The larva of Drusus dudor Oláh, 2017, including an updated key to larval Drusinae Banks, 1916 (Insecta, Trichoptera, Limnephilidae)

Abstract The caddisfly Drusus dudor Oláh, 2017 (Limephilidae: Drusinae) was described from the Northwestern Italian Alps. We provide a detailed description of the larva, based on material from the Italian Province of Piemonte. Information on the morphology of the 5th larval instar is given, and the most important diagnostic features are illustrated. The larva is included in an updated key to larval Drusinae where D. dudor keys together with Drusus aprutiensis Moretti, 1981, D. camerinus Moretti, 1981, D. croaticus Marinkovic-Gospodnetic, 1971, D. mixtus (Pictet, 1834), and D. nigrescens Meyer-Duer, 1875. The species can be reliably separated by the morphology of the pronotum, the shape of the metanotal sclerites, and by morphological details of abdominal sternum I.


Introduction
The limnephilid subfamily Drusinae is characterized by larvae restricted to high-gradient, turbulent, running waters in hard-substrate channels. Drusinae species occur across the Eurasian mountain ranges from the Iberian Peninsula to the Iranian Highlands. In their larval stages, Drusinae have evolved an amazing diversity of body morphologies coupled with unique feeding strategies; the mechanisms behind these diversifications are not clear, but they are likely due to evolutionary opportunities formed by variable and limited food availability in space and time, the ever-changing hydraulic stress of their lotic environment, or the presence of predators . Omnivorous shredders (e.g., Drusus alpinus Meyer-Duer, 1875) reflect a potentially basic evolutionary level within the subfamily as they share many characters with their limnephilid congeners in other subfamilies. This entails teeth along the edges of the mandibles without further specializations of the body and characterizes an omnivorous, shredder feeding type (Graf et al. 2009). Species of this group prefer springs and spring brooks. Species in the second group that comprises epilithic grazers only, develop mandibles without terminal teeth as an adaptation to their feeding ecology (e.g., Drusus nigrescens Meyer-Duer, 1875; Drusus bolivari (McLachlan, 1880); Vieira-Lanero et al. 2005). Finally, larvae of filtering carnivores exhibit toothed, shredder mandibles, filtering spines on legs and the first abdominal sternum, and unique head capsule complexities, unknown in other caddisfly larvae. In this last group, species with bare, flattened, or concave heads are basal to a strongly supported clade comprising all other carnivorous Drusinae . These larvae develop a modified setation or flocculent hair cover on their strongly modified head capsule (e.g., Drusus discolor (Rambur, 1842)). In addition, all filtering carnivores develop four distinct cephalic setae that likely serve as sensory organs to detect the optimal hydraulic niche for filtering.
About 75% of the known Drusinae species are endemics limited to a single or very few mountain ranges, and potentially undergo accelerated evolutionary processes including speciation and diversification; this is reflected by the high species diversity of this group: in his 'Trichoptera World Checklist', Morse (2019) lists a total of 174 extant Drusinae species with only a fraction of them known in the larval stage. In the present paper we address this lack in larval taxonomy by describing the larva of Drusus dudor and by including this larva in a comprehensive larval key to Drusinae.
Abdomen. Abdomen cream colored ventrally and laterally, light purple dorsally, with chloride epithelia on abdominal segments II-VII. Abdominal segment I with 1 dorsal and 2 lateral fleshy protuberances (Fig. 4, lp, dp). Continuous transverse row of setae (some with small basal sclerites) present anterior of dorsal protuberance, comprising fused sa1, sa2, sa3 (Fig. 4), meeting dorsal sections of lateral protuberances. No setae posterior of dorsal protuberance (Fig. 4). Lateral protuberance without posterior sclerite; anterior of each lateral protuberance, a continuous band of anterolateral setae connects to dorsal and ventral sa3 setal groups (Fig. 10, bracket). Abdominal sternum I with basal sclerites of central sa2 setae fused, thereby creating multilobed pattern (Fig. 11). In total, 91-105 setae present on abdominal sterum I (Fig. 11). Abdominal segments II-VIII with 2 dorsal setae (Fig. 12, ds); only 1 posterolateral seta present on each half of abdominal dorsum IX (Fig. 12, pls). Abdominal dorsum IX with dark brown, semicircular sclerite bearing 8 long and several short setae (Fig. 13). Brown to yellow anal prolegs of limnephilid type (Fig. 12), lateral sclerite with 10 dark dorsal and row of 5 dark ventral setae, 3 of the latter very strong and prominent; ventral sole plate with black dorsal stripe and single anterior seta. Anal claw orange, with 1 small dorsal accessory hook (Fig. 14). All gills as single filaments. Dorsal gills present at most from abdominal segments II (presegmental) to VI (postsegmental positions). Ventral gills present from segments II (presegmental) to VII (postsegmental). In lateral row, gills present on segments II-III only (presegemental). Lateral fringe extends from anterior border of segment III to anterior third of segment VIII (Fig. 12, lf ).

Morphological separation of 5 th instar larvae of Drusus dudor from other European Trichoptera
Within the framework of the larval key by , larvae of subfamily Drusinae are separated from other Trichoptera species by the following characters: • 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. 6, ph); • fleshy protuberances at abdominal segment I present dorsally and ventrally (Fig.  4, dp, lp); • gills consisting of single filaments only; • transverse groove typical for other limnephilids with single filament-gills (tribes Chaetopterygini, Stenophylacini) lacking at the anterior third of the pronotum (Fig. 3)  Within the framework of Key 1 presented below (Master key for main groups of larval Drusinae), Drusus dudor belongs to Group C of the Drusinae grazer clade. Information for the identification of the five species belonging to Group C is given in Key 2 in the present paper.

Discussion
In their revision of subfamily Drusinae, Oláh et al. (2017) reduced the number of Drusinae genera from 8 to 2: the generic status of the monotypic genera Anomalopterygella Fischer, 1966, Cryptothrix McLachlan, 1867, Hadimina Sipahiler, 2002, Leptodrusus Schmid, 1955 and Monocentra Rambur, 1842 was downgraded and synonymised with Drusus Stephens, 1835. The same procedure was performed with the original five species included in Metanoea McLachlan, 1880, leaving only two Drusinae genera, Drusus and Ecclisopteryx Kolenati, 1848. In the framework of this revision based on male genital morphology, taxa were organized into 'species groups' and 'species complexes', and former species split into 'sibling groups'. Within this framework, D. dudor (Figs 58-63) is considered a sibling of D. apuanensis Oláh, 2017, D. lepidopterus (Rambur, 1842, D. liguriensis Oláh, 2017, D. piemontensis Oláh, 2017, and D. savoiensis Coppa & Oláh, 2017(Oláh et al. 2017. Drusus dudor is separated from these siblings by a pair of basolateral humps on the paraproct (Fig. 63, dotted ovals) and a single dorsal, sharply pointed spur on each paramere which lacks secondary spines on its base (Fig. 62). Drusus dudor is restricted to the Italian Province of Piemonte (Oláh et al. 2017). The species was collected in small, stony streams near Fondo (Piemonte, Italy) at elevations ranging from 1403 to 2222 m above sea level. The rheophilic larva inhabits springs and spring brooks where it can be observed on the surface of boulders and large stones. According to its mouthpart anatomy, D. dudor is a grazer, feeding exclusively on epilithic algae and biofilm. The 36 adults of the Drusus lepidopterus sibling group (= Monocentra lepidoptera Rambur, 1842) included in the ZOBODAT database (ZOBO-DAT 2019) were sampled between 9 March and 6 November.