The larva of Oecetis tripunctata (Fabricius, 1793) (Trichoptera, Leptoceridae)

Abstract Oecetis tripunctata is a widely distributed leptocerid in Europe, ranging from the Iberian and Apennine peninsulas and the Central and Western European highlands to the plains of Eastern Europe. The long, single-bladed mandibles are indicative for a predacious lifestyle. This paper describes the previously unknown larva of Oecetis tripunctata. Information on the morphology of the 5th larval instar is given, and the most important diagnostic features are illustrated. A synoptic key for the European species of Oecetis is also provided. In the context of existing identification keys the larva of Oecetis tripunctata keys together with Oecetis intima and Oecetis notata. Oecetis tripunctata is separated from the other two species by the fact that a double row of long setal fringes is lacking at the hind tibiae and that several long setae are present on the protrochantinus.


Introduction
From Europe, ten species of genus Oecetis McLachlan, 1877 are known (Malicky 2004(Malicky , 2005. However, the larvae of only seven species were described up to now: Oecetis furva (Rambur), O. intima McLachlan, O. lacustris (Pictet), O. notata (Rambur), O. ochracea (Curtis), O. struckii Klapálek (unvalid synonym = Paroecetis Lestage) and O. testacea (Curtis). References for the larval descriptions available are listed in Table 1. The larvae of O. canariensis Brauer, O. grazalemae Gonzalez & Iglesias and O. tripunctata (Fabricius) were unknown. Recently, however, W. Graf managed to collect larvae of O. tripunctata at the Thaya river in the northern part of Lower Austria. Oecetis tripunctata is a widely distributed leptocerid in Europe, ranging from the British Isles and the Iberian and the Apennine peninsulas and the Central and Western European highlands to the plains of Eastern Europe (Graf et al. 2008). Across Asia, this species ranges even to Bali, with many records in the countries between. Oecetis tripuncatata is probably the caddisfly species with the widest distribution range currently known worldwide (Malicky 2005(Malicky , 2009). This taxon was described by Fabricius (1793) in the genus Phryganea, redescribed as Setodes punctatella (Rambur, 1842), as Oecetis buitenzorgensis (Ulmer, 1951) and as O. alexanderi (Kumanski, 1976). Oecetis buitenzorgensis was declared as synonym of O. tripunctata (Fabricius, 1793) by Malicky (2009b) whereas O. alexanderi was declared as synonym of O. tripunctata by Kumanski (1988) (Yang and Morse 2000;Morse 2014). With our description of its larva, proposed here, the identification of eight out of ten European Oecetis species is now possible even without an adult male as frequently practiced within caddisflies studies.

Material and methods
The larvae were sampled on 18 December 2012 by Wolfram Graf in the Thaya river at Hohenau, a short distance upstream of its confluence with the March river in Lower Austria. The catchment of the Thaya is situated within the granite and gneiss complex of the Bohemian highlands. Due to its low slope, the river meanders strongly and has created some scenic, deeply carved valleys descending up to 150 m steeply downwards from the plateau of the surrounding highlands. The watershed area of the Thaya is 13.319 km² with an average discharge of 43.9 m³ s -1 and a Strahler stream order of seven. Some river stretches of the Thaya situated within the Czech Republic have been transformed in reservoirs used for irrigation, drinking water supply and hydroelectric power plants (Waringer 2003). A hand net was used to collect larvae of Oecetis tripunctata in the Thaya river at Hohenau in the northern part of the federal state of Lower Austria on 18 December 2012 (48°36'04"N, 16°56'10"E, 161 m above sea level). The material was preserved in 70% ethanol. A Nikon Labophot 2 microscope and a Nikon SMZ 1500 binocular microscope with DS-Fi1 camera and NIS-elements D 3.1 image stacking software for combining 8-42 frames in one focused image were used to study and photograph the larvae.
Species association was enabled by the fact that all other five Oecetis species reported from Austria (Malicky 2009a: Oecetis furva, O. lacustris, O. notata, O. ochracea, O. testacea) are well known in the larval stage (e.g., Wallace et al. 2003;Waringer and Graf 2011), and the new taxon is morphologically very different from the other species. Additionally, close to our larval collecting site more than 300 adults of O. tripunctata were sampled by mobile light traps.

Oecetis tripunctata (Fabricius, 1793)
Description of the 5th instar larva Diagnosis. Mandible sickle-shaped, with only one cutting edge; head capsule without distinct dark patches; hind tibiae without double row of long setal fringes; number of long setae on protrochantinus is > 1; basal setae on 2nd and 3rd tarsal claw rudimentary.
Biometry. Body length of 5th instar larvae ranging from 2.7 to 4.5 mm, head width from 0.66 to 0.70 mm (n = 15).
Head. Head capsule surface very smooth, roundish and hypognathous with pale yellow coloration. Light muscle attachment spots on frontoclypeus and parietalia very indistinct (Figs1-2). White ring present around eyes (Fig. 1). In addition to complete set of primary setae, head capsule densely covered by pale secondary setae, especially at anterolateral corners, dorsally of eyes and over frontoclypeus (Figs1,4,5). Frontoclypeus elongated, narrow, with very shallow central constriction at eye level (Figs1, 5). Subocular ecdysial line running from foramen occipitale to lateral section of parietalia, ventrally to the eyes. Anteriorly of the eyes the subocular ecdysial line bends dorsally, eventually meeting frontoclypeal suture (Fig. 1e). Antennae slender, approximately six times longer than its width at widest section, situated at extreme anterior end of parietalia and originating from a socket-like ridge (Fig. 5a). Labrum light brown, quadrangular, with anterior median notch (Fig. 1); in addition to 6 pairs of primary setae, with numerous secondary setae on dorsal surface (Fig. 1ss). Maxillary palps very long, distinctly protruding labrum (Fig. 2mp). Ventral apotome trapezoidal in shape (Fig. 2a), pale yellow with light brown anterior border; apotome not separating parietalia posteriorly. Mandibles single-bladed, sickle-shaped, with only 1 cutting edge; with sharp terminal tooth and 1-2 subapical secondary teeth (Fig. 3).
Legs yellowish, with very numerous setae, especially on coxae, trochanters and femora (Figs 7-9); tibiae and tarsi undivided and without central constrictions. Femur of foreleg much wider than those of mid-and hind legs. Claw of mid leg curved and not hook-shaped (Fig. 8). Long setal fringes for swimming lacking on hind legs; distal section of hind trochanter broadened and club-like (Fig. 9).
Case. Larval case 3.0-3.7 mm long (n = 15), curved, tapered (width at anterior opening 1.2-1.5 mm and at posterior opening 0.6-0.7 mm), consisting of mix of mineral particles of unequal grain size (Fig. 12a). Posterior case opening partly closed by terminal silken membrane with oval foramen 0.2 mm wide and arranged transversally; ventral lip of membrane slightly protruding, creating an upward-directed twist of the foramen (Fig. 12b).
Synoptic key for the currently known European Oecetis larvae (final instars; Table 1 (Rambur, 1842) (Table 1). Oecetis tripunctata is easily separated from the other two species by the fact that a double row of long setal fringes is lacking at the hind tibiae (Fig. 9) and that several long setae are present on the protrochantinus (Fig. 6).

Biological remarks
Our collection time of the larvae is in accordance with the reported spring to summer emergence and flight periods of the species; the emergence period is short and mostly limited to two months or less (Graf et al. 2008). In a light trap study from the nearby river March, we observed O. tripunctata to be on the wing only from June 27th to August 3rd (Waringer and Graf 2006).
As pointed out by Wiggins (1996), Oecetis larvae are bottom-dwellers covering a wide range of habitats from lentic to lotic environments and may be even collected from brackish  . 13)?
waters (e.g., O. intima; Lepneva 1964). In Europe, the preferred habitats are lowland rivers with low current velocities, e.g. the Raab and the March systems in Austria. The long, single-bladed predatory jaws of O. tripunctata and most other known Oecetis larvae are unusual among cased caddisflies; they are used for catching worms and chironomid larvae which are ingested whole (Wallace et al. 2003). Interestingly, O. struckii, once attributed to genus Paroecetis Lestage, is unique in that the mandibles are double-edged (Fig. 13, arrows). Nevertheless, Wiberg-Larsen and Waringer (1998) reported also for this Oecetis species only animal remains in the foregut (Testacea, Hydrachnellae, Oribatei, Cladocera, Asellus, Chironomidae).