo. de México, Chapingo, 28 x 2000, R. A. Zagoya (MNHN, 70% ethanol). Homotomidae Homotoma Wcus (Linné, 1758), Switzerland, Ticino, Cadro, 12 vi 1976, D. & L. Matile (MNHN, 70% ethanol). Psyllidae—Psyllinae Psylla buxi (Linné, 1758), France, Grignon (78), vi 1950, J. Balazuc (MNHN, 70% ethanol). Psyllidae—Spondyliaspidinae Glycaspis brimblecombei Moore, 1964, USA, California, Alameda, 15 ii 2000, D. Dahlsten (MNHN, 70% ethanol). Dissections and light microscopy Specimens were Wrst relaxed and cleared by boiling in saturated potassium hydroxide, rinsed, and then dissected in distilled water. Head and abdomen were Wrst removed, as well as major parts of the thorax. To manipulate the specimen during observation more easily, only parts of the notum and pleurae connected with the wing base articulation were kept. The cuticle was stained with chlorazol black according to the method of Carayon (1969) and examined in glycerin in an excavated slide in order to orientate objects easily. A dissecting microscope Leica MZ125 was used for the observations.

Materials and methods

Scanning electron microscopy

Terminology

Specimens were progressively dehydrated in a graded series of increasing concentrations of ethanol in water to a Wnal concentration of 95% and then air-dried. Specimens were mounted on standard SEM stubs using double-sided tape, sputter-coated with gold-palladium, and examined with a JEOL JSM-840 scanning electron microscope.

Terminology for wing axillary sclerites and articulation folds follows mainly Matsuda (1970), Brodsky (1994) and Yoshizawa and Saigusa (2001), and that of associated structures of the notum and the pleurae accords with Ouvrard et al. (2002).

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Results The general organization of Psylloidea is described below. Variations between species under study are summarized in Table 1, and state conditions are given for each species. Forewing articulation environment (Fig. 1a) Anteriad of the wing articulation area, the lateral edge of the notum does not reach down laterally as in the pronotum. However, a long and narrow process extends downwards and slightly backwards to the mesepisternum, with which it articulates underneath the wing base: this is the prealar bridge (prb). The notum articulates with the wing base via two extensions called the anterior notal wing process (anwp, lateral extension of the scutum) and the posterior notal wing process (pnwp), hardly visible on preparations. The anal vein lies in this groove when the wing is folded back. The median notal wing process described by Yoshizawa and Saigusa (2001) in Sternorrhyncha is, however, absent in Psylloidea. The X-shaped scutellum (scl2) is much smaller than the scutum (sc2) and bears a long thin axillary cord (axc2) that runs laterally to the wing base. This axillary cord is an important landmark to determine the posterior limits of both the mesonotum and the wing articulation. The posterior edge of the prealar bridge delimits anteriorly a small region, also delimited medially by the anterolateral edge of the scutum and posteriorly by the anterior notal wing process, which contains two tubercle-like sclerites: the parapterum (ppt) anteriorly and the tegula (tg) posteriorly. Forewing articulation sclerites As detailed earlier, the most anterior sclerite in this area is the tegula: it lies just backward of the prealar bridge. Shape and relative size compared to the close parapterum varies, from reduced to a very small scale in C. schini (Fig. 2a, b) and P. buxi (Fig. 2c) to a large and globulous in G. brimblecombei (Fig. 3a, b).

The Wrst axillary sclerite (1ax) shows a long anterior arm (a) in P. buxi (Fig. 2d), which articulates on the median side with the anterior notal wing process, and on the wing side with the basisubcostale (bsc). This arm is much more reduced in the other examined species: we observed in G. brimblecombei (Fig. 3a, b) and C. schini (Fig. 2a, b) that the Wrst axillary sclerite lacks the anterior arm present in P. buxi. The elongated body (b) of this Wrst axillary sclerite leans against the lateral edge of the scutum instead of articulating with a true median notal wing process, which is absent in Psylloidea. Finally, its distal rim articulates with the second axillary sclerite (2ax), which is the largest structure of this assemblage. The humeral plate (hp), usually diVerentiated at the base of the costal vein, backward or laterally to the tegula, is in psyllids completely fused with the basisubcostale. The basisubcostale is deWned by Brodsky (1994) as a sclerotization of the proximal extremity of the subcostal vein, also distinctly identiWed by its median longitudinal ridge as described by Yoshizawa and Saigusa (2001). The humeral plate shows an anterior bump in all observed species, except for G. brimblecombei (Fig. 3a, b), which displays an evenly rounded antero-lateral edge of the humeral plate. At the point of articulation between the anterior notal wing process, the anterior arm of the Wrst axillary sclerite and the basisubcostale, the latter presents a depression that can be interpreted as the cavity of a condyle articulation. The observed fusion between the basisubcostale and the humeral plate (originally found at the proximal end of the costal vein) is consistent with the fusion of costal and subcostal veins in psyllids. H. Wcus is the only examined species to present a groove between the humeral plate and the basisubcostale (Fig. 3c, d). The group humeral plate + basisubcostale is found anterior to the basiradiale (br). As its name indicates, this basiradiale is located proximally to the basal end of the radial vein. In psyllids, the proximal part of this vein is fused with the proximal parts of median and cubital veins to form vein R + M + Cu. Posteriorly, a particularly developed structure links this basiradiale to the anterior rim of the triangular distal median plate (dmp): this is the basiradial bridge (brb).

Table 1 Axillary characters and their states in the species observed by SEM in the present study Calophya schini

Glycaspis brimblecombei

Homotoma Wcus

Psylla buxi

Shape and size of the tegula compared to the pronotum

Scale-like, very small

Globular, large

Globular, small

Globular, small

Anterior arm of the Wrst axillary sclerite

Not present

Not present

Not present

Present

Anterior bump of the humeral plate

Present

Not present

Present

Present

Groove between humeral plate and basisubcostale

Not present

Not present

Present

Not present

Anterior lobe of the distal arm of the third axillary sclerite

Distant from the median distal plate

In contact with the median distal plate



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40 Fig. 1 a Scanning electron microscopy showing the thorax of Psylla buxi (Linné, 1758), dorsal view. b The dorsal limit of the mesopleuron of Psylla buxi (Linné, 1758), internal view (the two arrows point out the two processes of the basalare)

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Implicit posterior margins of the basisubcostale proximally, of the basiradiale and of the basiradial bridge more distally are fused with the anterolateral margin of the second axillary sclerite to form together with the humeral plate, the basisubcostale and the basiradiale a continuous compact ovoid structure. Anteriorly, this second axillary sclerite bears an antero-median projection, just caudally to the point of articulation. As the antero-lateral limit of the second axillary sclerite is hardly visible, the particular microsculptures of this sclerite allow recognising it. It is well contrasted from the smooth surface of the basiradial bridge (Fig. 2b). Caudally, the third axillary sclerite (3ax) is unusually weakly sclerotized in psyllids and therefore diYcult to identify in microscopic preparations; it contrasts strongly with the group of fused sclerotized sclerites anteriorly. It marks the posterior limit of the wing articulation. From the three arms described by Brodsky (1994), two are recognized here. The posterior arm (e) joins the posterior notal wing process (Fig. 2d). The distal arm is characterized by an anterior lobe (c) oriented toward the distal median plate, and a posterior lobe (d) oriented toward the anal vein (A). The anterior lobe of the distal arm is in contact with the median distal plate in G. brimblecombei (Fig. 3a, b), whereas it is distant in P. buxi (Fig. 2d) and C. schini (Fig. 2b). The proximal median plate (pmp) is a triangular sclerite diYcult to observe in psyllids due to extreme concavity of the area situated posterior to the second axillary. Usually, this plate is related to the third axillary sclerite posteriorly, to the lateral edge of the second axillary sclerite on the proximal side, and to the proximal edge of the median distal plate on the wing side. In psyllids, this median proximal plate has a vertical orientation, as shown in Fig. 2d where only one pointed angle of this sclerotized triangle (pmp) is visible in P. buxi. Furthermore, the weak sclerotization of the third axillary does not allow to situate clearly the posterior side of this triangular sclerite. Finally, the distal median plate is characterized by the division into a proximal part (dmp1) and a distal part (dmp2), all of it having also a triangular shape. The anterior edge of this triangle lies along the posterior edge of vein R + M + Cu; the median edge articulates with the lateral edge of the median proximal plate; and the distal edge runs along vein R + M + Cu anteriorly to the beginning of the anal vein posteriorly. Below the wing articulation, a single small sclerite is present, the basalare (bas), which bears dorsally two typical horns or processes (Fig. 1b), dorsally oriented, on which lie several axillary sclerites. The basalare lies on a particular structure of the pleuron described in Ouvrard et al. (2002), the anepisternal disk (adk), only visible internally.

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Forewing articulation fold and Xexion lines (Fig. 4) To be complete, the description of the wing articulation must also depict the diVerent folds present in the area that are important topological landmarks. The horizontal hinge (as deWned by Brodsky 1994) is a fold (concave) that can be interpreted as a hinge for horizontal movements of the wing. In psyllids, this hinge is clearly visible: the two strap hinges being the notum and the Wrst axillary sclerite. The second hinge (convex), called vertical hinge, allows the wing to move forward and backward, and is described by Brodsky (1994) as a meeting point of three structures. First, the neutral humeral fold that originates between the tegula and the humeral plate, from where it runs between the antero-lateral basiradial and the posteroproximal second axillary sclerite. Then, it joins the concave (inferior) fold of the third axillary sclerite and the convex (superior) jugal fold. A last hinge allows the wing to be twisted along its longitudinal axis, and runs along the distal edge of the triangle distal median plate: this is the torsional hinge.

Discussion Stabilisation of terminology and homology assumptions Parapterum vs. tegula. The parapterum and the tegula are improperly called anterior and posterior basalares by Crawford (1914) and Taylor (1918). The tegula has also often been confused with a putative posterior parapterum. In fact, Lefeuvre (1969) describes the tegula in Blattaria as an oval prominence covered by many trichoid sensillae. Referring to the deWnition given by Chapman (1969) for tegula, i.e. a plate belonging to the articulary membrane, close to the humeral plate, at the base of the costal vein, the most posterior sclerite is then the tegula, which is conWrmed by the presence of trichoid sensillae. Crampton (1914) erroneously synonymizes the terms tegula and parapterum as well as pterygode and épaulet, and calls “tegula” any more or less globular sclerite situated just anteriad the wing base of the mesothorax. Posterior notal wing process origin Furthermore, there is still doubt whether the posterior notal wing process belongs to the scutellum or to the scutum: the postero-lateral scutal suture (plss), or oblique suture sensu Brodskiy (1992), takes its rise from the postero-lateral edge of the scutum, anteriad of the posterior notal wing process, and isolates the postero-lateral scutal angle which bears the posterior wing process.

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䉳 Fig. 2 Scanning electron microscopy showing the forewing articulation of a, b Calophya schini Tuthill, 1959. Large (a) and close (b) dorsal view. c, d Psylla buxi (Linné, 1758). Large (c) and close (d) dorsal view

Basiradial bridge signiWcance Caudally to the basiradiale, we associate the basiradial bridge, characterized by the absence of any cuticular microsculpture, with the sclerotized bridge deWned by Brodsky (1994) linking together the subcostal and the radial veins. This is in contrast to the interpretation of this structure by Yoshizawa and Saigusa (2001) as being the distal fragment of the second axillary. Third axillary sclerite interpretation Yoshizawa and Saigusa (2001) consider the posterior lobe of the distal arm of the third axillary sclerite as an independent sclerite called basanale, but our observations show that it is at least partially fused with the third axillary sclerite, as stated by Brodsky (1994). The anterior arm of this third axillary has not been observed on our preparations. Also, no connection has been observed between this sclerite and the second axillary sclerite. Yoshizawa and Saigusa (2001) assume that this absence of connection is due to artefact during specimen preparation for observation, but it seems rather that no real articulation exists between the third and the second axillary sclerite in Psylloidea. The third axillary sclerite and its associated muscle are of particular importance in Neoptera because they allow the wing to fold in the resting position (Hörnschemeyer 2002). Yoshizawa and Saigusa (2001), based on Wootton (1979), describe additional fold- and Xexion-lines to those given in our Results. These are: (1) the convex axillary Xexion-line running between the Wrst and the second axillary sclerites and joining caudally the concave axillary fold-line, (2) the anterior axillary fold-line running between the basiradial bridge and the Wrst distal median plate, and (3) the distal axillary Xexion-line running between the distal median plates (1 and 2) and the vein R + M + Cu. The terminology of the axillary fold- and Xexion-lines used in Brodsky (1994) and Yoshizawa and Saigusa (2001) is compared in Table 2. The horizontal hinge is considered by Wootton (1992) as the principal wing hinge.

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a true process and not an independent sclerite is in accordance with Matsuda (1979) for whom this prealar bridge is deWnitely of prescutal nature. In Cicadidae, this bridge appears bilobed (Taylor 1918). C. schini (Fig. 2a, b) and P. buxi (Fig. 2d) share the Xattened, scale-like tegula and the subdivided distal median plate. The two characters may constitute synapomorphies linking the two groups (Calophyidae and psyllid assemblage sensu Burckhardt 2005). The tegulae are polyneuronal sensory organs that seem to have a minor role in insect Xight (Fischer and Ebert 1999; Frye and Gray 2005). As psyllid Xight is weak (Clark 1962), observed variations in size and shape are likely to occur (by a relaxation of selective constraints), but are almost impossible to code in a morphological character matrix, and a morphometric study may help. G. brimblecombei diVers from the other three species in the evenly rounded humeral plate (Fig. 3a, b), rather than angular (Figs. 2b, c, 3d) which may be autapomorphic. P. buxi is the only species with a long anterior arm on the Wrst axillary sclerite (Fig. 2c, d). According to Hörnschemeyer (2002) and Yoshizawa (2002), such a long anterior projection of the Wrst axillary sclerite is close to the wing-base ground-plan in insects. Despite this, within Psylloidea this appears to be a derived condition, the generalized condition of psyllids being short and compact. More taxa should be examined for testing these hypotheses. Note also that only three of the four angles described by Brodsky (1994) for this Wrst axillary can be (with diYculty) observed here, and that the overall shape of this sclerite is not triangular as in most Neoptera (Hörnschemeyer 2002; Wootton 1992) but rather rod-like adpressed between the notum and the second axillary sclerite. Variation in Wrst axillary sclerite may be phylogenetically signiWcant, and investigations must be carried out in that sense despite the small size of this structure. The reduction of the anterior arm, also called “narrow neck/head area” by Hörnschemeyer (2002) or “neck region” by Yoshizawa (2002), and observed in C. schini (Fig. 2b) and G. brimblecombei (Fig. 3b) is considered by these authors as a derived state. The length ratio between the anterior and the posterior basalare processes could be prospected for phylogenetic purpose, but SEM is deWnitely required. The second distal median plate shows also variations in size and degree of sclerotization, and may be absent in H. Wcus (Fig. 3c, d) and G. brimblecombei (Fig. 3a, b), whereas it is clearly present in P. buxi (Fig. 2d).

InterspeciWc variability Phylogenetic implications The present study shows that there is some variation in the wing base morphology between the examined psyllid species. In some species like Apsylla cistellata a suture separates the prealar bridge from the prescutum. However, it is unknown if this is a true suture or the trace of an internal ridge, even if the latter interpretation making this structure

The present study has led to the discovery of new autapomorphies for the superfamily Psylloidea (Table 3): absence of subalare; absence of the median notal wing process; absence of the anterior arm of 3ax and no articulation with 2ax; a weakly-sclerotized 3ax; two-horned basalare. The

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Fig. 3 Scanning electron microscopy showing the forewing articulation of a, b Glycaspis brimblecombei Moore, 1964. Large (a) and close (b) dorsal view. c, d of Homotoma Wcus (Linné, 1758), the arrow

points out the groove between humeral plate and basisubcostale. Large (c) and close (d) dorsal view

absence of an isolated basanale and jugum is discussed later. The subalare is deWned in Psocomorpha by Yoshizawa (2005) as a relatively large, oval sclerite placed just

dorsal to the epimeron, posterior of the pleural wing process, and of tergal or laterotergal origin for Matsuda (1963) or of pleural origin according to Ivanov and Kozlov (1987).

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Fig. 4 a, b Schematic drawing showing fold- and Xexion-lines in the forewing articulation of Psylla buxi (Linné, 1758), dorsal view (thick grey lines for fold-lines; thick hatched grey lines for Xexion-lines)

Its reduction, but not its loss, has also been observed in Hymenoptera (Hörnschemeyer 2002), and Owen (1977) stated that no muscles were inserted on this sclerite in dipteran Culicidae implying that it plays a minor role in the Xight of this group. The role of the third axillary sclerite in moving the wing up and backward has been described by Rheuben and Kammer (1987). These authors show the importance of the point of articulation between the anterior arm of the third axillary sclerite and the posterior margin of

the second axillary sclerite, which is close to the rotation point of the wing during this movement. The weak sclerotization of the third axillary sclerite, the absence of an anterior arm and articulation point between the second and third axillary sclerites in Psylloidea suggests that the up movement of the wing is accomplished by other structures and muscles than those invoked by Rheuben and Kammer (1987). According to Brodsky (1994), in Neoptera the major role in wing movement is played by the basalar

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Table 2 Comparison of terminology of axillary fold- and Xexionlines Brodsky (1994)

Yoshizawa and Saigusa (2001)

Concave (inferior) fold of the third axillary sclerite

Concave axillary fold-line

Convex (superior) jugal fold

Convex axillary fold-line

Horizontal hinge

Basal hinge

Humeral fold (distal branch)

–

Neutral humeral fold

Anterior axillary fold-line

Torsional hinge

Distal axillary Xexion-line (?)

Vertical hinge

–

–

Convex axillary Xexion-line

Table 3 Characters of the wing base and their states in Psylloidea compared to other Paraneoptera Other Paraneoptera

Psylloidea

Median notal wing process

Present

Absent

First axillary sclerite

Triangular

Rod-like

Humeral plate

DiVerentiated at base of costal vein

Fused with basisubcostale

Third axillary sclerite

Sclerotized

Weakly sclerotized

Three arms

Two arms

Proximal median plate

Horizontal

Vertical

Distal median plate

One plate

Divided into two plates

Two horns of the basalare

Not present

Present

sclerite, instead of the pleural wing process playing the role of fulcrum in the generalized wing apparatus. This is particularly true in Psylloidea, with the striking development of the two upward-oriented horns of this sclerite, associated to the anepisternal disk (Fig. 1b), in order to counterbalance the weakness of the incomplete pleural suture (Ouvrard et al. 2002). The terms basanale and jugum are usually used in morphological descriptions of Lepidoptera (Maggenti and Gardner 2005) and Diptera (Knight and LaVoon 1970); the homology made by Yoshizawa (2005) and Yoshizawa and Saigusa (2001) with structures in Psocoptera and Psylloidea, respectively, are questionable. In our observations, no such structures are present in the posterior area of the axillary region.

Conclusion Starting from the work of Yoshizawa and Saigusa (2001) on the phylogenetic signiWcance of the wing base structure at family or genus level in Sternorrhyncha, the wing articulation in psyllids was analysed with respect to comparative

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morphology. The observations with both light and electron microscopy of the phylogenetically distant species C. schini (Calophyidae), G. brimblecombei (Psyllidae, Spondyliaspidinae), H. Wcus (Homotomidae) and P. buxi (Psyllidae, Psyllinae) suggest that there is at least some phylogenetic information in this body region. In particular, there is one gain, in addition to some loss characters supporting the monophyly of Psylloidea, and there are two potential synapomorphies linking the Calophyidae with the “psyllid assemblage”. Despite the small sample size the present study demonstrates the crucial need of detailed comparative morphological studies for elucidating phylogenetic questions at high taxonomic levels. Acknowledgments We are grateful to Rebeca Álvarez Zagoya, David Hollis and Kathleen L. Chan for providing some of the specimens used in this investigation. We also thank Imre Foldi and Gérard Mascarell for providing help and advice in scanning electron microscopy. Gilbert Hodebert made the line drawing and Laurent Fauvre helped in preparing the illustrations. This work was supported in part by the European Community’s Sixth Framework Programme to D.O. (Marie Curie Outgoing International Fellowship).

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