Description of two new species of the Exocelina broschii-group from Papua New Guinea, with revision and key to all representatives of this species group (Coleoptera, Dytiscidae, Copelatinae)

Abstract Two new species of Exocelina Broun, 1886 from Papua New Guinea are described herein: Exocelina mondmillensis sp. n. and Exocelina pseudomarinae sp. n. They are placed into the Exocelina broschii-group based on the shovel/fork-like ventral sclerites of their median lobe. While the former has rather distinct combination of the morphological characters (inconspicuous dorsal punctation, thin apex of the median lobe and ventral sclerite of the median lobe with two tips of different length), the latter is very similar to already described species Exocelina marinae (Shaverdo, Sagata & Balke, 2005). All described species of the group are revised and a key to their identification is provided. Important diagnostic characters (habitus, color, protarsomeres 4–5, median lobes, and parameres) are illustrated. Data on the distribution of all species of the group are given showing that its representatives occur only in Papua New Guinea and most of them are widely distributed in it central part.


Introduction
This paper continues our previous studies on the New Guinea species of the genus Exocelina Broun, 1886 (Balke 1998(Balke , 1999Shaverdo et al. 2005Shaverdo et al. , 2012Shaverdo et al. , 2013Shaverdo et al. , 2016 and deals with one of the five species groups of New Guinea Exocelina, the E. broschii-group. This group was introduced by Shaverdo et al. (2005) for three species from Papua New Guinea (E. broschii (Balke, 1998), E. hintelmannae (Shaverdo, Sagata & Balke, 2005) and E. marinae (Shaverdo, Sagata & Balke, 2005)) and defined by the following apomorphy: shovel/fork-like ventral sclerite of the median lobe. Monophyly of the group was also supported by the phylogenetic analysis, based on molecular data (Toussaint et al. 2014).
Here, we provide a detailed diagnosis of the E. broschii-group, describe two new species, review the known species providing new faunistic data, and present a key to the species and map of their distribution.
We provided electronic resources for the species treated here in the form of species pages, which were automatically created by ZooKeys on the species-id.net portal with the publication of this article. This wiki engine based site provides for example high resolution art work and can be improved interactively should new data become available. By providing these resources, we hope to help creating a more user-friendly, sustainable taxonomy as suggested by Riedel et al. (2013).
Including the results of this work, 91 Exocelina species are described from New Guinea and 144 worldwide.

Material and methods
The present work is based on the material from the following collections:

BMNH
The Natural History Museum, London, UK NARI Papua New Guinea National Insect Collection, Port Moresby, PNG NHMW Naturhistorisches Museum Wien, Vienna, Austria ZSM Zoologische Staatsammlung München, Munich, Germany All specimen data are quoted as they appear on the labels attached to the specimens. Label text is cited using quotation marks. Comments in square brackets are ours. We extracted DNA and obtained DNA sequence data for some of the species/ specimens, marked with individual DNA extraction numbers (e.g., "264 DNA M. Balke"). All types of the herein described species are provided with red labels. The female specimens, identification of which is difficult or sometimes impossible, were included in the type series only when collected with males of respective species and did not differ morphologically from them. If two or more morphologically similar species were collected together (i.e., males found together), their females were not included in the types series of the respective species but were instead mentioned under additional material. Species descriptions are based on the whole type series.
Some of the species treated herein are very similar to each other and, based on low overall genetic divergence, most likely also very recent (Toussaint et al. 2014). We have used constant morphological difference based on examined series as an indicator of interrupted gene flow and as an operational criterion to delineate biological species, but suggest that extensive population genetic work using genomic data might reveal many additional lineages that represent putative species in a highly structured geographic and geological setting.
Measurements were taken with a Wild M10 stereomicroscope choosing the smallest and the largest specimens within and among the populations. The following abbreviations were used: TL (total body length), TL-H (total body length without head), MW (maximum body width), and hw (handwritten). The number of ventral setae on the male protarsomere 5 is given for only one specimen of each species, which was mounted on a glass slide (see below) for drawing. This character was found not very useful for the species identification since it is possible to make a general statement of the setation pattern (short/long, dense/sparse) but not to count them with certainty at the magnification of normal dissecting scopes. The potential phylogenetic information content of this character will be studied in a further work.
Drawings were made with the aid of a camera lucida attached to a Leica DM 2500 microscope. For detailed study and drawing, protarsi, and genitalia were removed and mounted on glass slides with DMHF (dimethyl hydantoin formaldehyde) as temporary preparations. The drawings were scanned and edited, using the software Adobe Illustrator CS5.1. Arrangement of the figures follows the species descriptions.
The terminology to denote the orientation of the genitalia (ventral for median lobe and dorsal and external for paramere) follows Miller and Nilsson (2003). Left and right lobes of the ventral sclerite of the median lobe are indicated according figure view, not their original orientation. The terminology on the structure of the prosternum follows Larson et al. (2000). Administrative divisions of Papua New Guinea follow information from Wikipedia (2016).

Diagnosis of the Exocelina broschii-group sensu Shaverdo et al. (2005)
The representatives of the E. broschii-group share the following diagnostic characters: -beetles small or middle-sized (TL-H 3.2-4.15 mm); -habitus oblong-oval (broadest approximately at elytral middle), with rounded pronotal and elytral sides, body outline continuous; -pronotum short, trapezoidal, with posterior angles not drawn backwards; -coloration brown to piceous, mainly uniform, sometimes with paler head and pronotum and darker elytra; -microreticulation and punctation of dorsal surface very fine to strongly impressed, so that beetles shiny to matt dorsally; -metacoxae and abdominal ventrites 1-5 (and 6 in males) with thin, almost longitudinal striae/strioles; -pronotum and elytra without striae or strioles; -pronotum with lateral bead; -male antennomeres not modified, antennomere 2 larger than antennomere 3; -male protarsomeres 1-3 not expanded laterally; -male protarsomere 4 cylindrical, narrow, with large anterolateral hook; -male protarsomere 5 not modified: long and narrow, without expansion and concavity, ventrally with two sparse rows of relatively short setae; -median lobe of aedeagus with continuous outline in ventral and lateral view; -ventral sclerite of median lobe not deeply divided in the middle, apically forming a shovel/fork-like structure with two apices; -apical part of median lobe with numerous setae; -paramere without notch on dorsal side; -paramere with long setae occupying whole dorsal side.
Variability: Beetles vary in size, kind of dorsal punctation (Figs 1-3), and shape of ventral sclerite of the median lobe ( Fig. 9A-E). Dorsal punctation in great majority of the specimens is fine (Fig. 3), but some specimens have slightly coarser punctation, and very few (the types and one specimen from Simbai area, Madang, see Figs 1, 2) have distinct punctation, similar to that of E. pseudomarinae sp. n. There are populations (e.g., from Simbai area, Madang, and Akameku-Brahmin, Eastern Highlands) with the left lobe of the ventral sclerite of the median lobe very narrow (Fig. 9A, B). However, this character is not very stable even in one population. Taking into consideration the other different shapes of the ventral sclerite observed (Figs 8B, 9C-E) as well as the fact that it is differently sclerotized in different specimens and because of that is variable in shape, we treated all the material as E. broschii. Such variability in shape of the ventral sclerite of the median lobe is also characteristic for other species of this group (Figs 9, 10, 12).

Exocelina hintelmannae
Female: Without evident differences in external morphology from males, except for not modified protarsi and abdominal ventrite 6 without striae.
Variability: Beetles vary in shape of the median lobe and its ventral sclerite: mainly shape and length of its left lobe (Fig. 9F, G).
Distribution. Papua New Guinea: Simbu, Eastern Highlands, and Gulf Provinces (Fig. 14). Additions to the description (original description in Shaverdo et al. 2005, p. 270). Male: Median lobe with apex slightly curved in lateral view and more or less rounded in ventral view. Its ventral sclerite with unequal apical lobes: left lobe very long (very often with broken apex) and right lobe short, relatively narrow (Fig. 10).

Exocelina mondmillensis
Diagnosis. Beetle medium-sized, brown to piceous, with reddish head and pronotal sides, shiny; median lobe with slightly curved downwards apex but thin in lateral view and ventral sclerite with two unequal apices (left one long, narrow and curved and right one short, broad and more or less rounded). The species is similar to E. marinae and E. pseudomarinae sp. n. in shape of the ventral sclerite of median lobe, but distinctly differs from them in having fine, inconspicuous punctation and weak microreticulation on the dorsal surface and a thin tip of the median lobe. From E. broschii and E. hintelmannae, it can be distinguished by the shape of the median lobe (thin apex) and its ventral sclerite (very long left lobe), and from the former also by finer elytral punctation.
Surface sculpture: Head with dense punctation (spaces between punctures 1-3 times size of punctures), evidently finer and sparser anteriorly; diameter of punctures smaller than diameter of cells of microreticulation. Pronotum with much sparser and finer punctation than head. Elytra with very sparse and fine punctation, almost invisible. Pronotum and elytra with weakly impressed microreticulation, dorsal surface shiny. Head with microreticulation stronger. Metaventrite and metacoxae distinctly microreticulate, metacoxal plates with longitudinal strioles and transverse wrinkles. Abdominal ventrites with distinct microreticulation, strioles, and very fine and sparse punctation.
Structures: Pronotum with distinct lateral bead. Base of prosternum and neck of prosternal process with distinct ridge, slightly rounded anteriorly. Blade of prosternal process lanceolate, relatively narrow, slightly convex, with distinct lateral bead and few setae; neck and blade of prosternal process evenly jointed. Abdominal ventrite 6 broadly rounded or slightly truncate.
Male: Antenna simple (Fig. 6). Protarsomere 4 with large, thick, strongly curved anterolateral hook-like seta. Protarsomere 5 ventrally with anterior row of 18 and posterior row of 4 short setae (Fig. 11A). Median lobe with thin apex, slightly curved in lateral view and pointed in ventral view. Its ventral sclerite with two unequal apices (left one long, narrow and curved and right one short, broad and more or less rounded). Paramere with subdistal setae denser and thicker than proximal setae . Abdominal ventrite 6 with 4-7 lateral striae on each side.
Holotype: TL-H 4.05 mm, TL 4.45 mm, MW 2.15 mm. Female: Without evident differences in external morphology from males, except for not modified protarsi and abdominal ventrite 6 without striae.
Variability: Beetles vary mainly in shape of the ventral sclerite of the median lobe as shown in Fig 12. Distribution. Papua New Guinea: Western Highlands, Enga, and Madang Provinces (Fig. 14).
Etymology. The name refers to Mondmill, the type locality, where the species was found in great numbers. The name is an adjective in the nominative singular. Diagnosis. Beetle medium-sized, brown to dark brown, with reddish head and pronotal sides, submatt; median lobe with apex strongly curved downwards in lateral view and ventral sclerite with two unequal apices (left one long, narrow and curved apically and right one short, broad and more or less strait). The species is similar to E. marinae Shaverdo, Sagata & Balke, 2005 from which distinctly differs in larger size, sparser and finer punctation and weaker microreticulation of the dorsal surface, and strongly curved apex of the median lobe, which is similar to that of E. hintelmannae Shaverdo, Sagata & Balke, 2005. The specimen of E. marinae from Tari-Koroba, though large in size and with the same distribution, has a distinctly stronger sculpture on the dorsal surface and a median lobe with only a slightly curved apex in lateral view and a narrower right lobe of the ventral sclerite. Therefore, it can be easily distinguished from E. pseudomarinae sp. n.
Surface sculpture: Head with very dense punctation (spaces between punctures 1-2 times size of punctures), evidently finer and sparser anteriorly; diameter of punctures smaller than diameter of cells of microreticulation or equal to it. Pronotum and elytra with punctation sparser and finer than on head, but very distinct. Pronotum and elytra with distinct microreticulation, dorsal surface submatt. Head with microreticulation stronger. Metaventrite and metacoxae distinctly microreticulate, metacoxal plates with longitudinal strioles and transverse wrinkles. Abdominal ventrites with distinct microreticulation, strioles, and sparse punctation, coarser on two last abdominal ventrites.
Structures: Pronotum with distinct lateral bead. Base of prosternum and neck of prosternal process with distinct ridge, slightly rounded anteriorly. Blade of prosternal process lanceolate, relatively narrow, slightly convex, with distinct lateral bead and few setae; neck and blade of prosternal process evenly jointed. Abdominal ventrite 6 broadly rounded or slightly truncate.
Male: Antenna simple (Fig. 7). Protarsomere 4 with large, thick, strongly curved anterolateral hook-like seta. Protarsomere 5 ventrally with anterior row of 20 and posterior row of 7 short setae (Fig. 13A). Median lobe with strongly curved apex in lateral view and more or less rounded in ventral view. Its ventral sclerite with two unequal apices (left one long, narrow and curved apically and right one short, broad and more or less strait). Paramere with subdistal setae denser and thicker than proximal setae ( Fig. 13B-D). Abdominal ventrite 6 with 6-8 lateral striae on each side.
Holotype: TL-H 4.05 mm, TL 4.4 mm, MW 2.1 mm. Female: Without evident differences in external morphology from males, except for not modified protarsi and abdominal ventrite 6 without striae.
Distribution. Papua New Guinea: Hela Province. This species is known only from the type locality (Fig. 14).
Etymology. The name points to similarity of the new species to E. marinae. The name is a noun in the nominative singular standing in apposition.

Key to species of the Exocelina broschii-group
The key is based mostly on male characters. In many cases females cannot be assigned to a species due to the similarity of their external and internal structures (for female genitalia see Figs 17a and 17b in Shaverdo et al. (2005)). Some species are rather similar in external morphology, therefore, in most cases, the male genitalia need to be studied for reliable species identification. Numbers in parentheses refer to the order of species descriptions given above.

1
Dorsal surface of the body matt, with strongly impressed microreticulation and dense coarse punctation or submatt, with finer microreticulation and punctation (Figs 5, 7). Median lobe with apex broad, slightly or strongly curved in lateral view and ventral sclerite with two apical lobes of different length and shape: left one long (it can be broken apically) and narrow and right one shorter and broader (Figs 10, 13) (Figs 1-4, 6). Median lobe with apex thin, slightly curved in lateral view and ventral sclerite with two apical lobes of different length and shape (Figs 11B,12) or with apex broad, slightly or strongly curved in lateral view and ventral sclerite with two apical lobes of more or less equal length and shape .. Dorsal surface of body matt, with strongly impressed microreticulation and dense coarse punctation (Fig. 5); apex of median lobe slightly curved in lateral view (Fig. 10D-F, fig. 12a in Shaverdo et al. (2005)) ..........(3) marinae -Dorsal surface of body submatt, with punctation evidently sparser and finer and microreticulation weaker (Fig. 7); apex of median lobe more strongly curved in lateral view (Fig. 13C)