Setodes acutus, Navas, 1936

Bemmoussat-Dekkak, Soumya, Ibrahimi, Halil, Sartori, Michel & Karaouzas, Ioannis, 2024, Larval description of Setodes acutus Navás 1936, (Trichoptera: Leptoceridae) from Algeria, including a discriminatory matrix to the known Setodes larvae from the Maghreb region, Zootaxa 5437 (4), pp. 589-596 : 590-594

Description of the fifth instar of Setodes acutus ( Figs 1–14 )

Biometry. Last larval instar length of Setodes acutus 5.8–5.9 mm (n = 4) ( Fig. 1 ).

Head. Head capsule longer (head length: 0.63–0.65 mm, n = 4) than wide (head width: 0.41–0.45 mm, n = 4); yellowish with light brown patches, pale ring around each eye with muscle scars on posterior half of frontoclypeal apotome ( Figs 2, 3 ). Frontoclypeal apotome elongate, nearly parallel-sided, with pair of small brown indentations at mid length near tentorial pits; aboral end of frontoclypeus almost hemispherical, posterior ecdysial suture straight, symmetrical, positioned left of midline; with dark color on each side of anterior clypeus region separated by dark yellow; dark color also along sides of frontoclypeus and across midline subanteriorly and subposteriorly forming elongate oval, with mesal region inside this oval and anterior of lateral indentations dark yellow, transverse region at posterior apex of frontoclypeus pale yellow; parietals with narrow, dark brown band on either side of frontoclypeal apotome from anterior margins to ecdysial suture and light brown on either side of ecdysial suture, lateral postgenae also light brown ( Figs 2, 3 ). Parietals dark brown on either side of ventral apotome in ventral view; ventral apotome rectangular, dark brown, with pair of small sclerites at posterior end before curved, dark margins of occipital foramen ( Fig 4 ). Antennae near distal parietal borders, long (more than 6 × longer than wide) and each with single terminal seta ( Fig. 6 ).

Thorax. Pronotum rectangular in dorsal view, divided into pair of sclerites by mid-dorsal ecdysial line; each sclerite light brown with rows of setae of varying lengths on anterolateral borders and with numerous pale setae arranged transversely across surface posterior to each pronotal half ( Figs 5, 6, 7 ). Mesonotum with pair of setal rows on anterolateral corners; mesonotal sclerites trapezoidal in dorsal view, pale, divided into pair of sclerites by median ecdysial line ( Figs 5, 6, 7 ). Metanotum membranous except for small and weakly sclerotized anteromesal plate (sclerite of setal area 1, or sa 1, of Wiggins 1996; Figs 5–7 ), one pair of setae at sa 1, one pair at sa 2, and four pairs at sa 3 ( Fig. 7 ). Metasternum bearing one transverse row of 4–6 pairs of setae.

Forelegs and midlegs arising ventrolaterally below pro- and mesepimera, hind legs arising laterally at anterior ends of long, dark metepisterna/metepimera ( Fig. 6 , black arrow). Forelegs short and stout ( Figs. 8, 9 ); each foretrochantin rectangular with elongate extension at apex ( Fig. 10 , dashed circle); each foretibia with apicoventral angle projecting ventrad and provided with small spine ( Figs. 8, 9 ). Hind legs longer than midlegs and projected forward, each with many setae on femur, tibia, and tarsus, without articular membrane at tarsi ( Fig. 11 ). Constriction or translucent section absent from hind tibia ( Fig. 11 ).

Abdomen. Long, white to yellow, tapering distally; abdominal segment I bearing dorsal and pair of lateral humps; dorsal hump with small brown, seta-bearing sclerites interspersed at each side ( Fig. 5 , black arrow). Dorsum of abdominal segment VIII with pair of long setae ( Fig. 12 ). Abdominal tergite IX mostly translucent, semicircular posteriorly, bearing 6 to 7 pairs of long bristles concentrated submesally and additional smaller setae on each side along posterior margin ( Fig. 12 ). Anal prolegs weakly sclerotized, each with two arched rows of 9 strong, golden erect spines ( Figs 13, 14 , arrows). In caudal view, anal slit with pair of weakly sclerotized perianal pads laterally, each covered with transverse row of 4 or 5 small, obliquely flattened spinules directed mesad ( Fig. 13 ). Anal claws each with three little accessory hooks. Each anal proleg bearing 4 long, dark setae ( Figs 13, 14 ). At base of claw insertion, one long seta projecting caudoventrad with rows of spinules ( Fig. 14 ).

Case. Case made of small rock fragments, slightly curved, not tapered ( Fig. 1 ), without closing membrane posteriorly.

Discussion

Morphological diagnosis of fifth instar larvae of Setodes acutus Navás 1936 from those of other known Maghreb Setodes species.

The Maghreb is part of the West Palaearctic Region; the larvae of only three Setodes species are known from the Maghreb ( Coppa & Tachet 2007). In Algeria, only two Setodes species are known; S. acutus and S. argentipuntellus . The larvae of these species are very similar, sharing most of their morphological features, with only a few diagnostic characters to distinguish them ( Coppa & Tachet 2007; Rinne & Wiberg-Larsen 2017; Wallace 1981; Waringer & Graf 2011; Weinzierl 1999). The larva of S. acutus can be distinguished from the other known larvae of the Maghreb region mainly by the following characters; these characters and several more are provided in a discriminatory matrix ( Table 1 ):

i) Head capsule of S. acutus yellowish , pale, with brown markings ( Fig. 2 ), as in S. punctatus and S. argentipunctellus , versus white in S. viridis .

ii) Aboral end of frontoclypeus apotome of S. acutus ( Figs 2, 3 ) wider than in S. argentipunctellus ( Fig. 15 ).

iii) Ventral apotome of S. acutus ( Fig. 4 ) more nearly rectangular than in S. argentipunctellus ( Fig. 16 ); uniformly brown in both S. acutus and S. argentipunctellus ; pale in S. punctatus , black in S. viridis .

iv) Setodes acutus with four setae in middle of the metanotum around sa 1 sclerite, similar to S. argentipunctellus ( Fig.17 ); about 20 setae in S. punctatus ; pair of tufts each with 4 setae in S. viridis .

v) Metasternum with row of 4 pairs of setae on middle ( sa 1) and with pair of tufts each with four to five setae on anterolateral part ( sa 3) of S. acutus ; 6–30 pairs in S. punctatus , 2–6 pairs in S. argentipunctellus , and 5 pairs in S. viridis .

vi) Foretrochantins of S. acutus ( Fig. 10 ) broader and each with its apical extension shorter and more pointed than in S. argentipunctellus .

vii) Dorsal hump of abdominal segment I of S. acutus with small brown seta-bearing sclerites interspersed with pair of longer setae on each side of hump ( Fig. 5 ); more than one pair of setae on each side of this hump in S. punctatus ; dorsal hump without sclerites in S. argentipunctellus ( Fig. 17 )

viii) Abdominal tergite IX of S. acutus weakly sclerotized and bearing 6 or 7 pairs of long terminal setae posteromesally ( Fig. 12 ); 5 pairs of short setae in S. punctatus , 3 or 4 pairs of long setae in S. argentipunctellus ( Fig. 19 ), and S. viridis with one pair of shorter dorsolatersal setae.

ix) Anal prolegs of S. acutus ( Figs 13, 14 ) weakly sclerotized, each with two arched rows of 9 golden, erect spines, similar to those of S. argentipunctellus ( Fig. 20 ); one row in S. viridis ; three pairs of sclerotized plates bordered with many teeth in S. punctatus .

Ecology

In Algeria, larvae of S. acutus were found in two streams, one in the northwestern Tafna River Basin ( Bemmoussat-Dekkak et al. 2021) and the second in the northeastern Aures Region. The altitudes are 916 and 1,060 m a.s.l., respectively. As for the congeneric species ( Graf et al. 2008), S. acutus has a short emergence period; pupae and adults were found during July–September in Bouailef Stream ( Dambri et al. 2020). In addition, at this locality, the stream width is 60 cm with slow current and water depth about 0.5 cm ( Figs. 21, 22 ). The substrate of the stream bottom is composed of pebbles, gravel, and sand. The water temperature of the stream on the day of sampling was 27°C, conductivity was 560 μS/cm, and dissolved oxygen was 5.60 mg /L. Setodes acutus is sympatric with Hydropsyche maroccana Navás 1936 and Hydroptila vectis Curtis 1834 .

According to Bemmoussat-Dekkak (2022), S. argentipunctellus was recorded at the same site with S. acutus at Chouly Wadi Yebder stream; this sampling site was characterized by moderate flow and water depth about 20 cm. Water temperature was moderate with an annual average of 18°C and dissolved oxygen was 8.41 mg /L. Pupae of this species were found attached to rocks and pebbles among aquatic vegetation. Adults are on the wing from July to August.

According to Merrill & Wiggins (1971) and Wiggins (1996), larvae of the North American species Setodes incertus ( Walker 1852) are burrowers in sand, orienting the case vertically with the upper end nearly level with the substrate surface; they sometimes also attach both ends of the case to a rock in fast current. The open ends allow the larva to reverse direction inside the case to burrow into the sand without reversing direction of the case itself or to attach both ends of the case to a stone. The spinous posterior end of the larva helps to deter intruders entering the open end when the larval head is at the other end of the case, a structural and behavioral complex considered to be an example of phragmosis ( Bishoff et al. 2023). As for S. incertus , the case of S. acutus is cylindrical, untapered, and with both ends open and unrestricted by silk and the apex of the abdomen is armed with strong spines, suggesting that S. acutu s and other species of Setode s may exhibit similar behaviors.

Figures

FIGURES 1–14. Setodes acutus Navás 1936, 5th instar larva and its case. 1, habitus of larva in its case, dorsal; 2, head, dorsal (Scale bar: 0.5 mm); 3, head, dorsal; 4, head, ventral (Scale bar: 0.5 mm); 5, dorsal hump of abdominal segment I (lateral humps collapsed; Scale bar: 0.5 mm); 6, head and thorax, dorsal (Scale bar: 1 mm); 7, thorax, dorsal; 8, left foreleg, posterior; 9, left foreleg, posterior; 10, right foretrochantin with elongate apical extension (dashed circle, black arrow); 11, hind leg, anterior view; 12, abdominal tergites VIII and IX, dorsal; 13, anal prolegs and anal claws, ventral (Scale bar: 0.5 mm); 14, anal prolegs and anal claws, double row of spines (black arrows), left ventrolateral.

FIGURES 15–20. Setodes argentipunctellus McLachlan 1877, 5th instar larva. 15, head, dorsal (Scale bar: 0.2 mm); 16, head, ventral (Scale bar: 0.2 mm); 17, head and thorax, dorsal; 18, right foreleg, anterior; 19, abdominal tergites VIII and IX, dorsal; 20, anal prolegs and anal claws, caudal (Scale bar: 0.5 mm)

FIGURES 21, 22. Collection site of S. acutus; Bouilef stream, NE Algeria.

Tables

TABLE 1. Discriminatory matrix for the known Setodes larvae from the Maghreb region.

Species	Head parietal coloration	Frontoclypeal apotome coloration	Mesonotum	Metanotum/ Metasternum	Foretrochantin	Hind legs	Setae and sclerites on abdominal tergite IX	Anal prolegs
S. acutus	Yellowish with pale brown marks and with pale rings around eyes	Pale yellow with dark color at each side of clypeus forming “horseshoe” shape extending posteriorly from anterior end	Pair of sclerites pale yellow with conspicuous brown spots	Membranous, with weak anteromedian sclerite/Row of 4–6 setae on each side at median position	Rectangular, with short round extension at distal end	Without articular membrane.	Mostly translucent sclerite, bearing 6 pairs of long setae concentrated submesally	Anal prolegs each with 2 rows of strong, posteriorly directed spines; with weak anal lobe spinules
S. argentipunctellus	Uniformly brown with pale rings around eyes	Same color as head capsule	Pair of sclerites brown with darker rounded spots	Membranous, with weak anteromedian sclerite/Row of setae at median and posterior positions (5 on each side)	Rectangular, with short round extension at distal end	Without articular membrane.	Weakly sclerotized, bearing pair of rows of setae concentrated submesally	Anal prolegs with 2 rows of strong, posteriorly directed spines; with dense anal lobe spinules
S. punctatus	Unpigmentedpale, no pale rings around eyes	Same color as head capsule	Pair of sclerites unpigmented	Membranous, with weak sclerites anteriorly/ Double row of long setae posteriorly (15 on each side) directed forward	Rectangular, with short round extension at distal end	With articular membrane at tarsi.	Three pairs of sclerites each bearing few relatively short setae	Anal region surrounded by tooth-edged plates
S. viridis	White, no pale rings around eyes	Same color as head capsule	Pair of sclerites pale	Membranous, with weak anteromedian sclerite/ Row of 5 setae on each side at median position	Rectangular, with elongate extension at distal end	Without articular membrane.	Mostly translucent sclerite, bearing one spinule row	Anal prolegs each with single row of strong, posteriorly directed spines