Research Article |
Corresponding author: Rafał Gosik ( r.gosik@poczta.umcs.lublin.pl ) Academic editor: Miguel Alonso-Zarazaga
© 2022 Rafał Gosik, Peter Sprick.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Gosik R, Sprick P (2022) Morphology of immature stages, biology, and systematic position of the Violet seed weevil, Orobitis cyanea (Linnaeus, 1758) (Curculionidae, Conoderinae, Orobitiditae, Orobitidini). ZooKeys 1121: 59-82. https://doi.org/10.3897/zookeys.1121.86888
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The mature larva of the weevil species Orobitis cyanea (Linnaeus, 1758), one of only two Palaearctic members of the supertribe Orobitiditae, is re-described, while the pupa is described for the first time. The biology of this species was studied at two sites in Germany. It was reared from seed capsules of Viola canina L. (Violaceae), and feeding holes were observed on V. riviniana Rchb. Adults of Orobitis cyanea and O. nigrina Reitter, 1885, specialists of Viola, show a well-developed escape mechanism, to which contribute a smooth surface, a rounded, nearly spherical body shape, and a seed-imitating thanatosis behaviour. The molytine weevil Leiosoma cribrum (Gyllenhal, 1834), the only other known weevil specialist of Viola in Europe, has a smooth surface, also, and is the most spherical species of the genus. The unique characters of the larva and pupa of Orobitis cyanea are discussed in regard to the systematic position of this taxon.
Blacus, escape mechanism, life cycle, mimicry, parasitoid, thanatosis
The subfamily Conoderinae Schoenherr, 1833, in the broad sense of some current classifications (
To date, the systematic placement of Orobitidinae has been changed many times.
However,
In view of the difficulty in clarifying of the taxonomic position of this widespread Palaearctic species, the critical morphological differences to other Conoderinae, some important discrepancies between previously published information on the larval stage (
On 3 July 2020, Orobitis cyanea was detected in stands of Viola canina L. in nutrient-poor grassland on a military training area near the village of Scheuen in the Celle district of Lower Saxony (Niedersachsen) (Fig.
Larvae: 10 exx. 03.07.2020, Scheuen (Celle), military training area, dry, nutrient-poor grassland on sandy soil, in Viola canina seed capsules (Fig.
Pupae ♀: 1 ex. 03.07.2020, 2 exx. 19.06.2021, Scheuen (Celle), military training area, dry, nutrient-poor grassland on sandy soil, in Viola canina seed capsules (Fig.
Before description, all the specimens were fixed in 75% ethanol and examined under an optical stereomicroscope (Olympus SZ 60 and SZ11) with calibrated oculars. The following measurements of the larva were made: body length (BL), body width (BW) (at the third thoracic segment), head capsule width (HW) and head capsule height (HH, measured from the apex to the epistoma). 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). Drawings and outlines were made using a drawing tube (MNR–1), installed on a stereomicroscope (Ampliwal), and were processed with computer software (Corel Photo-Paint X7, Corel Draw X7).
Slide preparation basically followed
The photographs were taken using an Olympus BX63 microscope and processed with Olympus cellSens Dimension software. The larvae selected for SEM imaging (scanning electron microscope) were first dried in absolute ethanol (99.8%), then rinsed in acetone, treated by CPD (Critical Point Drying) and finally gold-plated. TESCAN Vega 3 SEM was used to examine selected structures.
The general terminology and chaetotaxy follow
BL: 1.00–4.00; BH: 0.57–1.43; HW: 0.37–0.58 (all measurements are given in mm). The detailed results of measurements and the Growth Factor calculation are listed in Table
Measurements and Growth Factor calculation in Orobitis cyanea larvae (measurements are given in mm, n–number of specimens; HW is relevant to GF calculation; abbreviations: BL–body length, BW–body width, HW–head width; HH–head height).
Instar | HW | HH | BL | BH | GF |
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1st instar | 0.371; 0.381 | 0.351; 0.551 | 1.001; 1.051 | 0.571; 0.601 | |
2nd instar | 0.462; 0.471 | 0.402; 0.421 | 3.002; 3.161 | 0.832; 1.001 | 1.23 |
3rd instar (mature) | 0.572; 0.583; | 0.462; 0.502; 0.531 | 3.001; 3.662; 4.002 | 1.001; 1.161; 1.332; 1.431; | 1.24 |
General habitus and chaetotaxy. Live larva pure white, with yellow head capsule (Fig.
Head and antenna. Head capsule (Fig.
Orobitis cyanea mature larva, head and antenna (SEM micrograph) A frontal view B lateral view C ventral view D antenna. Abbreviations: at – antenna, sb – sensillum basiconicum, Se – sensorium, ss – sensillum styloconicum, setae: des – dorsal epicranial, fs – frontal, les – lateral epicranial, pes – postepicranial.
Mouthparts. Clypeus (Fig.
Orobitis cyanea mature larva, maxillolabial complex and apical part of maxilla A maxillolabial complex, ventral aspect B apical part of left maxilla, photo. Abbreviations: setae: dms – dorsal malar, ligs – ligular, mbs – malar basiventral, mps – maxillary palp, pfs – palpiferal, prms – prelabial, pms – postlabial, stps – stipal, vms – ventral malar.
Orobitis cyanea mature larva, maxillolabial complex (SEM micrographs) A maxillolabial complex, ventral aspect B prementum, ventral aspect C apical part of distal maxillary palp D apical part of labial palpomere E, F surface of ligulae. Abbreviations: ds–digitiform sensillum, sa – sensillum ampullaceum, tra – terminal receptive area, setae: dms – dorsal malar, ligs – ligular, mbs – malar basiventral, mps – maxillary palp, pfs – palpiferal, prms – prelabial, pms – postlabial, stps – stipal, vms – ventral malar.
Body. Prothorax small, pronotal shield not pigmented; mesothorax slightly smaller than metathorax. Meso- and metathorax each divided dorsally into two lobes (prodorsal and postdorsal lobes almost equal in size). Pedal lobes of thoracic segments isolated, conical, prominent. Abdominal segments I–III of similar size, slightly smaller than metathorax (Figs
Orobitis cyanea mature larva, lateral view, habitus and chaetotaxy. Abbreviations: Th. I–III–thoracic segments 1–3, Abd. I–X–abdominal segments 1–10, setae: ds–dorsal eps–epipleural, eus–eusternal, ps–pleural, pda–pedal, pds–postdorsal, prns–pronotal, prs–prodorsal, ss–spiracular, sts–sternal.
Orobitis cyanea mature larva, habitus and cuticle (SEM micrographs) A lateral view of head and thorax B lateral view of abdominal segments I–V C lateral view of abdominal segments VII–IX D lateral view of abdominal segment V (magnification) E structure of cuticle of dorsolateral part of prodorsum F structure of cuticle of ventrolateral part of prodorsum. Abbreviations: setae: ds–dorsal eps–epipleural, eus–eusternal, ps–pleural, pda–pedal, pds–postdorsal, prns–pronotal, prs–prodorsal, ss–spiracular, sts–sternal.
Chaetotaxy: distinctly reduced, most setae minute, thorn–like, only on dorsal part of abdominal segment IX very short, hair–like. Thorax (Fig.
Female: BL: 2.001; 2.161; 2.201; BW: 2.161; 2.331; HW: 0.551; 0.572; RL: 1.001; 1.051; 1.101; PW: 1.231; 1.301 (one pupa partially deformed). n–number of specimens.
General habitus and chaetotaxy. Body white, compact, almost round in outline (Figs
A search for immature stages at the site near Scheuen yielded several larvae and a few pupae on 19 June 2020 and 3 July 2021. They were found only in seed capsules of Viola canina (Fig.
In April and May, overwintering adults make small feeding holes in the leaves of their host plants, eating for maturation. At dry sites with early-flowering Viola species, such as V. canina L. or V. hirta L., eggs are laid mainly in April and May in the immature ovaries of the flowers. Larvae feed from young seeds, generating sufficient room to develop into the pupal stage at their feeding sites. Pupation occurred at both study sites inside the seed capsules in June and the first half of July. Adults left the seed capsules actively through feeding holes, or at the latest in July, by which time the seeds had ripened and the seed capsules burst open. Even in the Harz Mountains, the new generation had totally abandoned its place of development at the well-insolated site by mid-July, and some individuals were now occurring on their host plants; at that time, many adult weevils were present only in less exposed places. There were many feeding traces and adult weevils on the plants, but there were no more larvae or pupae inside the seed capsules. These were either still closed along the shady trench or had burst open on the sun-exposed slope.
Our observations regarding the feeding and development of Orobitis cyanea on Viola riviniana seed capsules are in accordance with those of
On the other hand, we confirm the information given by
Some specimens of a parasitoid wasp, Blacus sp. (Fig.
The method of larval instar determination worked out by
The number of larval instars in weevils is correlated primarily with the body size of a species. Thus, small species (head width of the mature larva below ~ 0.65 mm) usually have only three larval instars (
In some previously studied Entiminae species, GF usually varied between 1.38 and 1.44 (
When disturbed, Orobitis cyanea shows death-feigning or thanatosis behaviour and appears to imitate a Viola seed, which may be a form of mimicry: the dark part of the weevil may imitate the main part of the seed, the light part the elaiosome (
It seems worth mentioning that the only other European Viola-inhabiting weevil specialist, Leiosoma cribrum (Molytinae), occupies the top position in spherical body shape among all available Leiosoma species, where this could be tested (Table
Comparison of length-width ratio of the body of Viola-inhabiting species (A) and from species with other or unknown host plants (B). Measured from the front margin of the eyes to the apex of elytra and at the widest part of the elytra. *Groups defined by
Host | Species/Species group* | Ratio | Data source |
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A | Orobitis cyanea | 1.51:1 | own data |
O. nigrina | 1.58:1 | own data | |
Leiosoma cribrum | 1.72:1 | own data | |
B | Leiosoma cribrum group (five further species) | 1.99:1 – 2.24:1 |
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Leiosoma oblongulum group (four species) | 2.00:1 – 2.16:1 |
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Leiosoma scrobiferum group (six species) | 2.22:1 – 2.43:1 |
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Species from undefined species groups: L. apionides, L. bosnicum, L. deflexum, L. kirschii, L. reitteri | 1.88:1 – 2.28:1 |
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Leiosoma cribrum is the most rounded, shortest, and smallest species of this genus, approximating mostly the nearly perfect spherical shape of both Orobitis species. All other Leiosoma species are more elongate, closest are L. reitteri Bedel, 1884, L. apionides (Wollaston, 1864) (both ~ 1.88:1), and L. deflexum (Panzer, 1795) (1.91:1). The bulk of the species ranges between ~ 2.00:1 in L. diottii Pedroni, 2018, L. osellai Diotti & Caldara, 2020, and L. senex Pedroni, 2018, and 2.42 – 2.43:1 in L. hernicum Pedroni, 2012 and L. komovicum Pedroni, 2018.
Even if only for a small part of Leiosoma species the host plant species are known (Sprick and Krämer-Klement, in press), it is noticeable that there should be some selection pressure to weevil specialists that live on Viola species, which are unable to fly, to improve the escape mechanisms by falling down, rolling away or imitate biotic or abiotic structures of the environment in which they live, e.g., seed in Orobitis, or soil or underground in Leiosoma. Night activity of Leiosoma species may be another behavioural adaptation to reduce the possible loss of adult weevils by unspecific predators.
In his comprehensive work,
The larva of Orobitis cyanea is easily recognised by the following features: 1) postdorsal folds of abdominal segments I–VII undivided; 2) abdominal segments VIII and IX without prodorsal folds; 3) anus T-shaped, with dorsal and lateral lobes; 4) body cuticle with asperities forming rows and circles; 5) all spiracles unicameral; 6) epicranial setae minute; 7) stemmata absent; 8) extremely elongate endocarina, almost reaching the epistome; 9) antennal sensorium elongate; 10) clypeus with prominent median depression and curved to inside anterior margin; 11) labrum extremely narrow with anterior margin deeply rounded inwards (concave); 12) clypeus with only one pair of cls; 13) labrum with two pairs of lrm; 14) labrum with one pair of ams, two pairs of als but no mes; 15) labral rods absent but presence of multiple rib like sclerotisations; 16) postlabium with two pairs of pms; 17) labial palpi uni-segmented; and 18) ligula divided.
Knowledge of the immatures of the various Conoderinae supertribes is uneven. This is mainly because the supertribes of this subfamily differ in species numbers, distribution, individual abundance and economic importance. Bariditae and Ceutorhynchitae have been relatively well studied (
Finding features common to all known larvae that would be diagnostic of Conoderinae sensu lato is not possible. Some larval characteristics present in all the supertribes belonging to this subfamily are in fact common to the family Curculionidae (
In addition, the GF measurements indicate that there are three larval stages in Orobitis cyanea, as in Ceutorhynchitae (
The features given by
In the pupae, the differences between Orobitiditae and other Conoderinae are more clearly visible: setae on head, rostrum, pronotum and abdomen always clearly visible vs no setae entirely; pupal urogomphi, which are more or less developed or reduced in Conoderinae, are completely absent in Orobitiditae.
In general, endophagous larvae have significantly shorter segmental setae than exophagous larvae (
It is worth noting that a structure similar to “ligula with depression in middle” has been described as characteristic of the larva of only one species of Baridini, namely Aulacobaris johanni (Korotyaev, 1988) (
Both larva and pupa of Orobitis cyanea display many diagnostic features and at the same time differences from other Conoderinae species that it is difficult to find arguments supporting the current systematic position of this species. We consider, therefore, that there is ample justification for retaining Orobitiditae as a separate subfamily (as suggested by
The study of immatures of the two Neotropical Orobitiditae species could well provide new data, but at this stage, the placement of Orobitiditae within an enlarged concept of Conoderinae is not supported. Finding features unique to immatures of Orobitis is rather easy, but associating them with any other Curculionidae group is problematic. Therefore, leaving Orobitiditae as the subfamily Orobitidinae, as suggested by
We thank Andreas Marten for his assistance with the fieldwork, Marek Wanat and Michael Morris for their help with the literature queries. We would like to express our cordial thanks to Werner Ulrich for his help in identifying the parasitoid. We are indebted to Peter Senn for the linguistic editing of the text.