Chostonectes nebulosus (Macleay, 1871)

Tsyrlin, Edward, Carew, Melissa & Alarie, Yves, 2020, Re-description of larvae of Chostonectes nebulosus (Macleay, 1871) (Coleoptera: Dytiscidae, Hydroporinae, Hydroporini, Sternopriscina) with an identification key to the known larvae of Chostonectes Sharp, 1882, Zootaxa 4718 (3), pp. 436-446 : 439-443

Larvae of Chostonectes nebulosus (Macleay, 1871)

( Figs 1–3 )

Material examined. Australia: Victoria: 5 larvae, 2 adults , Australia, Victoria, Binks Road , 37.520S 144.239E, pond, 23 Dec 2018, E. Tsyrlin leg.; 9 larvae, 11 adults, same data, but on 20 Jan 2019 .

Molecular analysis. Neighbour joining analysis showed that the two specimens of C. nebulosus grouped together with a 100% match between them ( Fig. 3A ). The specimens of C. johnsonii and C. gigas separated from C. nebulosus by an average genetic distance of 15.2% and 20.1%, respectively. The average genetic distance between C. johnsonii and C. gigas was 19.6%.

Diagnosis: Larvae of C. nebulosus look similar to those of C. gigas and C. johnsonii recently described by Alarie (2019). They all possess the well-developed lateral projections of the nasale and a short siphon. C. nebulosus can be recognized using the features mentioned below.

Head width and length smaller ( Table 2 ); HL/HW> 1.75 (instar II) and> 1.65 (instar III); NW/FRMW> 1.5 (instar II) and> 2 (instar III); HL/LAS> 2.6 (instar II) and> 2.5 (instar III); relatively long and narrow lateral projections of nasale ( Fig. 3C ), with less than seven ventral tubercles distributed irregularly and not extending to base of lateral projections.

Redescription of instar III. Colour: Dorsally brown with yellow legs and head appendages; frontoclypeus pale with two light brown maculae mesally; frontal and lateral projections pale yellow; parietale yellowish laterally; head appendages predominantly pale yellow, A4 light brown, distal third of mandible light yellowish brown; thoracic and abdominal tergites light yellow; AB7 and AB8 brown except tip of siphon yellow; urogomphus predominantly brown, yellow proximally and over mid-length.

Body: Body shape sub-cylindrical, fusiform. Measurements and ratios are shown in Table 2 .

Head: Head capsule ( Fig. 2C ). Occipital suture present. Width of nasale at least two times greater than minimum width of anterior projection. Lateral projections of nasale pointed downward at approximately 13 degrees in relation to horizontal plane, bearing one apical seta and five or six ventral tubercles ( Fig. 3C ); parietale with 7–9 temporal spines ( Fig. 2C ).

Thorax: Mesopleural region with a spiracular opening on each side.

Abdomen: Mesopleural region of segments I–VII with a spiracular opening on each side.

Chaetotaxy: Position and number of secondary setae ( Table 3 ); natatory setae present on dorsal margin of femora, tibiae, and tarsi ( Figs. 2 D–2F).

Description of instar II. As instar III except as follows:

Body: Measurements and ratios that characterise the body shape are shown in Table 2 .

Colour: One large light brown patch in middle of frontoclypeus.

Head: Parietale with 5–6 lateral spines.

Thorax and abdomen: Spiracles absent.

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Figures

FIGURE 1. Dorsal view of Chostonectes nebulosus A—larva, B—adult; scale bar is 4 mm for adult and 3.2 mm for larva.

FIGURE 2. A—selected head measurements and features of Chostonectes larvae. WSAN—width of spatulate apex of nasale; MWN—minimum width of nasale; B—mandible measurements; C—head capsule of C. nebulosus instar III showing distinctive colour pattern and distribution of setae; D—posterior view and E—anterior view of middle leg of C. nebulosus instar III; E—details of femur.

FIGURE 3. A—neighbour joining tree for 12 sequences of a 658 bp fragment of CO1 using Kimura 2-parameter distance and 1000 bootstrap replicates. Dorsal view of head illustrating distinctive features of each species outlined in text; B—dorsal view of last abdominal segment of Chostonectes nebulosus instar III, showing shape of siphon and distribution of setae; C—side view of lateral projections of nasale of Chostonectes nebulosus and C. johnsonii.

Tables

TABLE 3. Number of secondary setae on the legs of Chostonectes nebulosus (CNEB) compared to those of C. gigas (CGIG) and C. johnsonii (CJOH) (Alarie et al. 2019); CO, coxa; FE, femur; TA, tarsus; TI, tibia; TR, trochanter and ‘Total’ is the total number of setae.

		CNEB		CGIG			CJOH
Segment	Sensillar	Instar II	Instar III	Instar II	Instar III	Instar II	Instar III
	series	(n = 2)	(n = 2)	(n = 4)	(n = 5)	(n = 1)	(n = 7)
ProCO	D	3	5	3–5	6–11	5	4–7
	A	0	3	0	3–6	0	1–4
	V	1	4	1–2	7–9	1	4–8
	Total	4	12	4–7	18–26	6	10–18
ProTR	Pr	1	1–2	1–2	2–4	1	1–2
	Di	0	0	0	0–2	0	0–1
	Total	1	1–2	1–2	3–5	1	1–3
ProFE	NS(PD)	7	23–24	11–14	31–33	7	21–25
	AD	4–6	9–11	3–9	13–17	3	8–11
	AV	0	16–17	11–19	26–39	9	16–22
	PD	0	0–1	0	1–3	0	0
	PV	3	7–8	2–4	12–17	3	9–12
	Total	21–23	55–61	27–39	88–106	22	57–65
ProTI	NS(PD)	8–10	19–20	15–18	32–36	9	15–21
	AD	3	5	4–6	6–9	5	4–8
	AV	0	2–3	0–1	2–4	0	0–1
	PV	2	3–4	3–5	5–6	2	3–6
	Total	13–15	30–31	24–27	47–54	16	23–34
ProTA	NS(PD)	5–7	13–15	9–11	22–27	7	14–19
	AD	3–4	6–7	5–7	4–8	4	4–8
	AV	0	0	0–1	0–1	0	0
	PV	2–3	2–5	3–5	5–7	4	4–6
	Total	10–13	21–27	19–22	36–42	15	23–32
MesoCO	D	3–5	4–6	3–5	8–13	5	6–10
	A	0	4	0	4–8	0	1–5
	V	2	5	1–2	8–12	2	4–8
	Total	5–7	13–15	5–7	23–33	7	12–21
MesoTR	Pr	1	2	1–2	2–4	1	2–3
	Di	0	0	0	0–2	0	0–1
	Total	1	2	1–2	2–6	1	2–4
MesoFE	NS(PD)	6–7	23–24	8–11	30–34	9	18–24
	AD	3–7	15–18	6–11	17–26	7	12–18
	AV	6	13–18	7–11	30–38	7	17–19
	PD	0	0	0	1–4	0	0
	PV	5	13	6–10	24–29	6	13–17
	Total	21–24	64–73	27–40	109–128	27	65–76
MesoTI	NS(PD)	9–13	23	17–22	40–46	12	24–26
	AD	6–7	7–9	6–11	11–17	6	9–14
	AV	3–4	8	5–7	8–11	3	5–7
	PD	0	0	0	0–3	0	0–3
	PV	2	6–7	3–4	4–6	4	4–7
	Total	21–25	45–46	33–42	67–80	23	45–56
MesoTA	NS(PD)	8–9	18–20	10–15	30–37	7	21–24