Research Article |
Corresponding author: Krzysztof Pawlęga ( krzysztof.pawlega@up.lublin.pl ) Academic editor: Miguel Alonso-Zarazaga
© 2019 Krzysztof Pawlęga, Jacek Łętowski, Ewelina Szwaj, Tomasz Gosławski.
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:
Pawlęga K, Łętowski J, Szwaj E, Gosławski T (2019) The morphology of the immature stages of Squamapion atomarium (Kirby, 1808) (Coleoptera, Brentidae) and notes on its life cycle. ZooKeys 892: 143-160. https://doi.org/10.3897/zookeys.892.36027
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The immature stages (egg, mature larva and pupa) of Squamapion atomarium (Kirby, 1808), as well as its development cycle and the phenology of its developmental stages, are described for the first time. The larva and pupa of S. atomarium have typical morphological features of the subfamily Apioninae. Morphological data on the immature stages were compared with the only fully described Squamapion species, S. elongatum (Germar, 1817). The larvae of the two species differ in body size and shape, head shape, setae length, the chaetotaxy of the mouthparts, and individual types of setae on the pronotum and thorax. In the case of the pupa, there are also differences in body size and in the type of setae and chaetotaxy of the head, pronotum, metanotum and abdomen.
Apioninae, biology, central Europe, egg, host plant, life cycle, morphology, weevil
The genus Squamapion Bokor, 1923 belongs to the tribe Kalcapiini Alonso-Zarazaga, 1990 in the subfamily Apioninae Schönherr 1823 and family Brentidae Billberg 1820. The adult morphology, ecology, distribution and systematics of the Apionidae family have been presented in detail by
The biology of only one species and the morphology of its immature stages are known – Squamapion elongatum (Germar, 1817) (
This study is a continuation of research on representatives of this genus found in Poland. The authors describe the morphology of the third larval instar and pupa as well as issues concerning the development and ecology of Squamapion atomarium (Kirby, 1808).
According to the literature, this species prefers warm, sandy areas and is usually found in xerothermic grasslands (
The research material comprised developmental stages (egg, larvae, and pupa) of S. atomarium, isolated in the laboratory from field-collected plants described in the literature as hosts. The choice of study sites was based on faunistic data on the occurrence of S. atomarium as well as our own observations of potential habitats in the Lublin region of Poland (
Adult specimens were placed in plastic containers covered with mesh – separately for T. serpyllum L. and T. pulegioides L. Wet filter paper was placed on the bottom of the containers to maintain a suitable moisture level, together with thyme. The stems were searched for signs of oviposition and eggs about every three days. Then immature stages were grown in Petri dishes in a growth chamber, in the following conditions: daytime minimum 25 °C, daytime maximum 35 °C, minimum at night 15 °C, maximum at night 20 °C, humidity (60%), light duration – day 14 h, night 10 h. Immature stages were also grown in 125 ml plastic containers stored under room conditions (25 °C with a 14:10 photoperiod). Filter paper soaked in water was placed on the bottom of the container to maintain moisture, together with thyme stems with galls. The closed containers were monitored daily for mould. This method produced better results in terms of larvae survival than the use of the Petri dishes proposed by
The immature stages obtained by the methods described above were preserved in 70% ethyl alcohol. Two methods were used to prepare microscope slides, as described by
The terminology of
An analysis was made of the growth of the heads of individual larval instars based on Dyar’s law (1890), and the growth rate (GF) was determined based on
AbI, AbVII, AbVIII, AbIX, AbX – abdominal segments 1, 7–10, ThI, ThII, ThIII – thoracic segments 1–3, prns pronotal setae, pda pedal s., ps pedal s., eus eusternal s., lsts laterosternal s., prs prodorsal s., pds postdorsal s., as alar s., ss spicular s., eps epipleural s., ds dorsal s., les lateral epicranial s., fs frontal s., des dorsal epicranial s., pes posterior epicranial s., at antenna, Se sensorium, sb sensillum basiconicum, ss sensillum styloconicum, oc ocellus, enc endocarina. lrms labral setae, cls clypeus s., ams anteromedial s., als anterolateral s., mes median s., lr labral rods., mds – dorsal malae s., dms dorsal maxillary s., pfs palpiferal s., sts stipal s., mpxs maxillary palp s., mbs malar basiventral s., prms prelabium s., pms postlabium s., lgs ligular s., lbp labial palpus, as apical setae, ls lateral s., pls posterolateral s., sos suborbital s., rs rostral s., fes femoral s., ur urogomphi.
Figs
Measurements (in mm). First larval instar (L1) – body length 0.46 (0.39–0.55), width 0.21 (0.19–0.23). Head width 0.13 (0.12–0.14).
Second larval instar (L2) – body length 0.68 (0.64–0.74). Body widest at abdominal segment III (0.34). Average pronotum width 0.27 (0.15–0.21). Head width 0.19 (0.17–0.21). Stemmata present.
Mature larva (third instar, L3) – body length 1.36 (1.09–1.72). Body widest at abdominal segment III (0.66, 0.50–0.86). Width of pronotum 0.47 (0.40–0.55). Head width 0.33 (0.30–0.38).
General
L3. Cylindrical, C-shaped, pale yellow with no distinct sclerotizations (Fig.
Head
(Fig.
Mouthparts. Labrum – anterior margin slightly arched. Dorsal side with 3 thorn-shaped labral setae (lrms), of which lrms2 long and longer than others; lrms1 closer to centre, below mid-height of labrum, lrms2 and lrms3 anterolaterally (Fig.
Labium cup-shaped (Fig.
Thorax. Thoracic segments with well visible pedal area. Thoracic setae longer than others. Prothorax with 7 pronotal setae varying in length (prns), 1 pleural seta (ps) and on pedal area 3 pedal setae (pda) (Fig.
Abdomen. Tergites I–VII with 2 folds and 1 seta (prs) on prodorsum. Postdorsum with 2 pds and 2 ss of varying size; 1 epipleural seta (eps) slightly below ss. Pleurum with 1 ps. Sternum with 1 laterosternal seta (lsts). Tergit VIII with gentle folds and with 1 prs, 1 dorsal seta (ds) and 1 eps. Tergit IX without folds, with 1 ds and 1 ps. Sternum and pleurum of segments VIII–IX with 1 ps and 1 lsts. Segments I–VII with unicameral spiracles, others without spiracles (Fig.
Figs
Measurements (in mm). Body length 1.51 (1.24–1.63), width 0.84 (0.72–0.93) (Figs
Colouration. Colour creamy-white.
Head. Eyes large, with 1 supraorbital seta (sos) between them. Rostrum long, extending to end of tarsi of mesolegs, not very wide, with 1 rostral seta (rs) below base of antennae, shorter than sos. Antennae relatively long, club with conical papillae. Antennae sub-parallel to protibia (Figs
Thorax. Pronotum wider than long; sides with 2 lateral setae – long ls1 and shorter ls2; 1 apical seta on apex (as), half length of ls1 (Figs
Abdomen. Chaetotaxy very sparse. Each segment with 1 short dorsal seta located close to lateral margin. Each of lateral parts of abdominal segments I–VII with 1 pair minute lateral setae. Spiracles located between tergites and pleurites, clearly visible on segments I–VI, on others absent (Fig.
Figs
Host plant. The life cycle of S. atomarium was described based on field data and laboratory observations. Thymus serpyllum and T. pulegioides were confirmed as host plants (Fig.
Life cycle. Adults, following overwintering and maturation feeding, begin copulation and egg laying in the first half of May. Increased egg laying was observed at the end of May, and single eggs were still noted in early June. Adults usually feed in the evening, by gnawing round holes in the leaf that do not exceed 1 mm in diameter. The fertilized female gnaws a cavity in the stem and lays one egg in it (Fig.
Parasitoids. In the second half of July, endoparasitic hymenopterans of the superfamily Chalcidoidea were very active, which is manifested by the high level of parasitism of L3 larvae. On average 7 of 10 third-instar larvae exhibited symptoms of parasite infection: dark red discolouration on the thoracic tergites and pleurites and swelling of the abdominal segments caused by the growth of the intruder larvae (Fig.
Figs
Deviations of the mean dimensions of the heads of individual larval stages from the theoretical dimensions are shown in Figures
Among species of the genus Squamapion, only S. elongatum (Germar, 1817) has previously been described, and the existing data on S. atomarium concern only its habitat and host plants, with an equal role ascribed to T. serpyllum and T. pulegioides (
The morphology of the L3 larva and pupa of S. atomarium does not differ from the typical characters of the subfamily Apioninae (
Thus the immature stages of the species are generally very similar in their morphology to those described by
In the comparative analysis of the egg and L3 larva of S. atomarium and S. elongatum, the two species are distinguished by differences in the size of both the egg and the L3 larvae – in S. atomarium they are about half the size as in S. elongatum (
Character comparison between L3 Squamapion atomarium and Squamapion elongatum.
Trait | Species | ||
Squamapion atomarium | Squamapion elongatum | ||
Body mm (length/width) | ca 1.36/ca 0.66 | ca 2.78/ca 1.24 | |
Setae | shorter, with pointed ends | longer | |
Head | slightly egg-shaped | oval | |
Antennae | 4 sensilla | 2 sensilla | |
Number of setae on maxillary palpus | basal segment | 1 seta, 1 sensillum | 1 seta, 2 sensilla |
distal segment | none | 1 short sensillum | |
Labrum/epipharynx | ams | 2 pairs (ams1–2) | 3 pairs(ams1–3) |
als | 2 pairs | 3 pairs | |
lr | large, widening towards outer margin of epipharynx | narrow | |
Labium with pms | 3 pairs(pms1–3) | 2 pairs(pms1 and pms3) | |
Number of conical papillae dms | 4 | 5 | |
Number of setae prns on pronotum | 7 | 5 | |
Number of setae pda | 3 | 2 | |
Number of setae ss | 2 | 3 |
The case of the pupa is similar. There are clearly visible differences between species in body size and chaetotaxy. The body of the pupa of S. atomarium is shorter than that of S. elongatum (1.5–2.0 times) and has far fewer abdominal setae (Table
Character comparison between the pupa of Squamapion atomarium and Squamapion elongatum.
Trait | Species | ||
Squamapion atomarium | Squamapion elongatum | ||
Body mm (length/width) | ca 1.51/ca 0.84 | ca 2.67/ca 0.94 | |
Colour | creamy-white | whitish-grey | |
Head setae | 1 pair sos, 1 pair rs | 1 pair vs, 1 seta rs | |
Pronotum | 1 pair as (as1), 2 pairs ls (ls1,2) | 2 pairs as (as1,2), 1 pair ls | |
Metanotum | 2 pairs of setae | 3 pairs of setae | |
Abdomen | lateral part | only 1 pair of setae of I–VII abdominal segments | absent |
dorsal part | absent | AbI-III: 7 pairs, | |
AbIV-VI: 5 pairs, | |||
AbVII: 3 pairs, | |||
AbVIII: 1 pair | |||
Urogomphi | flattened bifurcation, straight | crescent-shaped, narrow |
The study and descriptions of additional species of the genus Squamapion will make it possible to distinguish and describe its generic characters.
Analysis of the growth rate and the ratio of actual and theoretical average head sizes produced some discrepancies that may have been influenced by the fact that the individuals were not divided by sex or collection site, and thus may have represented different populations.
The authors would like to thank Professor Bernard Staniec and Jarosław Pawelec of the Maria-Curie Skłodowska University in Lublin for allowing us to take photos using scanning electron microscopy and the CPD procedure. The authors would like to thank the English native Sara Wild – for the correction of the manuscript.