Plecia adiastola Hardy & Takahashi, 1960

Sutou, Mitsuaki, 2006, Immature stages of three Japanese species of the genus Plecia Wiedemann (Diptera: Bibionidae), Zootaxa 1223, pp. 1-18 : 5-6

Plecia adiastola Hardy & Takahashi, 1960

Description

Egg: Shiny brown with smooth surface ( Fig. 1 A), 0.53–0.65 mm long (n = 20).

Final­instar larva: Genae with 6 setae anteriorly and 18 sensory pits dorsally including 8 sensory pits along posterior stem of ecdysial suture ( Fig. 1 G: 4–7). Frontal apotome with 2 setae anteriorly, 2 minute setae posteriorly, and 2 sensory pits between them ( Fig. 1 G: 1–3). Clypeus with 4 setae and 6 sensory pits dorsally ( Fig. 1 G). Labrum with 2 minute setae and 2 sensory pits dorsally, about 10 small papillae and fine setae on its anterior membranous part ( Fig. 1 G). Epipharynx ( Fig. 1 H) with many fine setae, small conical processes, 8 differentiated small papillae and 4 sensory pits. Mandible ( Figs. 1 I, J) with 3 large and 2 small teeth, well­developed prostheca attached inside, 3 setae and 2 dorsal sensory pits on its basal half. Cardo with 3 setae and sensory pit ventrally ( Fig. 1 M). Galeolacinia nearly quadrilateral in shape, with conical projection and 2–3 small papillae on anterior margin ( Figs. 1 L, M); dorsal surface with many fine setae, spinules, and 5–7 sensory pits ( Fig. 1 L); ventral surface with 1 seta ( Fig. 1 M). Stipes composed of 3 parts: proximal isolated appendage, middle sclerite with seta and sensory pit, and distal membranous ring with sensory pit ( Fig. 1 M). Maxillary palpus with sensory pit and 11–13 apical papillae, 1 or 2 of which slightly longer than others ( Fig. 1 M). Central membranous area of labium with 12–14 small papillae and fine setae on its anterior margin ( Fig. 1 K). Hypopharynx nearly wing­like in shape with some pubescence ( Fig. 1 K). Body dark brown, arrangement of processes as shown in Table 1 a, but following deformations were found in some specimens: number of lateral processes of T3 only 4, posterior row of ventral processes of A1 and A2 absent. Plate­like ventral swelling of prothorax ( Fig. 1 C) with 3 processes, central one smaller than lateral ones. Body cuticle covered with many spinules ( Fig. 1 D). Width of head capsule 0.68–0.92 mm (mean ± SD = 0.78 ± 0.06 mm, n = 67; Fig. 7 a); Body length 11.7–16.7 mm (mean ± SD = 14.2 ± 1.5 mm, n = 66).

Pupa. Male ( Figs. 2 A, B): Vertex uneven in frontal view; antenna arched, extending obliquely; maxillary palpus extending laterally; rostrum short. Extremities of hind legs reaching posterior margin of 2nd abdominal segment or anterior margin of 3rd abdominal segment. Arrangement of body processes as shown in Table 1 b, but venter of A3 with 2 setae in some specimens. Abdominal spiracles flat, not protruded from body surface. Body length 6.3–10.0 mm (mean ± SD = 8.5 ± 1.0 mm, n = 26). Female ( Fig. 2 C): similar to male, but compound eye smaller, abdomen thicker than that of male. Body length 7.6–11.1 mm (mean ± SD = 9.5 ± 0.7 mm, n = 24).

Specimens examined

Egg: Laid by females reared from larvae or pupae collected on 29.III.1998 and 17.IV. 2001 in Tokiwadai, 35°28’N 139°35’E, 60 m alt., Hodogaya ward, Yokohama, Kanagawa Pref. and on 29.IV. 2000 in Mt. Inao, 31°07’N 130°53’E, 600 m alt., Sata, Kagoshima Pref. Final­instar larva: 14 examples, 6.V.2003, Mukaiyama, 38°14’N 140°52’E, 70 m alt., Taihaku ward, Sendai, Miyagi Pref.; 6 examples, 29.III.1998, 21 examples, 17.IV.2001, 18 examples (10 examples in NSMT), 5.XII.2002, 14 examples, 4.IV.2003, Tokiwadai, Yokohama; 5 examples, 9–10.III.2000, Shiroyama, 31°35’N 130°32’E, 60 m alt., Kagoshima, Kagoshima Pref. Pupa: 13♂, 14Ψ (5♂, 5Ψ in NSMT), reared from larvae collected on 6.V. 2003 in Mukaiyama, Sendai; 2♂, 2Ψ, 10.V.1998, Tokiwadai, Yokohama; 4♂, 5♂ and 3Ψ, reared from larvae collected on 29.III.1998 and 17.IV.2001 respectively in Tokiwadai, Yokohama; 3Ψ, reared from larvae collected on 9–10.III. 2000 in Shiroyama, Kagoshima; 3♂, 2Ψ, 29.IV.2000, Mt. Inao, Sata.

Ecological notes

The distribution of this species appears to be restricted to Japan. In the plains of central Japan, the adults emerge from mid­May to early June, and the final­instar larvae develop beyond the winter (univoltine). The larvae and pupae are usually gregarious in soil. The number of individuals in a single cluster ranged from 23 to 96 in the present study. The larvae of this species prefer damp soil in a forest, e.g., directly under rotting wood. According to Krivosheina (1998), the larvae of the closely related species P. thulinigra live in rotting wood at the final stage of decomposition in Far East Russia. Many males of P. adiastola were observed walking on piles of decayed wood trying to mate with females (29. V. 2000, Hachioji, Tokyo). Flight activity was rarely seen in this species, but several males were once observed flying during daylight hours (25–26. V. 2001, Yokohama).

Figures

FIGURE 1. Plecia adiastola. A, egg. B – M, final­instar larva. B, habitus, lateral view. C, plate­like ventral swelling of prothorax. D, spinules on dorsum of fourth abdominal segment. E, prothoracic spiracle. F, eighth abdominal spiracle. G, head, dorsal view. H, epipharynx. I, right mandible, dorsal view. J, right mandible, inner view. K, labium and hypopharynx, ventral view. L, left maxilla, dorsal view. M, left maxilla, ventral view. Abbreviations (full terms in parentheses): ant p (antennal plate), a st (appendage of stipes), cd (cardo), cly (clypeus), ec su (ecdysial suture), e st (eye spot), fr ap (frontal apotome), gala (galeolacinia), ge (gena), hyp (hypopharynx), lb (labium), lr (labrum), md (mandible), mxp (maxillary palpus), prs (prostheca), st (stipes). Scales: G, 0.5 mm. D – F and H – M, 0.1 mm.

FIGURE 2. Plecia adiastola, pupa. A, male, frontal view. B, male, lateral view. C, female, lateral view. Abbreviations (full terms in parentheses): ant (antenna), mxp (maxillary palpus), pro sp (prothoracic spiracle), rt (rostrum).

FIGURE 7. Width of head capsule of final­instar larva. a: P. adiastola; larvae were collected from several locations. b: P. hadrosoma; larval exuviae were collected from a cluster of pupae. Two peaks in b indicate that the size difference between males and females (males <females) is apparent in the final­instar larva.