Thecagaster coronata ( Morton, 1916 )

Müller, Ole, Brochard, Christophe, Ikemeyer, Dietmar, Snegovaya, Nataly, Lemke, Martin & Schneider, Thomas, 2025, Description of the larvae and exuviae of Thecagaster charpentieri (Kolenati, 1846) and Thecagaster brevistigma (Selys, 1854) with a re-description of the exuviae of Thecagaster coronata (Morton, 1916) (Odonata: Cordulegastridae), Zootaxa 5653 (2), pp. 211-228 : 222-226

Thecagaster coronata ( Morton, 1916)

General: The exuviae of T. coronata display the typical morphological characteristics of Cordulegastridae larvae ( Fig. 12 ). Female exuviae range in total length from 42.2 to 48.6 mm (N=2), while male exuviae measure between 38.2 and 44.5 mm. The body is uniformly pigmented brown without distinct darker markings. The cuticle is covered with fine, sparsely distributed setae, which can be found on the abdominal sternites and tergites. Faint darker colour patterns are present on the upper side of the thorax. Additionally, longer setae and hairs are conspicuously found along the posterior edge of the thorax, parts of the legs, the tergites of the abdomen, the anterior edges of the sternites, and the appendages.

Head: From dorsal perspective, the lateral margins of the head are strongly roundet behind the eyes, resulting in the head not having a strictly rectangular shape, occiput irregularly haired on the sides and rear margin, with some strong lateral bristles pointing forward and upward ( Fig. 13A ). The clypeus is brown, and the front margin of it is lined with a row of short setae. The compound eyes are notably large, forming the antero-lateral corners of the head. The antennae consist of seven segments.

The labium exhibits the characteristic structure observed in Cordulegastridae , extends, in resting position, just past the middle coxae ( Figs. 13B–13C, 13E ). The prementum is elongated, with a length-to-width ratio of 0.97. The anterior margin of the ligula is bordered by a dense array of short setae and displays two slightly uneven, blunted projections at its centre ( Fig. 13D ). A series of evenly spaced, short setae extends along the lateral margins of the prementum. The mesial margins of the palpal lobes are irregular and bear sharply pointed teeth, which are heavily sclerotized and dark brown at their tips ( Figs. 13B–13C, 13E ). The movable hooks are elongated and robust, with dark brown apices. From a dorsal perspective, the prementum bears 3–5 long setae on each side and 2–3 shorter ones centrally ( Fig. 13E ). The palpal lobes possess 4–6 palpal setae, with their outer margins bordered by rows of densely arranged setae ( Fig. 13E ).

Thorax: The pronotum is broad and extends to approximately the vertical midline of the eyes. The epaulette measures over one-third the length of the pronotum ( Fig. 12 ). It is gently curved backward, with only slight tapering toward the tip, and the posterior margins appear slightly thickened when viewed from the rear. The forewing sheaths extend to the fourth abdominal segment. The tarsi have three segments and a two-part claw. The segments are densely covered with bristles, with strong spines at the proximal ends, especially on the ventral side. In addition, dorsal sides of the segments 2 and 3 of hind tarsi are covered with long, flexible bristles ( Fig. 14D ).

Abdomen: All sternites are sparsely covered with bristles, with the bristle density significantly decreasing on the more caudally positioned sternites. Each sternite and each tergites has a dense row of short bristles along its posterior edge. Longer hairs are found along the sides of the abdominal segments ( Fig. 12 ). The female gonapophyses (ovipositor) protrudes slightly beyond the posterior end of abdominal segment 9 ( Fig. 14C, 14G ). In males, the projection is about one quarter the length of the epiproct, clearly notched mesially, with parallel lateral margins and distal curved margins ( Fig. 14E ). The cerci are short, reaching only about one-third the length of the paraprocts. The male cerci are more robustly developed than the female cerci ( Figs. 14E–14F ). The paraprocts are as long as the epiproct and form a pointed anal pyramid when closed. Both sexes lack lateral and dorsal spines ( Figs. 14A, 14C ).

Measurements: total length 38.2–43.1 mm males N=3, 42.2–43.5 mm females N=4 (approximate value); head width 8.0 mm male N=1, 8.1 mm female N=1) (approximate value); length of mentum 6.0 mm male N=1, 6.7 mm female N=1; basal width and distal width of mentum: 2.1/ 6.2 mm male N=1, 2.3/ 6.9 mm female N=1; length of wing case of hindwing: 8.5 mm male N=1, 8.5 mm female N=1; length femur foreleg (without trochanter) 4.7 mm male N=1, 5.2 mm female N=1; length tibia foreleg 5.8 mm male N=1, 6.2 mm female N=1; length tarsus foreleg 2.6 mm male N=1, 3.1 mm female N=1; length tarsal segments (example male N=1) T1 0.7, T2 0.6, T3 1.4 mm; length abdomen: 27.5 mm male N=1, 29.9 mm female N=1; abdomen maximum width, 7.6 mm male N=1, 8.4 mm female N=1; lengths cerci 1.2 mm males (N=1), 1.2 mm females (N=1); cerci basal width 0.4 mm male (N=1), 0.4 mm female (N=1); lengths paraproct (inner margin in ventral view) 3.0 mm male (N=1), 3.4 mm female (N=1); lengths epiproct (in dorsal view): 3.5 mm male (N=1), 3.6 mm female (N=1); lengths male projection: 0.7 mm (N=1), female gynapophyses: 2.1 mm (N=1); length of antennae 2.8 mm (example male N=1).

Differential diagnosis

All exuviae and larvae of the three species described here show the typical morphological characteristics of Cordulegastridae . All measurements are summarized in Table 2 and important differential characters are highlighted in grey. All three species studied show the characteristics of the former “ bidentata group”, now assigned to the genus Thecagaster , the most important of which is the absence of lateral spines on all abdominal segments (especially on segments 8 or 9).

Thecagaster coronata differs from T. charpentieri and T. brevistigma by having relatively long and slender cerci. In contrast, specimens of T. charpentieri and T. brevistigma possess shorter and broader cerci. Thecagaster charpentieri can be distinguished from T. brevistigma by its shorter paraprocts and slightly longer cerci ( Table 3 , key).

Figures

FIGURE 12. Reconstruction of the last instar of larva of Thecagaster coronata—male (bar scale 10 mm). Drawing by Ole Müller.

FIGURE 13. Head of larva of Thecagaster coronata. A—head, dorsal view, B—head, frontal view, C—labium, ventral view, D—anterior margin of ligula in ventral view, E—labium, dorsal view. Drawing by Ole Müller.

FIGURE 14. Details of the abdomen and the thorax of the larva of Thecagaster coronata (without bristles). A—abdomen, dorsal view, male, B—abdomen—S1–4 ventral view, male, C—abdomen—S6–10 ventral view, female, D—tarsus, hind leg, E—anal pyramid, dorsal view, male, F—anal pyramid, dorsal view, female, G—abdomen, lateral view, female. Drawing by Ole Müller.

Tables

TABLE 2. Data sets of the morphological characteristics of the Thecagaster species studied here (dimensions in mm, without differentiation by sex, sample sizes in brackets). The currently most important morphological differences in this study, which were also used for the serious proposal of a key (Table 3), are highlighted in grey.

	T. charpentieri	T. coronata	T. brevistigma
total length	37.5–41.7 (3)	38.2–43.5 (7)	36.8 (1)
abdomen length	23.0–27.4 (4)	27.5–29.9 (2)	24.5 (1)
wing case HW length	8.3–10.2 (4)	8.5 (2)	10.0 (1)
abdomen S5 width	7.4–8.0 (4)	7.6–8.4 (2)	7.3 (1)
cerci length	1.0–1.2 (4)	1.2 (2)	1.0 (1)
cerci width	0.4–0.5 (4)	0.4 (2)	0.4 (1)
paraproct length (ventral)	2.6–3.0 (4)	3.0–3.4 (2)	2.9 (1)
epiproct length (dorsal)	2.3–3.3 (4)	3.5–3.6 (2)	3.2 (1)
projection length	0.6–0.7 (3)	0.7 (1)	0.7 (1)
gynapophyses length	2.0 (1)	2.1 (1)	-
gynapophyses width (basal)	1.6 (1)	1.6 (1)	-
femur foreleg length	4.1–5.4 (3)	4.7–5.2 (2)	4.3 (1)
tibia foreleg length	5.5–6.1 (3)	5.8–6.2 (2)	5.2 (1)
tarsus foreleg length	2.7–2.9 (2)	2.6–3.1 (2)	3.0 (1)
head width	7.5–8.5 (3)	8.0–8.1 (2)	7.7 (1)
mentum length	6.0–7.0 (3)	6.0–6.7 (2)	6.1 (1)
mentum width basal	2.2–2.5 (3)	2.1–2.3 (2)	2.0 (1)
mentum width distal	5.9–7.2 (3)	6.2–6.9 (2)	6.1 (1)

used for the serious proposal of a key (Table 3), are highlighted in grey.