Oecetis tripunctata (Fabricius, 1793)

Waringer, Johann & Graf, Wolfram, 2014, The larva of Oecetistripunctata (Fabricius, 1793) (Trichoptera, Leptoceridae), ZooKeys 445, pp. 117-126 : 119-122

Taxon classification Animalia Trichoptera Leptoceridae

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 (Figs 1-2). 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 (Figs 1, 4, 5). Frontoclypeus elongated, narrow, with very shallow central constriction at eye level (Figs 1, 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).

Thorax.

Pronotum covering posterior section of head, light yellowish-brown, semitransparent, without distinct markings and muscle attachment spots (Fig. 5); dense continuous row of straight, pale setae along anterior border; pronotal surface densely covered by high number of pale setae (Figs 1, 4). Pleural sclerites pale, semicircular, with thin, blackish-brown ventral margins; anteriorly, with pale, large, ear-like protrochantin bearing numerous pale setae (Fig. 6p). Prosternal horn absent.

Mesonotum completely covered by two sclerites, yellowish and paler than pronotum, with distinct markings and muscle attachment spots (Figs 4, 5); dense cover of pale setae on the surface and along anterior border. Pleural sclerites pale, with thin, blackish-brown ventral margin (Fig. 4). Mesoventer without setae.

Metanotum without sclerotization except by pleural sclerites and with dense dorsal setal cover; pleural sclerites arrangement as on mesonotum (Fig. 4).

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).

Abdomen.

First abdominal segment with one dorsal (Fig. 4dp) and two lateral protuberances (Fig. 4lp), the latter with very pale and inconspicuous lateral sclerites. Lateral fringe present from segments 3-7, consisting of extremely short, pale hairs. Dorsum of 9th abdominal segment with 6-8 setae (Fig. 10ds). Anal prolegs medium brown, anal claws dark brown, each with two small dorsal accessory hooks (Fig. 11ah). Anal region without rows of spines and tooth-edges figs (Fig. 11).

Gills single-filamented; dorsal gills present at most from 2nd segment (presegmental position) to 3rd segment (presegmental position); ventral gills only at 3rd segment (presegmental); lateral gills absent.

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)

Within genus Oecetis , Oecetis tripunctata keys together with Oecetis intima McLachlan, 1877 and Oecetis notata (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 Oecetis 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 waters (e.g., Oecetis 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 Oecetis 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, Oecetis 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 ).