Heterarthrus vikbergi Liston, Mutanen & Viitasaari sp. nov.

Description.

Female (Figs 4, 14, 51 - 55, 57).

Body length. 3.0-4.0 mm.

Colour (Figs 4, 51 - 55). Head (Figs 52-53): mostly pale, shading from yellow-brown dorsally to whitish ventrally. Contiguously dark (black) are occiput, a patch covering postocellar area and frontal field, extending laterally over vertex, and frontally narrowly more or less along antennal furrows to dorsal tentorial maculae. Frontal groove black. Apex of mandible more or less darkened. Apical maxillary and labial palpomeres slightly dark. Antenna (Fig. 55): scape and pedicel whitish; flagellomeres yellowish, 1 and 2 more or less darkened.

Thorax (Figs 4, 51, 53): black. Whitish are postspiracular sclerite, tegula, broad posterodorsal margins of pronotum, metascutum at least laterally, a fleck on medioposterior of mesepisternum, margins of metepimeron. Legs entirely pale (yellowish), except for very narrowly darkened bases of coxae, and more or less darkened tarsomeres 4 and 5. Wings (Fig. 4): fore wing membrane darkened broadly below pterostigma, and within anal cell, but hyaline distal of apex of pterostigma. Hind wing membrane extensively darkened, but hyaline are apical ca. 0.3, extreme base, and jugal lobe. Venation largely dark. Pale are: C, Sc, small anterior part of fore wing pterostigma basally and apically, all veins in approx. basal 0.25 of fore wing (but R completely dark) and basal 0.2 of hind wing (but 3A completely dark).

Abdomen (Figs 4, 51, 54): black. Obscurely pale are more or less hypopygium (Fig. 54), and margins of downturned parts of terga (Fig. 51).

Structure. Postocellar area about 4 × as broad as long (Fig. 53). Inner margins of eyes in frontal view subparallel (Fig. 52). Antenna: 11-12 antennomeres; about 1.5 × as long as greatest width of head (Fig. 4); pedicel slightly broader than scape. Ratio of lengths of flagellomeres 1-7 approximately as: 100, 62, 58, 58, 51, 44, 42 (Fig. 55). Frontal groove broad, shallow, poorly defined (Fig. 52). Malar space 0.30-0.35 as long as height of compound eye.

Body shiny, without surface sculpture except for sculpture on narrow median part of median mesoscutal lobe. Pubescence pale: length varying from about 0.3 × diameter of median ocellus on upper head, to almost as long as median ocellus on inner orbits, lower head, and mesepisternum. Anterior of median mesoscutal lobe without transverse depressions (Fig. 53). Dorsum of thorax almost entirely glabrous, with only a few minute setae on mesoscutum and posterior of mesoscutellum. Upper ca. 0.6 of mesepisternum densely and evenly setose, lower 0.4 entirely glabrous.

Metatarsomere 1 about as long as combined lengths of following tarsomeres. Inner tooth of tarsal claw about as high as basal lobe and 0.6-0.7 as long as outer; teeth very close together; basal lobe well-developed, acute. Metatibial spurs slightly shorter than apical width of metatibia.

Terga 1-8 unsculptured, with sparse, extremely short setae. Terga 9-10 dull, with distally increasingly dense sculpture, and longer setae. Valvulae 3 (Fig. 14) in dorsal view parallel-sided, at least 3 × as long as basal width, apically densely setose; all setae shorter than width of sheath. Lancet (Fig. 57) with 13-14 sawteeth. Each tooth with 6-10 small denticles. Compact groups of ctenidia on venter of lancet above saw teeth 9-10, occupying more than half the length of the basal 2 annular sutures, but proximally the groups become progressively smaller.

Male (Figs 5, 56, 58-63).

Body length. 3.0-3.5 mm.

Colour (Figs 5, 58-62). Head and ventro-lateral parts of body markedly paler than female.

Head (Figs 59, 60): dark dorsal patch extends only slightly anterior of median ocellus; lower head entirely pale. Antenna (Fig. 56): scape and pedicel white, flagellum yellowish.

Thorax (Figs 5, 58): dorsum black, except for tegula, upper edge of pronotum, lateral part of metascutum. Underside of thorax predominantly pale (whitish) except for extreme anterior of pronotum, more or less edges of propleuron, mesepimeron, flecks on the metapleuron, and underside of mesepisternum. Metascutum entirely dark. Legs entirely pale, except for more or less darkened apical tarsomere of mid and hind leg.

Abdomen (Figs 5, 58, 61, 62): pale, except for a broad, black median stripe on terga 1-8.

Structure. As female except for: Antenna ca. 1.8-1.9 as long as widest width of head (Fig. 5). Terga 1-5 largely unsculptured, with sparse, extremely short setae. Terga 6-8 dull, with distally increasingly dense sculpture. Penis valve: Fig. 63.

Variability.

Female and male. Approximately equal numbers of specimens with 10 or 11 antennomeres: sometimes each antenna of the same specimen appears to have a different number of antennomeres. Relative length proportions of the apical 3 antennomeres highly variable. Fore wing vein 3r-m present or absent, or partly obsolete: frequently unequally developed in each wing of the same specimen. Mesoscutellar appendage black or slightly brown. Female: fleck on medioposterior of mesepisternum variable in size, and sometimes nearly invisible. Lateral part of metascutum always more or less white, but interior from pale brown to black.

Male: usually but not always present is a small pale fleck on anterior of each median mesoscutal lobe, and a pale streak on posterio-lateral edge of lateral lobe.

Larva, full-grown. Length 8-10 mm. Pro-, meso- and metathorax ventrally each with dark median fleck, decreasing in size towards posterior; abdominal sterna one and two also with faint dark markings (Fig. 66). Teeth on sclerotised ring around anal prolegs mostly not clearly separated from each other (Fig. 67).

Holotype: ♀. Finland: Karelia borealis, Tohmajärvi 6906:3673 [Finnish grid: = 62.299N, 30.374E], larva 24.08.2017 on Populus balsamifera L., M. Mutanen leg. Deposited in the ZMUO.

Paratypes: Total: 41♂, 35♀. All same collection data as holotype, except for dates. 5♂, 6♀, larvae collected 26.07.2016. 36♂, 29♀, larvae collected 24.08.2017. Deposited in the Finnish Museum of Natural History, Helsinki, Finland; HNHM; National Museum of Nature and Science, Tsukuba, Japan; NHRS; Private Collection Matti Viitasaari, Helsinki, Finland; Private Collection Veli Vikberg, Janakala, Finland; SDEI; Tartu University Zoological Museum, Tartu, Estonia; United States National Museum, Washington DC, USA; ZMUO.

Other material (immature stages).

Two mature larvae (or pronymphs?), collection data as for holotype: DEI-GISHym31985 preserved during ecdysis, with more darkly pigmented cuticle attached (Fig. 66); and DEI-GISHym31986, moulted [weakly pigmented] (Figs 67, 68).

Diagnosis.

The female of H. vikbergi differs from all other described Heterarthrus species in its combination of narrow and proportionately long valvulae 3 in dorsal view (Fig. 14), the nearly entirely pale fore wing veins C and Sc (which contrast with the otherwise mostly dark venation), and the only partly strongly darkened fore wing membrane (Fig. 4). All other described Heterarthrus species have at least the antennal flagellum largely black, whereas male H. vikbergi have completely pale antennae (Fig. 56), and females only flagellomeres 1 and 2 more or less dark (Fig. 55). Furthermore, the extensively pale and tricoloured head of H. vikbergi is highly distinctive (Figs 52, 59): only the Mediterranean H. cypricus Schedl, 2005 has a similarly extensively pale head (but bicoloured: orange and black). For further diagnostic characters of adults, see key to species. Compared to the description of the larva of H. ochropoda by Lorenz & Kraus (1957), H. vikbergi differs in having a dark marking on venter of abdominal segment 1 (Fig. 66), and the teeth on anal leg ring closer to each other (Fig. 67).

Etymology.

The new species is named in honour of Veli Vikberg, whose life-long work on the taxonomy, biology and distribution of sawflies has greatly advanced our understanding of the group.

Host plants and biology.

All specimens were reared from leaf-mines on Populus balsamifera . Mines were rather abundant on less than ten approximately 20-year old trees, planted in three short rows along the roadside in Tohmajärvi village (Fig. 64). The positions of mines in pressed leaves (Fig. 65) indicate that oviposition is normally in the leaf edge. In this sample, only one mine / larva is present in each leaf. During rearing, many cocoons detached from the leaves, but it is not clear if under natural conditions this takes place before or after leaf-fall. Diameter of 14 cocoons recovered from the rearing pots was 5-7 mm.

Distribution.

Only known so far from the type locality, in eastern Finland. The species is unlikely to be indigenous there (see Discussion).