Holocentropus insignis Martynov 1924

Description of the fifth instar larva of Holocentropus insignis

Head. Head capsule longer than broad in dorsal view with maximum width of 0.94–1.24 mm (N = 258; Table 1). Dorsal and lateral sides with smoky muscle attachment spots (Figs. 2, 3). Dorsal side with striking colour pattern, with dark bands of pigment alongside frontoclypeal suture and coronal suture (Fig. 2). Pale median stripe of frontoclypeus lacking pigment from posterior to anterior part. No light areas alongside constriction of frontoclypeus. Dark stripes of frontoclypeus at its immediate point of constriction clearly darker than other pigment on head. Parietals with light areas around eyes. Ventral side light in colour, with smoky muscle attachment spots on posterior half (Fig. 4). Submental sclerites present and fused. Labrum light brown, with lighter patch medially (Fig. 2). Left mandible with three teeth on each of dorsal and ventral blades, and sharp apical tooth; central concavity of left mandible with brush of setae near ventral blade. Right mandible with central concavity lacking brush of setae and with only two teeth on dorsal blade (Fig. 5). Shape of prelabio-hypopharyngeal lobe elongate and tapered to apex, basal portion of dorsal surface of lobe and ventral surface sclerotised, prementum not distinguishable. Labial palpi present and appressed to sides of prelabio-hypopharyngeal lobe.

Thorax. Anterior and posterior margins of pronotum distinctly brown, contrasting with lighter background. Light brown spots dispersed over pronotum. Pronotal hind angles extending ventrally and coming into contact on sternum behind procoxae. Meso- and metanota membranous and with primary setae only. Shape of foretrochantins acute and elongate, each fused with its episternum and without suture (Fig. 3). Dorsal and distal margins of all coxae black. Proximal margins of first pair of legs black; second and third legs brown (Fig. 6). Tarsi of first legs about as long as tibiae; brown-yellow compared to lighter tibiae, femora, trochanters, and coxae; each with short fringe of ventral setae. Tarsi of second and third legs about as long as tibiae; colour same as rest of legs; lacking fringes, but each with distal crown of feathered setae ventrally. All legs with numerous setae on coxae, trochanters, and femora; tibiae and tarsi with only small numbers of setae. Additional setae present on anterior faces of all femora. Claw of each leg with basoventral spine (Fig. 6). Length ratios of first, second, and third legs on each side 1:1.2:1.34.

Abdomen. Terga and sterna of abdomen similar in colour. Each side of segments II–VIII with lateral fringe of setae, but on segment IX only one lateral seta present. Terga I–IV and IX each bearing one pair of submesal setae, terga V–VIII each with two pairs of submesal setae. Pair of submesal setae inserted on each of sterna I–VIII, but two pairs on segment IX. Basal segment of each anal proleg longer than distal segment; basal segment lacking secondary setae; anal claw curved at right angle, without blunt teeth on inner margin, and without dorsal accessory spine (Fig. 7); membrane (joint) of anal proleg and claw with distinct dark X-figure in dorsal view (Fig. 8).

Morphological distinction of fifth-instar larva of Holocentropus insignis from those of other European

Trichoptera.

Larvae of the family Polycentropodidae are mainly predacious (sometimes filtering) and build various kinds of fixed retreats and associated capture nets. They share the following set of morphological features (Lepneva 1970; Edington & Hildrew 1995; Wiggins 1998; Chamorro & Holzenthal 2011):

- transportable case absent, constructing fixed retreat of silk, variously shaped;

- labrum sclerotized, anteriorly rounded;

- labium elongate, extending to anterior margin of head; anterior apex blunt;

- three pairs of legs approximately of same size;

- sclerites of each foretrochantin (episternum + epimeron) fused, anterior apex pointed;

- pronotum sclerotized, meso- and metanota membranous;

- no gills on abdominal segments I–VIII; anal papillae may be apparent;

- anal prolegs long, flexible; each with basal segment at least as long as distal;

- anal claws well developed, occasionally with slender dorsal accessory hooks.

Distinction of the genus Holocentropus McLachlan 1878 from other genera of family Polycentropodidae:

- proximal segment of anal prolegs lacking prominent setation (lacking also in Neureclipsis);

- tarsus of each first leg not distinctly shorter than tibia (shorter in Polycentropus);

- anal proleg claws without blunt teeth on inner margin (blunt teeth present in Cyrnus);

- anal proleg claws not obtusely angled (obtuse in Plectrocnemia);

- membranous connection (joint) of each anal proleg and claw (dorsal view) with strikingly dark sclerotized Xfigure (occurring in Holocentropus, Polycentropus, Plectrocnemia, Cernotina).

Diagnosis: Since the larva of H. varangensis is unknown, we are limited to compare the larva of H. insignis with those of the three European species of the genus for which larvae are known. Within the genus Holocentropus, H. insignis bears morphologically the strongest resemblance to H. picicornis, both having distinct dark bands alongside the frontoclypeal suture (Figs. 2, 12, 13), whereas such bands are absent in H. dubius (Fig. 10). Holocentropus stagnalis also has dark bands alongside the frontoclypeal suture, but the head is relatively broader than that of H. insignis and H. picicornis; furthermore, the median stripe of the frontoclypeus is darkly coloured anteriorly in H. stagnalis (Fig. 11), whereas this pigmentation is absent in the other two species (Figs. 2, 12). Holocentropus picicornis is separated from H. insignis by the light areas alongside the constriction of the frontoclypeus (Fig. 12), completely lacking in H. insignis (Fig. 2), and the conspicuous dark spots laterally and ventrally on the genae (Figs. 12, 13, 14) are only faint or smoky in H. insignis (Figs. 3, 4). The close relationship between the two species is confirmed by barcoding data using mitochondrial DNA (Fig. 9).

Key to the known European larvae of the genus Holocentropus

This key is largely based on that of Rinne & Wiberg-Larsen (2017).

1. Dorsal surface of head with distinct dark bands alongside frontoclypeal suture (Figs. 2, 3, 11–13)....................... 2

- No distinct dark bands alongside frontoclypeal suture (Fig. 10).......................................... H. dubius

2. Pale median stripe of frontoclypeus with dark pigmentation at its anterior end (Fig. 11); head relatively broad (ratio length/ width 1.02–1.16)............................................................................. H. stagnalis

- Pale median stripe of frontoclypeus without any darker pigmentation at its anterior end (Figs. 2, 3, 5, 12, 13); head more elongate (ratio length/width 1.11–1.35)........................................................................ 3

3. Small, pale patches present alongside constriction of frontoclypeus (Fig. 12); dark stripes of frontoclypeus at its immediate point of constriction same color as other pigment on head (Fig. 12); genae laterally and ventrally with distinct dark muscle attachment spots (Figs. 12–14)................................................................. H. picicornis

- No small, pale patches present alongside constriction of frontoclypeus (Fig. 2); dark stripes of frontoclypeus at its immediate point of constriction clearly darker than other pigment on head (Fig. 2); genae laterally as well as ventrally with light, smoky muscle attachment spots only (Figs. 3, 4)........................................................... H. insignis

Distribution and habitat. Holocentropus insignis has a northern distribution in Europe, the most southern sites being in northern Belgium and the Netherlands. Furthermore, it is recorded from northern Germany, Denmark, Lithuania, Latvia, Estonia, Finland, Sweden, Norway, and European Russia (Neu et al. 2018; Fig. 15). Holocentropus insignis has been recorded from other regions in European Russia (Kola Peninsula, Karelia, and the Leningrad and St. Petersburg regions) and Siberia (Yakutia) also, but no exact coordinates are available (Ivanov 2011).

Most records concern adults, and consequently the larval habitats cannot be established with certainty. However, larval records from the Netherlands, Denmark, and Finland nearly all seem to be from similar habitats, being rather shallow, small (typically 50–1000 m 2) pools or ponds with acid water, rather rich in humic substances, and an abundant growth of submerged Sphagnum mosses (Fig. 1). These sites were typically located in oligotrophic fens, often with an emergent vegetation dominated by Eriophorum angustifolium and Carex spp. Danish sites have a pH ranging from 4.3-4.6 and the water column is dominated by Sphagnum denticulatum or S. cuspidatum (Table 2). The collection site in the Netherlands (Fochteloërveen) is a treeless, water-saturated, raisedbog relict with a dominance of Sphagnum spp., E. angustifolium, Carex rostrata, and Calluna vulgaris. The submerged vegetation of a second (Notterveenplas, Wierden; 1 larva), third (Deldenerzijdevennen, Oele; 3 larvae), and fourth (Zandveen, Dwingeloo; 1 larva) collection site in the Netherlands are dominated by S. cuspidatum, S. denticulatum, and Warnstorfia fluitans. pH at a Dutch site was measured only once and was 4.7.

Kubiak & Peters (2010) suggested that H. insignis might be classified as tyrphophilic (i.e., associated with raised bogs). However, historical and recent records from Belgium, Denmark, the Netherlands, and Germany (Wiberg-Larsen 1986; Lock et al. 2013; Tempelman & Lock 2012; this study) only partly confirm the association with raised bogs. Only half of all fifteen new records are from pristine or degraded raised bogs, the other half stemming from oligotrophic fens located in heathland areas. According to Rinne (unpublished), the main habitats in Finland are not raised bogs, but rather oligotrophic (acid) ponds, smaller lakes, and wet mires with growth of Sphagnum mosses. In the northern provinces, however, species may also inhabit more eutrophic mires, groundwater-fed springs, and locally even calciferous Palustriella springs. In the northernmost provinces, larvae have also been collected amongst Sphagnum mosses from very small (<5 m 2) pools of so-called “palsa-mires” that may show some resemblance to raised bogs, but are being shaped by permafrost processes.

At almost all sites surveyed, H. insignis appeared to be the only representative of the family Polycentropodidae. In three cases only (Grene Sande, Denmark; Fochteloërveen and Zandveen, the Netherlands) it coexisted with H. dubius. A relevant question is how H. insignis is ecologically separated from its nearest relatives. According to our experience, H. dubius and H. picicornis can be found in both acid and neutral ponds and lakes, generally (but not exclusively) being larger water bodies than those of H. insignis. Thus, the latter seems to prefer tiny, very acid pools (see above). Holocentropus stagnalis, like H. insignis, also seems to prefer small ponds and pools, but is generally associated with water that is closer to neutral. None of the species appears to be adapted to habitats that might become dry during summer. The well-studied population in Stensbaek disappeared during the very dry summer of 1989 when the habitat totally dried for several months (Wiberg-Larsen, unpublished). Isolation-by-habitat is not the only way in which species might be ecologically separated. For example, Higler (1977) demonstrated a separation in time of development between H. dubius and H. picicornis, the former especially growing faster during July/August and thereby becoming bigger and able to feed on larger prey earlier. The growth pattern of H. insignis resembles that of H. picicornis, and thus life cycles may ecologically separate H. insignis and H. dubius where the species occur together.

Biological notes. In the shallow pool at Stensbaek, which was studied intensively over a one-year period, only 38 invertebrate taxa were recorded from the spatially dominating Sphagnum mats. Besides H. insignis, constituting 90% of all macroinvertebrate individuals, Chironomidae were by far the most prominent group (68% of all taxa exclusive of H. insignis). Surprisingly, a very high proportion of the macroinvertebrate taxa (i.e., number of taxa) could be classified as predators (45%), whereas the rest were either deposit feeders (48%) or grazers (7%). Numerically (number of individuals), however, predators and deposit feeders were equally abundant.

The life cycle of H. insignis is univoltine, as in most Trichoptera. In the shallow Stensbaek pool, larval development seems to be asynchronous, 2nd- to 5th-instar larvae occurring almost all year round (Fig. 16). However, 1st-instar larvae appeared and were only present from July to August, whereas pupae were exclusively found in May and June. Growth of the larvae was apparently related to temperature, primarily taking place from April to September at mean weekly air temperatures above 10°C (data from nearest meteorological station, data not shown).

Stomach analysis of H. insignis larvae (4th and 5th instar) showed that 53% of the identifiable material was invertebrate prey, 19% algae, the rest (25%) being fine particular organic matter and dead Sphagnum fragments (Fig. 17a). Chironomidae (Diptera) dominated among the invertebrate prey (64%), the rest being represented equally by microinvertebrates, Naididae, Hydrachnidia, and other H. insignis. The dominance of Chironomidae in the diet corresponds well with their representation in the Sphagnum mats (Fig. 17b).