﻿Description of immature stages of Gymnetron species (Coleoptera, Curculionidae, Curculioninae), with particular emphasis on the diagnostic morphological characters at the generic and specific levels

﻿Abstract The immature stages of the following five Palaearctic Gymnetron species are described for the first time: G.tibiellum Desbrochers des Loges, 1900, G.veronicae (Germar, 1821), G.rotundicolle Gyllenhal, 1838, G.melanarium (Germar, 1821), and G.villosulum Gyllenhal, 1838. These species belong to four different groups previously established according to a phylogenetic analysis: the first two belong to the G.veronicae group and the other three to groups respectively bearing their name (G.rotundicolle, G.melanarium, and G.villosulum groups). All these species exhibit several diagnostic characters distinguishing them from each other. Some characters that can be used to separate Gymnetron from other genera in the tribe are also suggested. Three highly significant characters for the larvae and three for the pupae were identified. For the larvae they are: (1) labial palpi with single palpomeres, (2) all spiracles unicameral, and (3) epipharynx with a single pair of mes or none at all. For the pupae they are: (1) the pronotum with prominent pronotal protuberances, (2) abdominal segment VIII with a conical abdominal protuberance dorsally, and (3) very short or even reduced urogomphi. The species studied here are compared with those Gymnetron species already known and with other genera in the tribe Mecinini. Keys to the larvae and pupae described here are provided. All the characters used for identification are illustrated by photographs or drawings.


Introduction
The genus Gymnetron Schoenherr, 1825 belongs to the tribe Mecinini (Curculionidae, Curculioninae) and includes some 35 Palaearctic species (Caldara 2008a;Alonso-Zarazaga et al. 2017) and 70 Afrotropical species (Caldara 2003). The adults of this tribe were recently subjected to morphological revision and phylogenetic analysis (Caldara 2003(Caldara , 2008a. Based on this analysis, nine Palaearctic species groups and 13 Afrotropical species groups were recognized. Within this tribe the genus Gymnetron seems more closely related to Rhinusa Stephens, 1829 than to other genera (Caldara 2001). Preliminary molecular studies appear to confirm this placement (Hernández-Vera et al. 2013;I. Toševski unpublished data).
Therefore, the aims of the present study are to describe larvae and pupae of five Gymnetron species in detail for the first time, to find characters that are diagnostic at the generic and specific levels, and finally, to compare the characters of the immature stages of this genus with other genera of the same tribe that might be phylogenetically informative.

Materials and methods
The material for this study, i.e., L3 larvae and pupae from each of the species studied was collected from their host plants together with the adult, and subsequently preserved in 2 ml screw-cap micro tubes (Sarstedt, Germany) filled with 96% ethanol at 4-6 °C. The insect taxa were identified by Roberto Caldara, those of the plants by Ivo Toševski.
Part of the larval and pupal material was preserved in glycol or Pampel fixation liquid (see Skuhrovec and Bogusch 2016) and used for the morphological descriptions. These specimens are now deposited in the Group Function of Invertebrate and Plant Biodiversity in Agro-ecosystems of the Crop Research Institute (Prague, Czech Republic). Slide preparation basically followed May (1994). The larvae selected for study under the microscope were cleared in 10% potassium hydroxide (KOH), then rinsed in distilled water and dissected. After clearing, the head, mouthparts and body (thoracic and abdominal segments) were separated and mounted on permanent microscope slides in Faure-Berlese fluid (50 g gum arabic and 45 g chloral hydrate dissolved in 80 g of distilled water and 60 cm 3 of glycerol) (Hille Ris Lambers 1950).
All the specimens described were fixed in 95% ethanol and examined under an optical stereomicroscope (Olympus SZ 60 and Nikon Eclipse 80i) with calibrated oculars. The following measurements of larval instars were made: body length (BL), body width (BW) (at the third abdominal segment) and width of the head capsule (HW) (see Gosik et al. 2016). The pupal measurements included body length (BL), body width (BW) (at the level of the mid legs), head width (HW) (at the level of the eyes), length of rostrum (RL) and width of pronotum (PW). All the measurements are given in Table 1 (mature larva) and Table 2 (pupa). The drawings and outlines were made using a drawing tube (MNR-1) installed on a stereomicroscope (Amplival) and processed by computer software (Corel Photo-Paint X7, Corel Draw X7). The thoracic spiracle was located on the prothorax near the boundary of the prothorax and mesothorax, as shown in the drawing, but this spiracle is of mesothoracic origin (Marvaldi et al. 2002;Marvaldi 2003). The drawings show the thoracic and abdominal spiracles. The lengths of all setae are visible in the figures. The numbers of setae of the bilateral structures are given for one side.
The sequence of the species follows that proposed by Caldara and Fogato (2013) and .

Morphology of immature stages
Genus Colouration. Pale yellow or dark brown head. All thoracic and abdominal segments white, cuticle smooth or with many reddish or brown asperities.
Vestiture. Setae on body thin, distinctly different in length (minute to very short or long).
Head capsule. Head almost oval or suboval, endocarinal line present. Frontal sutures on head distinct, extended to antennae. One stemma, in the form of a pigmented spot with convex cornea, both located on each side anterolaterally, above frontal suture. Dorsum of epicranium with three or five setae; des 1 located in central part of epicranium; des 2 lateral, sometimes absent; des 3 located anteriorly on epicranium close to frontal suture; des 4 often medially, sometimes absent; des 5 located anterolaterally. Frons with three to four fs, fs 1 absent, fs 2 located medially, fs 3 sometimes absent, fs 4 and fs 5 subequal. Head with two les, one or two ves, and two to six pes.
Antennae located at end of frontal suture on each side, membranous and distinctly convex basal article bearing one conical sensorium, relatively long.
Mouth parts. Labrum ~ 3-4× as wide as long, with three piliform lms, relatively long; anterior margin doubly sinuate. Epipharynx with two or three long digitate als; with two or three ams, and one or without mes; labral rods indistinct. Mandibles distinctly broad, bifid, teeth of unequal height; slightly truncate; both mds relatively long, piliform, located in distinct holes. Maxilla: stipes with one stps, two pfs and sensillum, with or without mbs; mala with four or five elongated digitate dms; three or four vms, of various length; all vms distinctly shorter than dms. Maxillary palpi with two palpomeres; basal palpomere with one mxps and one sensillum; distal palpomere with one sensillum and a group of conical, cuticular apical processes. Praelabium oval, with one prms; ligula with two ligs. Labial palpi with one palpomere; palpomere with one sensillum and short, cuticular apical processes. Postlabium with two or three pms, all located laterally; membranous area finely or distinctly asperate.
Thorax. Prothorax distinctly smaller than meso-and metathorax. Spiracle unicameral, situated between pro-and mesothorax (see Material and methods). Prothorax with seven to eleven prns; two ps; and two eus. Mesothorax with or without two prs; two or three pds; one long as; two or three ss; one eps; one ps; and one or two eus. Each pedal area of thoracic segments well separated, with three or five pda.
Abdomen. Abdominal segments I-III of almost equal length, next abdominal segments shortening gradually to the terminal parts of the body. Abdominal segment X reduced to four anal lobes of unequal size, the lateral lobes being distinctly the largest, and the dorsal and ventral ones very small. Anus located terminally; ambulatory ampullae bilobate to circular. Spiracles unicameral, seven abdominal spiracles located laterally. Abdominal segments I-VI with one or two prs; one or two pds; two ss; one eps; one or two ps; one lsts and one or two eus. Abdominal segments VII-VIII without, one or two prs; one or two pds; one or two ss; one eps; one or two ps; without or one lsts; and one or two eus. Abdominal segment IX with one or two ds; one or two ps; and one or two sts. Abdominal segment X with one or two setae (ts).
Body. Moderately stout, yellowish or brownish. Pronotal protuberances (p-pr) sclerotized, prominent, body covered with fine, knobby asperities; fused at base or well separated. Rostrum rather or moderately slender, ~ 4× as long as wide, extending to mesocoxae. Antennae rather short, clava smooth. Pronotum 1.5-2.2× as wide as long. Mesonotum slightly or sometimes distinctly smaller than metanotum. Abdominal segments I-V of equal length; segments VI-VIII tapering gradually to the terminal part of the body, segment IX distinctly reduced. Spiracles on abdominal segments I-V functional. Urogomphi reduced or short. Abdominal segment VIII with well visible conical abdominal protuberance dorsally (a-pr), extending the outline of the body.
Chaetotaxy. Sparse, setae of different lengths, transparent. Head with one or two os. Rostrum with or without one rs. Pronotum with one or two as, one or two ds, with two or without sls, one or three ls and three or four pls. Dorsal parts of meso-and metathorax with two or three setae. Apex of femora with one or two fes. Abdominal segments I-VIII with two or five setae dorsally. Each lateral part of abdominal segments I-VIII with one or two setae. Ventral parts of abdominal segments I-VIII with two or three setae. Abdominal segment IX with two setae ventrally.
Vestiture. Setae on body thin, yellowish, distinctly different in length (minute to very short or long).
Head capsule (Figs 1B,2A). Head suboval, endocarinal line present, extending for 2/3 of length of frons. Frontal sutures on head very broad and distinct. Stemma, in the form of a very small pigmented spot with convex cornea. Des 1 long, located in middle of central part of epicranium; des 2 medium; des 3 long, located anteriorly on epicranium close to border with frontal suture; des 4 short; des 5 long, located anterolaterally above stemma ( Fig. 2A). Fs 1 absent; fs 2 short, located medially; fs 3 short; fs 4 short, located anteriorly; and fs 5 long, located anterolaterally, close to antenna ( Fig. 2A). Les 1 and les 2 as long as des 5 ; one short ves. Epicranial area with six postepicranial setae.
Antennae membranous and distinctly convex basal membranous article bearing one relatively long conical sensorium and three sensilla of different types: two basiconical and one ampullaceum (Fig. 2B).
Thorax. Prothorax ( Fig. 3A) with 11 long and one short to minute prns, small pigmented dorsal sclerite present with five long and one short prns, this sclerite subdivided into two triangular plates medially; two long ps; and two short to very short eus. Mesothorax (Fig. 3A) without prs, two long and one short pds; one long as; two long and one very short to minute ss; one long eps; one long ps; and two short eus. Chaetotaxy of metathorax ( Fig. 3A) almost identical to that of mesothorax. Each pedal area of thoracic segments well separated, with three long and two short pda.
Abdomen. Spiracles on abdominal segments I-VI close to anterior margin, functional, spiracles on abdominal segment VII not functional. Abdominal segments I-VII (Fig. 3B, C) with two minute prs; two long pds; one long and one very short to minute ss; one short eps; one short ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segment VIII ( Fig. 3C) with two minute prs; two long pds; one very short to minute ss; one short eps; one short ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segment IX ( Fig. 3C) with two short ds; two short ps; and two very short sts. Abdominal segment X ( Fig. 3C) with two minute setae (ts).
Biological notes. The immature stages of G. tibiellum were collected from capsules of Veronica anagallis-aquatica L. Previously, nothing was known about the biology of this species. The adults are active from mid-April following the appearance of the host plants. Oviposition takes place from early June until mid-August. The presence of larvae inside the seed capsules is readily detected from the dark colour of the deposited frass. The biologies of G. tibiellum and G. veronicae are very similar but no competition between these two weevil species has been observed in over 500 dissected seeds capsules where they occur in syntopy.
Remarks and comparative notes. Gymnetron tibiellum is widely distributed in the south-eastern part of central Europe, Italy, the Balkans, Caucasus, Anatolia and the Middle East (Alonso-Zarazaga et al. 2017). The adults of this species are very closely related to G. veronicae, from which they differ by the shape of the rostra and the penis (Caldara 2008a). This close relationship was confirmed here by several characters which the immature stages have in common, although differences in several other characters of both larvae and pupae readily discriminate these two species. General. Body elongate, slender, weakly curved, rounded in cross section (Fig. 6A). Colouration. Head dark brown (Fig. 6B). All thoracic and abdominal segments white with numerous reddish or brown asperities (Fig. 6A).  Vestiture. Setae on body thin, orange, distinctly different in length (minute to very short or long).
Head capsule (Figs 6B,7A). Head suboval, flattened laterally, endocarinal line present, clearly extending to half the length of frons. Frontal sutures on head very broad and distinct. Stemma, in the form of a very small pigmented spot with convex cornea. Des 1 long, located in middle of the central part of epicranium; des 2 short, placed medially; des 3 relatively long, located anteriorly on epicranium close to border with frontal suture; des 4 short, placed above frontal suture; des 5 long, located anterolaterally (Fig. 7A). Fs 1 short; fs 2 absent; fs 3 located medially; fs 4 short, located anteriorly; and fs 5 long, located anterolaterally, close to antenna (Fig. 7A). Les 1 and les 2 as long as des 5 ; one ves minute. Epicranial area with four postepicranial setae.
Antennae membranous and distinctly convex basal membranous article bearing one relatively long conical sensorium and six sensilla different in length (four basiconica and two ampullacea) (Fig. 7B).
Thorax. Prothorax (Fig. 8A) with seven long and one short prns, small pigmented dorsal sclerite present with three long prns, this sclerite subdivided into two triangular plates medially; two long ps; and two short to very short eus. Mesothorax (Fig. 8A) with two very short to minute prs; one short and two long pds; one long as; one long and two very short to minute ss; one long eps; one long ps; and two short eus. Chaetotaxy of metathorax (Fig. 8A) almost identical to that of mesothorax. Each pedal area of thoracic segments well separated, with three long and two very short to minute pda.
Abdomen. Spiracles on abdominal segments I-VI close to the anterior margin and functional, spiracles on abdominal segment VII not functional, and abdominal segment VIII with atrophied spiracles. Abdominal segments I-VII (Fig. 8B, C) with two minute prs (segment VII with one prs); one long and one minute pds; one long and one very short to minute ss; one long eps; one relatively long ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segment VIII ( Fig. 8C) with one minute prs; one long pds; one very short to minute ss; one long eps; one relatively long ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segment IX (Fig. 8C) with one relatively long ds; one relatively long ps; and one short to very short sts. Abdominal segment X (Fig. 8C) with one very short seta (ts).
Chaetotaxy. Sparse, setae short to medium, transparent. Head with one medium os. Rostrum without setae (Fig. 10B). Pronotum with two elongate as, one ds, one sls, and three pls, all of almost equal length. Dorsal parts of meso-and metathorax with three setae of various length, situated medially. Apex of femora with two medium-sized fes ( Fig. 10A-C). Abdominal segments I-VIII with four medium to short setae placed in horizontal line medially. Each lateral part of abdominal segments I-VIII with two setae of various size. Ventral parts of abdominal segments I-VIII with three medium setae. Abdominal segment IX with two minute setae ventrally (Fig. 10A-C).
Biological notes. The larva was already known to feed on the ovary of Veronica beccabunga L, where it pupates and develops to the adult stage, and the adult was also collected on V. anagallis-aquatica L. and V. scutellata L. (Hoffmann 1958;Koch 1992;Sprick 1997). We can now confirm that at least V. anagallis-aquatica L. must be another host plant. The biology of this weevil species is the same as that of G. tibiellum.
Remarks and comparative notes. The adult of this species, widely distributed throughout Europe (Alonso-Zarazaga et al. 2017), is closely related to G. tibiellum, but with which it is sympatric only in south-eastern Europe. The two species differ mainly in the shapes of the rostra and the penis. Examination of the larvae confirms the relationship between them: they share the praedorsal segment on the abdominal segments with two pds, the epicranium with fs 3 and the labral setae in one line. However, the larva of G. veronicae differs from that of G. tibiellum by the cuticle of the body covered with numerous reddish or brown asperities and setae emerging from black spots, the dark brown not pale yellow head, and the epipharynx with two (not three) als and three (not two) ams. The pupae also have many characters in common (see the key), but clearly differ by the number of setae as, ls and sls on the pronotum, those on the meso-and metathorax, and on the dorsal parts of abdominal segments I-VII. General. Body relatively elongate, distinctly curved, rounded in cross section (Fig. 11A). Colouration. Head pale yellow (Fig. 11B). All thoracic and abdominal segments white, cuticle smooth (Fig. 11A).

Gymnetron rotundicolle
Vestiture. Setae on body thin, transparent, distinctly different in length (minute to very short or medium).
Head capsule (Figs 11B, 12A). Head suboval, endocarinal line present, shorter than half the length of frons. Frontal sutures on head of medium width, distinct. Stemma, in  form of distinct, black pigmented spot with convex cornea. Des 1 short, located in middle of central part of epicranium; des 2 short, located in middle of central part of epicranium; medium size des 3 located anteriorly on epicranium close to border with frontal suture; des 4 short, located between des 2 and des 3 ; des 5 of medium size, located anterolaterally (Fig.  12A). Fs 1 absent; fs 2 very short to minute, located medially; fs 3 absent; fs 4 medium, located anteriorly; and fs 5 relatively long, located anterolaterally, close to antenna (Fig. 12A). Les 1 and les 2 as long as des 5 ; two ves short. Epicranial area with four postepicranial setae (pes).
Antennae membranous and distinctly convex basal membranous article bearing one relatively long conical sensorium and three sensilla basiconica (Fig. 12B).
Mouth parts. Labrum (Fig. 12C) ~ 3× as wide as long, with three piliform lms, relatively long, of almost equal length; lms 1 located posteromedially, close to clypeus, lms 2 located anteromedially, and lms 3 located anterolaterally. Epipharynx (Fig. 12D) with three very long digitate als, almost identical in length, two piliform ams almost equal in length and one mes; labral rods indistinct, enlarged anteriorly. Mandibles (Fig. 12E) with two relatively long, piliform mds, located in distinct holes. Maxilla (Fig. 12F): stipes with one stps, two pfs, one mbs and sensillum, stps and pfs 1-2 long, mbs very short; mala with five relatively long, digitate dms; four vms, different in length, one setae very short, and three setae minute. Maxillary palpi with two palpomeres; length ratio of basal and distal palpomeres: 1:0.5. Praelabium (Fig. 12F) oval, with one relatively long prms; ligula with sinuate margin and two very short ligs and one sensillum; premental sclerite broad, readily visible at sides but almost invisible in middle. Postlabium (Fig. 12F) with three pms, very long pms 2 , and very short to short pms 1 and pms 3 , all located laterally; membranous area sparsely and finely asperate.
Abdomen. Spiracles on abdominal segments I-VI close to the anterior margin and functional, spiracles on abdominal segment VII not functional, and abdominal segment VIII with atrophied spiracles. Abdominal segments I-VI (Fig. 13B, C) with one very short to minute prs; one relatively long and one very short pds; one relatively long and one very short to minute ss; one relatively long eps; two very short ps; one very short lsts; and one very short to minute eus. Abdominal segments VII-VIII (Fig. 13C) without prs; with one relatively long pds; one very short to minute ss; one relatively long eps; two very short ps; one very short lsts; and one very short to minute eus. Abdominal segment IX (Fig. 13C) with one very short ds; one very short ps; and two very short sts. Abdominal segment X (Fig. 13C) with one very short to minute seta (ts).
Chaetotaxy. Sparse, setae short to medium, transparent. Head with one short os. Rostrum without setae (Fig. 14B). Pronotum with two as, one ds, and three pls equal in length. Dorsal parts of meso-and metathorax with three setae of different length, situated medially. Apex of femora with one medium fes (Fig. 15A-C). Abdominal segments I-VIII with two short, equally long setae dorsally: one situated medially, the other mediolaterally. All dorsal abdominal setae almost equal in length, short. Each lateral part of abdominal segments I-VIII with two setae of various length (one short, one minute). Ventral parts of abdominal segments I-VIII with three medium setae. Abdominal segment IX with two very short setae ventrally (Fig. 15A-C). Abbreviations: a-pr -abdominal protuberances, p-pr -pronotal protuberances, ur -urogomphi; setae: as -apical, d -dorsal, ds -discal, fes -femoral, l, ls -lateral, os -orbital, pls -posterolateral, v -ventral.
Biological notes. The adults of G. rotundicolle were previously recorded as collected on two species of Veronica: V. persica Poiret in Italy and Switzerland (Caldara 2008b;Germann et al. 2013), and V. chamaedrys L. in the Czech Republic and Slovakia (Krátký and Trnka 2012;Krátký 2013). The reports of Veronica hederifolia L. and V. opaca Fr. as host plants of this weevil are new data. The adults appear in early spring (mid-March), feeding on the upper leaves of newly growing shoots of the host. Oviposition takes place in the seed capsules, in which the larvae complete their development. The presence of larvae inside seed capsules can be detected from the dark colour of their frass.
Remarks and comparative notes. The first findings of this originally central Asian species in many countries of central and southern Europe (Italy, Switzerland, France, Germany, Czech Republic, Slovakia, Hungary, Poland) have been reported in many faunistic papers during the last 15 years (Strejček 2007;Caldara 2008b;Krátký and Trnka 2012;Krátký 2013;Germann et al. 2013;Reibnitz 2013;Podlussány et al. 2017;Wanat and Ruta 2018;Nolte and Haag 2019). These papers indicate with a high degree of certainty that this species only recently colonized areas where a few years ago it was absent, in contrast to its host plants (Caldara 2008b;Germann et al. 2013). General. Body elongate, slender, weakly curved, rounded in cross section (Fig. 16A). Colouration. Head pale yellow (Fig. 16B). All thoracic and abdominal segments smooth (Fig. 16A). Vestiture. Setae on body thin, yellow, distinctly different in length (minute to very short or long).
Antennae membranous and distinctly convex basal membranous article bearing one relatively long conical sensorium and four sensilla: three basiconica and single ampullaceum (Fig. 17B).
Thorax. Prothorax (Fig. 18A) with nine long and one minute prns; two long ps; and two very short eus. Mesothorax (Fig. 18A) with two minute prs; one medium and two long pds; one long as; two long and one minute ss; one long eps; one long ps; and one short eus. Chaetotaxy of metathorax (Fig. 18A) almost identical to that of mesothorax. Each pedal area of thoracic segments well separated, with one long, two medium and two very short to minute pda.
Chaetotaxy. Sparse, setae rather short to moderately elongate, transparent. Head with two os, different in length. Rostrum with one rs. Setae on head and rostrum straight, as long as those on prothorax (Fig. 20B). Pronotum with two as, two ls, two ds and four pls; ds 1-2 and ls 2 slightly shorter than other pronotal setae. Dorsal parts of meso-and metathorax with two setae placed medially. Apex of femora with two fes equal in length (Fig. 20A-C). Abdominal segments I-VIII with five short, equally long setae dorsally: first placed antero-medially, the others distributed in regular line along posterior margin of segment. All dorsal abdominal setae short, almost equal in length. Each lateral part of abdominal segments I-VIII with one elongated seta. Ventral parts of abdominal segments I-VIII with three medium setae. Abdominal segment IX with two very short setae ventrally (Fig. 20A-C).
Biological notes. Previously the larva of this species was observed on Veronica serpyllifolia L., on the stems where it produces a small uni-or bilocular gall in which metamorphosis takes place. The adult emerges from the gall at the end of summer and hibernates in the soil (Hustache 1931;Hoffmann 1958). The adult has also been collected on other Veronica species such as V. agrestis L., V. austriaca subsp. austriaca L., V. chamaedrys L., V. officinalis L, and V. teucrium (L.) D.A. Webb (Hoffmann 1958;Koch 1992;Sprick 1997). In Serbia, the development of G. melanarium is restricted to the seed capsules of Veronica austriaca subsp. jacquinii (Baumg.) Watzl, which is new information. Nearly 90% of the seed capsules are infested with one or two larvae. The larvae are seed feeders and development occurs in the basal part of the strongly flattened, glossy and glabrous seed capsules with no visible sign of larval presence. Oviposition takes place from mid-May onwards and the new generation of adults emerges during July.
Remarks and comparative notes. This species belongs to a group of very similar species characterized by slender subrectangular elytra, rostrum in lateral view tapered from the antennal insertion to the apex, and short protibiae in the female. There are no particular phylogenetic affinities with the adult (see Caldara 2008a) and pupal stages (abdominal protuberance short, triangular, head with 2 os) of the other species  Abbreviations: a-pr -abdominal protuberances, p-pr -pronotal protuberances, ur -urogomphi; setae: as -apical, d -dorsal, ds -discal, fes -femoral, l, ls -lateral, os -orbital, pls -posterolateral, rs -rostral, v -ventral. described here. By contrast, the larvae share several characters with G. rotundicolle, e.g., the praedorsal segment on abdominal segments with one pds, the epicranium lacking fs 3 , and the conical layout of the labral setae. General. Body elongate, slender, weakly curved, rounded in cross section (Fig. 21A). Colouration. Head dark brown (Fig. 21B). All thoracic and abdominal segments white with many reddish or brown asperities (Fig. 21A).

Gymnetron villosulum
Vestiture. Setae on body thin, orange, distinctly different in length (minute to very short or long).
Head capsule (Figs 21B,22A). Head suboval, flattened laterally, endocarinal line present, clearly extending to 1/3 of the length of frons. Frontal sutures on head very broad and distinct. Stemma, in the form of a very small pigmented spot with convex cornea. Des 1 short, located in middle of central part of epicranium; des 2 absent; relatively long des 3 located anteriorly on epicranium close to border with frontal suture; des 4 absent; des 5 long, located anterolaterally (Fig. 22A). Fs 1 absent; fs 2 relatively long, located medially; fs 3 absent; fs 4 relatively long, located anteriorly; and fs 5 long, located anterolaterally, close to antenna (Fig.  22A). Les 1 and les 2 as long as des 5 ; ves short. Epicranial area with two postepicranial setae.
Antennae membranous and distinctly convex basal membranous article bearing one relatively long conical sensorium and four sensilla basiconica (Fig. 22B).
Thorax. Prothorax (Fig. 23A) with six long and two very short to minute prns, small pigmented dorsal sclerite present with two long prns, this sclerite subdivided into two triangular plates medially; two long ps; and two short to very short eus. Mesothorax ( Fig. 23A) with two very short to minute prs, two long pds; one long as; one long and two very short to minute ss; one long eps; one long ps; and two short eus. Chaetotaxy of metathorax (Fig. 23A) almost identical to that of mesothorax. Each pedal area of thoracic segments well separated, with three long and one very short to minute pda.
Abdomen. Spiracles on abdominal segments I-VI close to the anterior margin and functional, spiracles on abdominal segment VII not functional, and abdominal segment VIII with atrophied spiracles. Abdominal segments I-VI (Fig. 23B, C) with one short and one minute prs; one long pds; one long and one very short to minute ss; one long eps; one relatively long ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segments VII-VIII (Fig. 23C) with one very short prs; one long pds; one long and one very short to minute ss; one long eps; one relatively long ps; one short lsts; and two very short and sometimes one additional minute eus. Abdominal segment IX (Fig. 23C) with one relatively long ds; two relatively long ps; and one short to very short sts. Abdominal segment X (Fig. 23C) with one very short seta (ts).
Chaetotaxy. Sparse, setae short to medium, transparent. Head with one medium os. Rostrum with one rs (Fig. 25A). Pronotum with one elongate as, one ls, and four pls all almost equal in length. Dorsal parts of meso-and metathorax with two setae of various length, placed medially. Apices of femora with one medium-sized fes (Fig. 25A-C). Abdominal segments I-VIII with two medium-sized setae (one placed medially, the other laterally). Each lateral part of abdominal segments I-VIII with one mediumsized seta. Ventral parts of abdominal segments I-VIII with two medium-sized setae. Abdominal segment IX with two minute setae ventrally (Fig. 25A-C).
Remarks and comparative notes. This species is common in the whole of Europe and Anatolia. The adult is closely related to G. miyoshii, a vicariant species living in eastern Asia (Caldara 2008a;Alonso-Zarazaga et al. 2017). The immature stages confirm this relationship, as they share the postdorsal segment on the abdominal segments with one pds and the dorsal epicranium without des 4 .

Key to the known mature larvae of Gymnetron species
The following key is based on the larvae of the five Gymnetron species described in this paper and one described by Jiang and Zhang (2015). Key to pupae of known Gymnetron species The following key is based on the pupae of the five Gymnetron species described in this paper.

Comparison with immature stages of known Mecinini
It has been suggested that the number of palpomeres of the labial palpi is one of the most important morphological characters of larvae in the Mecinini (Skuhrovec et al. 2018). Phylogenetically, the basal state in weevils is the presence of two palpomeres on the labial palpi (Marvaldi 1997). In Mecinus there are species in the plesiomorphic state (e.g., Mecinus collaris Germar, 1821; Mecinus janthinus group), but also such with one palpomere (Gosik et al. 2020). All the Gymnetron species examined here have one labial palpomere, as do the few species of Rhinusa described to date. In contrast, Cleopomiarus and Miarus generally have two palpomeres, although in some Cleopomiarus species the basal palpomere is not distinctly separated from the labium and can appear to be just a single palpomere (Skuhrovec et al. 2018). Another crucial generic-specific character in Mecinini larvae is the number of air tubes of the thoracic and abdominal spiracles. In Gymnetron all the spiracles are unicameral (Jiang and Zhang 2015). In the larvae of Mecinus species this character has two states: (1) all spiracles unicameral, as in Gymnetron and (2) the thoracic spiracle bicameral and the abdominal ones unicameral, as in some Rhinusa (Anderson 1973;May 1993;Ścibior and Łętowski 2018;Gosik et al. 2020). In contrast, all known larvae of Cleopomiarus and Miarus species have bicameral spiracles on the thorax and abdomen (Skuhrovec et al. 2018).
Another debatable state in the larvae is the number of epipharyngeal setae (especially ams and mes), which has not yet been completely resolved in Curculionidae (Gosik and Skuhrovec 2011;Stejskal et al. 2014;Trnka et al. 2015). In the Mecinini there are three als, two or three ams, and none or one mes. In our view, the final decision regarding the number of each seta is important, but not crucial, and the comparison between groups/genera should be made together for all three kinds of these epipharyngeal setae in order to make fewer errors when creating a differential diagnosis for the genera in the tribe.
The last important characteristic observed within the Mecinini tribe is the integument of the body covered with distinct asperities, both in the larval and pupal stages (Skuhrovec et al. 2018). This feature is very variable within each genus, probably owing to the distinctive environmental conditions within plant tissues.
With regard to the pupae, an uncommon character is the presence of two more or less sclerotized pronotal prominences, which can be smooth or serrated. Moreover, these pronotal protuberances (p-pr) are divisible into two parts with or without a stem from the pronotum and may have conical asperities or serrated margins. These prominences are present in all the Gymnetron species studied here, but also in some Rhinusa and a few Mecinus (Gosik 2010). The evolutionary significance of this character, which disappears altogether in the adult, is unclear.

Differences between immatures at the species level
All the larvae and pupae of every species studied here, and also the three described by Jiang and Zhang (2015), have several characters distinguishing them from one another. These differences confirm that most of them belong to different groups, as suggested by the study of the adults (Caldara 2008a). Three species, very closely related on the basis of the adult morphology (G. veronicae, G. tibiellum and G. auliense), also have several characters in common in the larvae (presence of fs 3 ; proand postdorsal folds of abdominal segments I-VI (VIII) with two prs and two pds; labral setae in one line) and in the pupae (sclerotized pronotal protuberances covered with conical asperities). The other two related species, G. villosulum and G. miyoshii, resemble each other more than the other species (in the larvae des 4 and mbs absent, postdorsal segment on abdominal segments with one pds; in the pupae pronotal protuberances smooth). The other species do not show clear relationships with each other or with the group of G. veronicae and G. villosulum. Only G. vittipenne could be related to the G. villosulum group, as also shown by the phylogenetic tree of the adults reported by Caldara (2008a).

Biological and evolutionary considerations
This study confirms that all the Palaearctic species of the genus Gymnetron with known biologies live only on Veronica. No other species belonging to the Mecinini live on this genus of Plantaginaceae. All the species usually seem to feed on various species of this genus, partly unrelated to each other and belonging to different subgenera as currently considered (Albach et al. 2004). They feed on the ovary or the stem of the plant, sometimes forming more or less voluminous galls. A recent study of Gymnetron and Rhinusa indicated a strong phylogenetic signal with respect to host plants but a weaker one with respect to the particular plant structures occupied by the insects in question on different plant structures (Hernández-Vera et al. 2013).