Biometry. Body length of 5th instar larvae ranging from 9.7 to 10.8 mm, head width from 1.76 to 1.90 mm (n = 2).

Head. Head capsule coarsely granulated, almost circular in shape, hypognathous (Figs 1, 3), dorsally chestnut to black brown, with blackish muscle attachment spots. Ventral parietalia sections, submentum, maxillolabial sclerites and premandibular areas yellowish (Figs 2, 3). Eyes surrounded by whitish ring (Fig. 3). In lateral view, head capsule bearing carina which extends from anterior eye margin to anterior corner of frontoclypeus (Fig. 3, black arrow). Complete set of 18 pairs of primary setae on head capsule (nomenclature sensu Wiggins 1998); no additional spines or spinule areas as known from other Drusinae larvae (e.g., Ecclisopteryx spp., Drusus trifidus McLachlan, 1868, most of the Drusus bosnicus group except Drusus ramae Marinković-Gospodnetić, 1971) present. Frontoclypeus bell-shaped, with narrow median constriction (Fig. 1). Antennae located dorsally on central section of lateral carinae (Fig. 3), each consisting of 1 short cylindrical base and 1 prominent lateral seta. On each parietal, 10 dorsal and 2 ventral primary setae present (Figs 1, 3). Each side of frontoclypeus bearing 6 primary setae, 3 of them along anterior border. Labrum yellowish brown, anterolateral margins with setal brush and primary setae 1–3; dorsally, setation consisting of primary setae 4–6 (Fig. 1). Yellow ventral apotome funnel-shaped with postgenal suture reaching approximately 29% of apotome length (Fig. 2). Black brown mandibles (sometimes brownish on distal half; Fig. 3) spoon-shaped, lacking terminal teeth along edges as well as ridges in central concavity (Figs 1, 3).

Thorax. Pronotum chestnut brown and very coarsely granulated, with adjacent series of granuli creating ribbed structures (Figs 3, 4). Posterior margin thickened and darkly striped; no pronotal transverse groove at end of anterior 3rd (Fig. 5). In lateral view, dorsal profile of pronotum low, with posterior 2/3rds being evenly rounded (Fig. 5). Along anterior pronotal border 2 setal rows present, including: i) dense fringe of short, curved, fine, yellow setae, ii) continuous row of widely-spaced long, straight, dark setae meeting at pronotal midline (Figs 1, 3, 4, 5). Each pronotal half bearing in total 35–45 dark setae of varying lengths. In addition, pronotal surface covered by high number of tiny, pale, curved, recumbent setae (Fig. 5); no spines as present in other Drusinae (e.g., Drusus trifidus). Prosternite inconspicuous, pentangular in shape, pale yellow, with light brown posterior border. Prosternal horn present (Fig. 3).

Mesonotum completely covered by 2 yellow brown to dark brown sclerites with anterolateral sections bearing darkest coloration. Median to dark brown muscle attachment spots present, lateral and posterior margins darkly sclerotized (Fig. 4). Counts for mesonotal setae (nomenclature sensu Wiggins 1998): anterior setal group sa1: 8–15, posterior group sa2: 25–30, lateral group sa3: 30–35 (Fig. 4). In addition, small number of tiny, pale, curved, recumbent setae present.

Metanotum partially covered by 3 pairs of yellowish grey sclerites (Fig. 4). Anterior metanotal sclerites (sclerites of setal area 1, sa1, sensu Wiggins 1998) very large, ovoid, tapering laterally. Medially, the 2 sclerites strongly divergent, widely spaced; their median separation nearly as high as their length along the longitudinal body axis (Fig. 4). Posteromedian sclerites (sclerites of setal area 2, sa2, sensu Wiggins 1998) small, triangular, with approximately 20 setae per sclerite, framing row of setae (Fig. 4). Lateral sclerites (sclerites of setal area 3, sa3, sensu Wiggins 1998) with approximately 25–30 setae concentrated in cranial section (Fig. 10). Groups of setae present between sa2 and sa3 (Fig. 4).

Legs light brown with numerous setae on coxae, trochanters, and femora; tibiae and tarsi sparsely setose. Femora with several proximodorsal setae (e.g. Fig. 8, black bracket), and with setation on anterior and posterior faces; fore femora with 4, mid and hind femora with 3 yellow ventral-edge setae; no minute spines along ventral edges present. Foreleg coxa, femur and tibia wider than those of mid- and hind legs. Fore and mid trochanters with setae only on proximal sections; fore trochanters additionally with distal ventral trochanteral brush. Mid- and hind tibiae with dorsal setae only on distal 3rd (Figs 8, 9).

Abdomen. Abdominal segment I with 1 dorsal and 2 lateral fleshy protuberances (Figs 4, 10). Continuous transverse row of setae present anterior of dorsal protuberance (comprising fused setal areas sa1, sa2, sa3, sensu Wiggins 1998), stretching laterally from dorsal sections of lateral protuberances; posterior of dorsal protuberance, another row of setae present (Fig. 4). All these setae with small basal sclerites. Lateral protuberances without posterior sclerites (Fig. 10). Anterior of each lateral protuberance a continuous band of anterolateral setae connected to each dorsal and ventral sa3 setal group (Fig. 10). Abdominal sternum I with fused setal areas sa1, sa2 and sa3, creating continuous field of setae, therein occurs pair of central large basal sclerites with irregular borders and small number of randomly distributed basal sclerites of smaller diameter (Fig. 6). Abdominal dorsum VIII with 2 long and 2 short posterodorsal setae (pds) (Fig. 11 pds); only 1 posterolateral seta present on each half of abdominal dorsum IX (Fig. 11, arrows). Abdominal dorsum IX bearing beige pentangular sclerite with 8 long and several short setae (Fig. 11). Beige anal prolegs are of limnephilid type with medium brown anal claws, each with 1 small accessory hook (Fig. 12).

All gills as single filaments (Fig. 10). Dorsal gills present at most from abdominal segments II-VII (presegmental positions). Ventral gills present from segment II (presegmental) to segment VII (postsegmental). In lateral row, gills present on segments II-III only (ventrolateral position). Lateral fringe extends from anterior border of segment III (Fig. 10f) to middle of segment VIII.

Case. Larval case 8.5–12.1 mm long (n= 2), curved, conical (width at anterior opening 2.9–3.2 mm, at posterior opening 1.9–2.2 mm), consisting of mineral particles (sand grains of mixed size; Fig. 13).

Within the framework of the larval key by Waringer and Graf (2011), Drusinae larvae are separated from other Trichoptera species by the following features:

• sclerites present on pro-, meso- and metanota; mesontum completely covered by 2 sclerites in close contact separated by a straight suture; metanotum incompletely sclerotized by 6 sclerites (Fig. 4);
• prosternal horn present (Fig. 3);
• fleshy protuberances at abdominal segment I present dorsally and ventrally (Figs 4, 10);
• gills consisting of single filaments only (Fig. 10);
• transverse groove lacking at the anterior 3rd of the pronotum (Fig. 5) except in Leptodrusus budtzi (Fig. 16).

Within the subfamily Drusinae, Drusus vinconi is characterised by the following set of morphological details:

• mandibles spoon-shaped (Figs 1, 3);
• head capsule without additional spines or spinules (Fig. 1);
• anterior-row setae present near dorsal pronotal midline (Figs 1, 3);
• dorsal gills present (Fig. 10);
• dorsal edge setae restricted to distal 3rd of mid and hind tibiae (Figs 8, 9);
• basal sclerites of setae on abdominal sternum I separated (Fig. 6);
• pronotum evenly rounded (Fig. 5).

At this position in the key, Drusus vinconi appears together with Drusus annulatus, Drusus biguttatus (Pictet, 1834), Drusus ingridae, Hadimina torosensis Sipahiler, 2002 and Leptodrusus budtzi. These species are easily distinguished by differences in dorsal profile, presence/absence of the lateral carina on the head capsule, number of proximo-dorsal setae on mid-and hind femora, origin of abdominal lateral fringe, and geographic distribution (Table 1).