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 (Cmoluch 1963, 1971, 1987, 1992; Gosik and Łętowski 2003; Łętowski et al. 2003; Łętowski 2008). The sites were as follows: 1. Okale near Kazimierz Dolny (51°18'11.0"N, 21°53'58.6"E), 2. Bochotnica (51°20'38"N, 22°00'05"E), 3. Lublin-Górki Czechowskie (51°15'47"N, 22°32'03"E), 4. Lublin (51°13'11.50"N, 22°32'04.38"E), 5. Trześniów near Lublin (51°16'20"N, 22°37'04.10"E), 6. Kolonia Pliszczyn (51°17'38"N, 22°37'38"E), 7. the Stawska Góra Reserve near Chełm (51°22'23"N, 23°24'11"E) and 8. the Żmudź Reserve (51°00'35"N, 23°40'14"E). Immature stages of S. atomarium were found at sites 3, 5 and 8. In the remaining sites, despite the presence of host plants, no specimens were found. The material was collected from May to August 2016 and 2017. To obtain immature stages of the species, plants were collected at the sites every 2–3 days. This frequency made it possible to study the development cycle of the species in its natural conditions. Breeding was also conducted in the laboratory. Squamapion atomarium adults were collected individually, directly from the host plant and from its immediate surroundings.

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 Scherf (1964). In order to track development and acquire larval stages, 5 stems were randomly selected, the galls were cut open, and developmental stages were isolated from them.

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 Łętowski (1991) and Gosik et al. (2010). To prepare the drawings, we used an OLYMPUS SZX12 and DP72 microscope at magnifications from 200× to 400× and a TESCAN VEGA3LMU scanning electron microscope (SEM) at magnifications from 500× to 2000×. The larvae for SEM images were subjected to critical point drying (CPD). Drawings based on the slides were made using Corel Draw 18.

The terminology of Marvaldi (1999, 2003) and Oberprieler et al. (2014) was used in the morphological descriptions of the larva and pupa for chaetotaxy, and the terminology of Zacharuk (1985) and Marvaldi (1998) for antennae. The number and distribution of setae are given for one side. Measurements of the head (following decapitation) were made on the head capsule, isolated from the body, with the mandibles closed. Measurements were made of 10 L₁, 4 L₂, 15 L₃ and 10 pupae. The larvae were not separated by gender for the measurements. The mean and standard deviation for each parameter were calculated using Excel.

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 Bednarz (1953).

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.

Fig. 1

Measurements (in mm, N = 5). Length 0.28 (0.22–0.31), width 0.18 (0.15–0.20).

General. Egg elliptical, shiny, smooth. Chorion soft, delicate (Fig. 1).

Colouration. Pale to dark yellow.

Figure 1. 

Egg of Squamapion atomarium.

Figs 2–6

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. 2). Cuticle microstructure of entire body with many small, sharply pointed cuticular structures. Thoracic and abdominal segments with characteristic, short setae. Body much narrower after abdominal segment VIII.

Figure 2. 

Mature larva (L₃) of Squamapion atomarium, lateral view.

Head (Fig. 3). Pale yellow, later dark yellow, slightly hidden in prothorax, longer than wide, slightly egg-shaped, widest at 2/5 of length. Epicranial suture visible. Endocarina (enc) distinct, long, together with epicranial suture extends 3/4 length of head (Fig. 3). End of frontal suture with distinct stemmata. Antennae (at) without articulations. Sensorium (Se) long, slightly narrowing apically. Antenna with 4 sensillae: 2 sb (sensillum basiconicum) and 2 ss (sensillum styloconicum) (Fig. 3). Epicranium with 2 lateral epicranial setae (les1,2). Les1 more than 4 times longer than les2. Dorsal part of epicranium with 5 visible dorsal setae (des1–5), des1,3–5 situated more or less along frontal suture, des2 on extension of line of pes (Fig. 3). Des3 very long, des4 very short and des1,2,5 short and more or less equidistant. Epicranium with 4 thorn-shaped posterior epicranial setae posterolaterally (pes1–4) –very short and more or less equidistant. Frons with 4 frontal setae (fs1,3–5) (Fig. 3). Fs1,3 short (shortest of all fs), fs1 situated by endocarina at about 1/3 length of frons, fs3 situated outermost of all fs, close to fs4. Setae fs4 long – longest of all fs. Setae fs5 slightly above anterior margin of stemmata.

Figure 3. 

Squamapion atomarium (L₃), – epicranium, dorsal view.

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. 4a). Epipharynx anteriorly with 2 anteromedial setae (ams), of which medial ams1 finger-shaped, outer setae ams2 thorn-shaped (Fig. 4b). Beside ams, 2 als on epipharynx, arranged more or less diagonally from corner to centre of labrum. Als1 slightly shorter than als2. Both wider at base and narrowing apically. Mes digitiform and placed antero-medially. Labral rods (lr) present, long, extending well beyond suture (Fig. 4a). Clypeus kidney-shaped, with slightly concave anterior margin. 1 short seta cls at lower margin, between them 1 sensillum (clss) (Fig. 4a). Mandible massive, highly sclerotization, light to dark brown in colour. Two teeth equal in size, curved. Dorsally 1 pair mandible dorsal setae (mds1,2) and 1 sensillum (Fig. 5). Setae close together, one above the other, each sensilla peripherally. Mds2 more than twice longer than mds1. Inner margin of inner teeth serrated. Maxillary stipes elongated, widening apically, narrowed at mid-length, with 4 distinct, hair-like setae (Fig. 6). In lower part 1 stipal seta (stps). In the upper part palpiferal setae pfs1 fairly short, placed centrally under maxillary palpus, pfs2 very long, placed on inner side, basioventral seta (mbs) short. Maxillary palpus (mp) 2–segmented, distal segment cylindrical, smaller than basal segment, with 10 nodular cuticular tubercles situated apically. Basal segment with rod-shaped sensorium, 1 minute maxillary palp seta (mpxs) and 1 pore. Malar part of maxilla with 7 dorsal maxillary setae (dms1–4, vms1–3) clearly visible, finger-shaped setae of equal length in comb-like arrangement.

Figure 4. 

Squamapion atomarium (L₃) labrum and clypeus: a dorsal view b ventral view.

Figure 5. 

Squamapion atomarium (L₃) – mandibulae.

Labium cup-shaped (Fig. 6). Base of prementum rounded. Postmentum with 3 pairs postmental setae (pms1–3), distributed evenly, one over the other, closer to outer part of postmentum, more or less parallel to its edges. First pair setae (pms1) situated closest to lower margin, shortest of all pms. Above it pms2, very long and longest of pms, thick, narrowing only at apex. Setae pms3 situated at 2/3 height of labium, similar in structure to pms2 but half their length. Labium with Y-shaped premental sclerite situated centrally. 1 pair sensilla at base of arms of this structure. At height of premental sclerite, dorsally, chitinized inverted comma-shaped labial rods with uneven edges. Labium with 1 pair simple palpi (lbp), with 7 palpillae apically, 1 inner seta at base and 1 outer sensillum. In front of palpi 1 pair long premental setae (prms). Behind palpi 2 pairs very short ligular setae (lgs) and 1 pair sensilla (Fig. 6).

Figure 6. 

Labium and maxillae (L₃) of Squamapion atomarium.

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. 2). Meso- and metathorax each with 1 relatively long prodorsal seta (prs), 2 postdorsal setae (pds), 3 alar setae (as), 3 spicular setae (ss), 3 pda on pedal area and 1 short eusternal seta (eus). Thoracic spiracle bicameral, located intersegmentally, between Th.I and Th.II (Fig. 2).

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. 2).

Figs 7–10

Measurements (in mm). Body length 1.51 (1.24–1.63), width 0.84 (0.72–0.93) (Figs 7–10).

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 7, 9).

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 7, 8); lower margin with 2 posterolateral setae (pls1,2), similar in length to as1. Mesonotum without setae. Metanotum with 2 setae, slightly shorter than ls1 (Figs 8, 9). Each femur with 1 femoral seta (fes) on convex base (Figs 8, 9).

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. 9). Segment IX terminally with 1 pair urogomphi (ur) with characteristic ends in form of flattened bifurcation (Fig. 7).

Figures 7–9. 

Pupa of Squamapion atomarium 7 ventral view 8 dorsal view 9 lateral view.

Figs 11–17

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. 11).

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. 12). Oviposition takes place primarily at the root collar, but it was also observed up to the fourth or fifth node, in both nodes and internodes (Fig. 13). After laying the egg the female does not seal the site with any secretion. The first instar larva (L₁) hatches on average 4 days after the egg is laid and moults after 10–12 days. The L₁ instar was observed as early as mid-May, but these were isolated specimens. Maximum emergence was observed from the second third of May. L₁ larvae were found until mid-June. The second larval instar (L₂) appeared at the end of May. The activity of L₂ larvae causes distinct galls about 1.32 mm long and about 0.75 mm wide. Furthermore, L₂ gnaws out an opening for oviposition on the opposite side of the groove, but does not gnaw through the skin. The second larval instar lasts on average 10 days, and then the larva moults again. L₃ larvae appeared as early as the last third of June and were noted until mid-July. The average duration of this stage is about 11 days. This stage continues feeding and the gall grows, reaching on average about ca 2.31 in length and ca 1.70 mm in width (Fig. 14). The third larval instar enlarges the opening in the stem. Then pupation takes place (Fig. 15). The pupal stage lasts 2–3 days on average. The first pupae appeared at the end of June. Finally, at a maximum 40 days after the egg is laid, adult individuals appear. An increase in the emergence of adults took place from mid-July. The entire life cycle of S. atomarium is presented in the diagram in Figure 16.

Figures 10–15. 

Squamapion atomarium 10 pupa 11 occurrence environment 12 a gall 13 the most common place to lay eggs at the root collar 14 place for laying eggs 15 larva in prepupal stage for pupation.

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. 17). The mature larva of the parasitoid usually occupied the space from the second or third thoracic segment to the eighth abdominal segment. The adult larva of the parasite is ca 0.75 mm long and ca 0.56 mm wide. The body of the pupa of the parasitoid is black with a metallic sheen and well chitinized. The parasites brought about the death of L₃ of S. atomarium.

Figure 16. 

Life cycle of Squamapion atomarium.

Figure 17. 

Larva of a chalcidoid endoparasitoid found inside the mature larva (L₃) of Squamapion atomarium.

Figs 18, 19

Deviations of the mean dimensions of the heads of individual larval stages from the theoretical dimensions are shown in Figures 18, 19. Analysis of the ratios of the head sizes of larval instars does not clearly result in a single growth factor. GF between L₁ and L₂ is 1.43 and between L₂ and L₃ it is 1.75.

Figure 18. 

Mean real and average theoretical head lengths of Squamapion atomarium larval stages.

Figure 19. 

Mean real and average theoretical head widths of Squamapion atomarium larval stages.