The larvae of Sericostoma bergeri Malicky, 1973 and Sericostoma herakles Malicky, 1999 (Trichoptera, Sericostomatidae)

Abstract This paper describes the previously unknown larvae of Sericostoma bergeri and S. herakles (Trichoptera: Sericostomatidae) restricted to European Ecoregion 6 (= Hellenic western Balkan). Information on the morphology of the larvae is given, and the most important diagnostic features are illustrated. Sericostoma bergeri and S. herakles can be easily separated from known sericostomatid larvae of Ecoregion 6 (Schizopelex huettingeri, Oecismus monedula, Sericostoma flavicorne and S. personatum) by the shape of the pronotum, presence or lack of a comma-like marking on the lateral protuberance, by the number of setae on abdominal dorsum IX, and by distribution patterns. With respect to the latter, S. bergeri is a micro-endemic of the Greek Islands of Euboea and Andros whereas S. herakles is an endemic of the Peloponnese. The species are integrated in a dichotomous key including the currently known Sericostomatidae larvae of the Hellenic western Balkan. In addition, ecological information on the two species is provided.


Introduction
From Europe, 18 species of genus Sericostoma Latreille, 1825 are known (Graf et al. 2008;Malicky 2004Malicky , 2005aMalicky , b, 2014, with four species also present in European Ecoregion 6 (= Hellenic western Balkan). From the latter, only Sericostoma flavicorne Schneider, 1845 and Sericostoma personatum (Kirby & Spence, 1826) were described in the larval stage to date (Pitsch 1993). Several years ago, however, Hans Malicky managed to collect larvae and adults of the two remaining Sericostoma species of Ecoregion 6: S. bergeri Malicky, 1973 from the Greek islands of Euboea and Andros and S. herakles Malicky, 1999 from the Peloponnese. This material enabled us to infer reliable diagnostic characters for the larval description and to use this information for integrating the two species in the key of the previously known Sericostomatidae larvae of the Hellenic western Balkan provided by Karaouzas and Waringer (2017).

Material and methods
Three final instar larvae and numerous adults of Sericostoma bergeri were sampled by H. Malicky on the Greek island of Andros at Refmata (37°52'N, 24°50'E, 220 m a.s.l.) on 21 October 1980. In addition, one final instar larva and numerous adults of S. herakles were obtained by the same collector at Kefalarion, Peloponnese, Greece (37°54'N, 22°31'E, 670 m a.s.l.) on 19 May 1974. Immature stages were picked from the mineral substrate with forceps, adults were collected using light traps, and the material was preserved in 70% ethanol. The larvae were studied and photographed using a Nikon SMZ 1500 binocular microscope with DS-Fi1 camera and NIS-elements D 3.1 image stacking software for combining 6-46 frames in one focused image. Species association was enabled by the fact that both larvae and adults were collected at the same locations where other Sericostomatidae were lacking.
Diagnosis. Pronotum with convex ventral border; anterolateral pronotal corner short and knob-like; with black comma-like marking on lateral protuberance; setal counts on abdominal dorsum IX 18-41.
Biometry. Body length of 5th instar larvae ranging from 16.0 to 17.5 mm, head width from 1.69 to 1.76 mm (n = 3).
Head. Head capsule roundish, dorsally medium to orange brown, posterolaterally and ventrally whitish; with slightly granulated surface and large, elongated, orange muscle attachment spots (Figs 1-3). Distinct whitish spade-like patch present around each eye (Fig. 3). With ridge extending from posterodorsal corner of white spade-like ring around each eye (Fig. 3) to anterior parietal margin where ridge creates an inwardly-bending groove bearing the antenna (Fig. 1, arrow). Frontoclypeus with shallow central constriction; a shallow secondary constriction situated subapically near anterior border (Fig. 1). Head capsule with complete set of 18 pairs of primary setae: 10 dorsal and 2 ventral primary setae on each parietal, 6 pairs of primary setae on frontoclypeus. Labrum medium brown, narrowly rectangular, with 6 pairs of primary setae. Submentum separating the genae incompletely, broadly shield-shaped, light brown, with darker brown rectangular anterior sclerotization (Fig. 2). Mandibles blackish brown, of shredder type, with 4 terminal teeth (Fig. 2).
Thorax. Pronotum dark brown (Fig. 5), in some specimens slightly paler on posterior half (Fig. 3). Without transverse ridge present in other caddisfly taxa (e.g., Limnephilinae), heavily sclerotized, with anterolateral corner creating a tiny, knob-like projection (Fig. 4, white circle). The two pronotal plates mesially meeting in a narrow, straight suture; surface smooth ( Legs medium to light brown (Figs 7-9). Foreleg short and stout, femur distally enlarged and flattened, thereby creating an edge interacting with tibia when bent inwards (Fig. 7). Coxa with ventral group of long black setae, trochanter with dense ventral brush of pale, flexible setae. Dorsal edge of femur with large groups of dark setae. Tibia with groups of long dark dorsal and ventral setae and with pale apical spine. Strong tarsal claw sickle-shaped, with stout pale basal spine. Midleg much more slender, coxa weakly sclerotized, femur not enlarged. Hind leg even more slender, tarsal claw elongated, setation less than in other legs (Figs 7-9).
Abdomen. Abdominal segment I with 2 flat, oblique lateral and 1 low, inconspicuous dorsal protuberances (Fig. 10); setation consisting of 1 pair of ventral sa1 setae (Fig. 11, arrows) and 1 lateral protuberance seta per side (Fig. 11 lp). With black comma-like marking on lateral protuberance (Fig. 10, arrow). Gills consisting of tiny single (rarely double or triple) filaments and in presegmental position only. Dorsal gills present at most from abdominal segment I to VI, ventral gills from segment II to VII and lateral gills from II to III. Lateral fringe lacking; however, with lateral rows of tiny serrate lamellae on each side of abdominal segments III to VII (Fig. 12), and with row of forked lamellae on each side of segment VIII (Fig. 13).    (Figs 14, 15pds). Dorsum of each anal proleg with cluster of 35-45 black setae (Figs 14, 15 aps). Lateral sclerite of anal proleg with 28-35 black setae of varying length (Fig. 14 ls). Anal proleg claw with sharply angled crook and dorsal accessory hook (Fig. 14, arrow).
Larval case. Cylindrical, tapering, curved, made of flat sandgrains of approximately uniform size, neatly arranged in a puzzle-like pattern, thereby creating a rather smooth surface (Fig. 16). Case length 15.7 to 17.6 mm, anterior width 3.3 to 4.1 mm, posterior width 2.6 to 2.8 mm (n= 3). Foramen posterior partly closed by a slightly conical, translucent silken membrane with round central hole 0.61 mm in diameter (Fig. 17).
Biometry. Body length of 5th instar larva 13.7 mm, head width 1.93 mm (n = 1). All morphological characters identical to those of S. bergeri except as noted below.
Head. Head capsule dorsally medium brown, whitish coloration on ventral section of parietalia restricted to small oval patches (Fig. 18).
In the context of the Sericostomatidae larvae of European Ecoregion 6 (Hellenic western Balkan region), Sericostoma bergeri, and S. herakles can be identified by the following dichotomic key.

Key to the final instar Sericostoma larvae of European Ecoregion 6 (Hellenic western Balkan)
1 Pronotum with straight ventral border (Fig. 23, dotted line); abdominal dorsum IX with 18-41 setae (as in Fig. 15 pds); without black comma-like marking on lateral protuberance (as in Fig. 24, arrow)  Abdominal dorsum IX with 18-41 setae (Fig. 15); black comma-like marking present on lateral protuberance (Fig. 20, arrow) [Malicky 2014]). The species is univoltine and on the wing from April to August, peaking in May; a single specimen has been collected as late as October (Malicky 2005b). The larvae of both species are detritivore-shredders, feeding on leaf litter from the woody riparian zone and aquatic vegetation, particularly mosses.