Redescription of the genus Megastilicus Casey (Staphylinidae, Paederinae) with the description of a new species

The myrmecophilous Paederinae rove beetle genus Megastilicus Casey, 1889 from North America is reviewed based on museum specimens. Prior to this study, the genus was monotypic with one species Megastilicus formicarius Casey, 1889 described. Here, we provide a redescription of the genus and the type species, designate a lectotype, and provide pictures of habitus and illustrations of the aedeagus and genital segments. Additionally, we describe a new species for the genus, Megastilicus iowaensis sp. nov., include an identifi cation key to the two species and present the distribution map of both of them, including new state records. We discuss the assignment of the genus to the subtribe Stilicina based on morphological features.


Introduction
Beetles of the genus Megastilicus Casey, 1889 are ant nest predators found with Formica ulkei Emery, 1893, F. subsericea Say, 1836and F. exsectoides Forel, 1886 in Northeast and Midwest USA and Central-East Canada. The genus was fi rst described by Casey in 1889 to accommodate a single species Megastilicus formicarius Casey, 1889. In his revision of North American Paederini Fleming, 1821, Casey (1905) assigned the genus to a subtribe Stilicina Casey, 1905 (referred there as Stilici), incorporated it into the key to genera of the subtribe and expanded the description of the species. Moore & Legner (1974) in their key to the Staphylinidae Latreille, 1802 of America north of Mexico provided a more detailed description and a drawing of the habitus. The genus was also included in Newton et al. (2000) key of American beetles in a couplet with Eustilicus Sharp, 1886.
It was already mentioned in the original description (Casey 1889) that the genus is myrmecophilous and perhaps a predator (Wickham 1900), but its biology and behaviour was studied in more detail by Park (1935). His studies revealed several defence mechanisms that allow the beetle to enter the nest and prey on the host. Such mechanisms are, e.g., their hard exoskeleton, the overall agility and a defensive abdominal secretion released when they are attacked by an ant. He also noted that the beetle is clearly not integrated into the nest and persecuted by its hosts (Park 1935). The genus was also mentioned in a summary of myrmecophilous rove beetles by Parker (2016).
Megastilicus is an example of a genus that has never been the subject of a modern revision, lacking a proper illustration of habitus and genital structures. This motivated us to make a redescription of the genus, and while studying multiple specimens, we additionally discovered a new species that we here describe.

Specimen preparation and photographs
Specimens were relaxed in warm water and dissected. Specimens were studied using a Nikon SMZ1500 and an Olympus SZX9 stereo microscopes. All measurements are given in millimetres and were made with an ocular micrometer mounted on a stereoscopic microscope. A Nikon DS-Fi1 camera was used for taking pictures of habitus, and photos were stacked using Zerene Stacker software by the PMax method (ver. 1.04, Zerene Systems LLC). The pictures were further edited in Adobe Photoshop CS6 while illustrations were made in Adobe Illustrator CS6 (ver. 13.0, Adobe Systems Inc.) based on photographs. The distribution map was drawn in SimpleMappr (Shorthouse 2010) and modifi ed in Adobe Photoshop CS6. Morphological terminology follows Bogri et al. (2020). Data from the specimen labels are cited verbatim (with original spelling retained) and are provided inside quotation marks (" "). The slash (/) separates labels in each specimen. Ant species given in square brackets were identifi ed by the fi rst author (DŻ), using the Key to New England Formica (AntWiki 2017). Identifi ers of the ant species named on original specimen labels are unknown.

Specimen repositories
Material from the following collections was used in the current study and is currently deposited there:

Diagnosis
Body robust, antennae short and stout. Body covered with short, black, stout setae. Dorsal surface of head, pronotum and elytra granulate. Head orbicular in shape.

Redescription
B . Medium size beetle; body robust, somewhat fl attened. Colour of specimens from pale brown / sandy to brown / reddish brown. Head, pronotum and elytra covered with short, black setae. Dorsal surface of body densely granulato-reticulate, abdomen smoother, and not granulate.
H . Orbicular, with rounded hind angles; widest in middle, tapering towards antennal insertions anteriorly and towards neck posteriorly; disc of head without punctation, with leather-like integument. Head wider and longer than pronotum. Antennae short, reaching only to anterior part of pronotum, compact, mostly transverse, with well-developed stems in between, pubescent. Labrum large, almost completely covering mandibles from above, notched, broadly emarginate in middle at apex, with two distinct lobes, each bearing long and pale macroseta and comb of setae near emargination; emargination bidentate; weakly sclerotised, connected with head by membrane. Mandibulae without prostheca, with two teeth on right and one on left mandibula. Maxillary palpomere 3 about twice as long as palpomere 2; palpomere 4 elongate and thin, around ⅓ of palpomere 3 length and ⅓ of palpomere 3 width. Labial palpomere 1 shorter and thinner than palpomere 2; palpomere 2 slightly elongate, expanded toward apex, twice as long as palpomere 1; palpomere 3 thin and long, longer than half of palpomere 2 and ¼ of palpomere 2 width. Mentum transverse. Submentum with pair of setae on sides. Ligula bilobed. Gular sutures fused. Eyes small, from ⅕ to ⅙ of head length. Temples long, more than half of head length. Shallow longitudinal depression on each side of head ventrally. Neck distinct and narrow, about ⅕ of head width.
L . Long and slender, uniformly covered with glossy black macrosetae. All tarsi 5-segmented. Tarsomeres 1-4 of protarsus not infl ated. Protibia with three fully developed protibial combs. One pair of empodial setae on each tarsus, equal to or slightly shorter than claws (never shorter than half of claw length). Meso and metatarsomere 1 longer than 2. Metatarsi almost as long as metatibiae, with tarsomeres 1-4 decreasing gradually in length, metarsomere 1 shorter than 2 and 3 combined, slightly longer than tarsomere 5. A . Sternite III with medial longitudinal keel. Abdomen widest at tergite V. Abdominal segments III to VII with pair of paratergites on each side; setae along length of abdomen. Tergite / sternite IX or X with dense setation. [Similar structure for female.] A . With parameres reduced and fused to median lobe; ventral process slightly longer than uneverted internal sac; in parameral view with apex of ventral process acute, sides sinuate; dorsal plate rather large and relatively weakly sclerotised (Figs 1D-E, 2D-E).

Diagnosis
Head longer than wide. Antennomere 4 elongate. Teeth on labrum blunt. Eyes smaller than in M. iowaensis sp. nov., ca ⅙ of head length. Median emargination of posterior margin of sternum VIII deeper and broader than in M. iowaensis sp. nov.

B
. Colour of specimens from pale brown / sandy to brown (Fig. 1A-B). Head and tip of abdomen darker. Body length from tip of head to tip of abdomen 5.5-6.0.
L . Two long setae on outer margin of metacoxa. Seta closer to abdomen longer and thicker than seta more distant from abdomen.
A . Sternum VII with apical margin straight. Tergum VIII with rounded posterior margin. Male with sternum VIII with moderately deep and broad median emargination of posterior margin; depth of emargination of ca ⅕ of sternum length; sides of emargination rounded (Fig. 1C); surface right anterior to emargination without setae, while margins with dense setation. A . With ventral process in parameral view narrower in middle; lateral plates of internal sac fused to sides of median lobe and indistinctive (Fig. 1D-E).

Occurrence
The species has previously been recorded from the Canadian provinces of Manitoba, Newfoundland and Labrador, and Ontario and the following US states: Illinois, Indiana, Iowa, Massachusetts, New Jersey and Ohio (A. Newton, unpublished database). New state records include: Missouri, New Hampshire, Pennsylvania and Texas.

Diagnosis
Head as long as wide. Antennomere 4 more transverse compared to elongated in Megastilicus formicarius. Teeth on labrum pointed. Eyes larger than in M. formicarius, ca ⅕ of head length. Median emargination of posterior margin of sternum VIII shallower and narrower than in M. formicarius.

Etymology
The species name ʻiowaensisʼ is derived from the name of the US state Iowa where this species has been collected.

B
. Colour of specimens pale brown / sandy ( Fig. 2A-B). Head darker. Body length from tip of head to tip of abdomen 6.0.
L . Two long setae on outer margin of metacoxa. Seta closer to abdomen shorter and thinner than seta more distant from abdomen.
A . Tergum VIII with slightly pointed posterior margin. Male with sternum VIII with shallow and broad median emargination of posterior margin; depth of emargination of ca ⅐ of sternum length; sides of emargination rounded (Fig. 2C); surface anterior to emargination without setae, while margins with dense setation.

A
. Wit h ventral process in parameral view widened in middle; lateral plates of internal sac welldeveloped and attached to sides of median lobe (Fig. 2D-E).