Holotype worker labelled “USA: 44.7560°N, 86.0711°W, Michigan: Sleeping Bear National Lakeshore, 180 m, 2013.05.27 – M509” and “Holotype Temnothorax pilagens Seifert et al.”; 3 paratype workers from the holotype nest and the same collecting data; 1 paratype gyne labelled “USA: 44.7560°N, 86.0711°W, Michigan: Sleeping Bear National Lakeshore, 180 m, 2013.05.27 – M502”; 4 paratype workers, each on a separate pin, labelled “USA: 44.8435°N, 86.0612°W, Michigan: North Bar Lake Dunes, 185 m, 2013.05.31 – Q534.3”, “USA: 44.8435°N, 86.0612°W, Michigan: North Bar Lake Dunes, 185 m, 2013.05.31 – Q534.1”, “USA: 44.8435°N, 86.0612°W, Michigan: North Bar Lake Dunes, 185 m, 2013.05.31 – Q534.2”, “USA: 44.8435°N, 86.0612°W, Michigan: North Bar Lake Dunes, 185 m, 2013.05.30 – Q520”; 1 paratype gyne labelled “USA: 44.7560°N, 86.0711°W, Michigan: Sleeping Bear National Lakeshore, 180 m, 2013.05.27 – M502”. Different codes after the date sequence refer to different nests. All material is stored in the Senckenberg Museum of Natural History in Goerlitz.

Comparative material of Temnothorax duloticus (Wesson, 1937) consisted of five workers and one gyne labelled “USA: 39.9927°N, 83.2575°W, Ohio: Prairie Oaks Metro Park, 270 m, 2013.06.04 – Z698” and three workers labelled “USA: 40.1469°N, 83.0381°W, Ohio: Highbanks Metro Park, Olentangy River, 239 m, 2013.06.07 - G827”.

Twenty morphometric characters currently being used in taxonomy of Palaearctic Temnothorax (Seifert 2006, Csösz et al. 2013) were investigated. In bilaterally recorded characters, arithmetic means of both body sides were calculated. All measurements were made on mounted and dried specimens using a pin-holding stage, permitting full rotations around X, Y, and Z axes. A Leica M165C high-performance stereomicroscope equipped with a 2.0 planapochromatic objective (resolution 1050 lines/mm) was used at magnifications of × 120–384. The mean relative measuring error over all magnifications was 0.3%. A Schott KL 1500 cold–light source equipped with two flexible, focally mounted light–cables, providing 30°–inclined light from variable directions, allowed sufficient illumination over the full magnification range and a clear visualization of silhouette lines. A Schott KL 2500 LCD cold–light source in combination with a Leica coaxial polarized–light illuminator provided optimal resolution of tiny structures and microsculpture at highest magnifications. Simultaneous or alternative use of the cold-light sources depending upon the required illumination regime was quickly provided by regulating voltage up and down. A Leica cross-scaled ocular micrometer with 120 graduation marks ranging over 52% of the visual field was used. To avoid the parallax error, its measuring line was constantly kept vertical within the visual field. Measurements of body parts always refer to real cuticular surface and not to the diffuse pubescence surface.

Z-stack photographs were made with a Leica Z6 APO photomicroscope equipped with 2.0 × planapochromatic objective and the automontage software Leica application suite version 3.

CL maximum cephalic length in median line; the head must be carefully tilted to the position with the true maximum. Excavations of occiput and/or clypeus reduce CL.

CS cephalic size; the arithmetic mean of CL and CW, used as a less variable indicator of body size.

CW maximum cephalic width; the maximum is found in Temnothorax and Leptothorax usually across and including the eyes.

EYE eye-size index: the arithmetic mean of the large (EL) and small diameter (EW) of the elliptic compound eye is divided by CS, i.e. EYE=(EL+EW)/(CL+CW). All structurally visible ommatidia are considered.

FCDV tangens of divergence angle of frontal carinae measured along a 50 µm section from FRS level caudad. A cross-scaled ocular micrometer and full magnification is used.

FRS distance of the frontal carinae immediately caudal of the posterior intersection points between frontal carinae and the lamellae dorsal of the torulus. If these dorsal lamellae do not laterally surpass the frontal carinae, the deepest point of scape corner pits may be taken as reference line. These pits take up the inner corner of scape base when the scape is fully switched caudad and produce a dark triangular shadow in the lateral frontal lobes immediately posterior of the dorsal lamellae of scape joint capsule (fig. 1 in Seifert 2006).

MGr Depth of metanotal groove or depression, measured from the tangent connecting the dorsalmost points of promesonotum and propodeum; here given as per cent ratio of CS.

MH in workers: with mesosoma in lateral view and measured orthogonal to “longitudinal mesosomal axis”, MH is the longest measurable section line of mesosoma at mesopleural level (not height above all). “Longitudinal mesosomal axis” in lateral view is defined as straight line from the centre of propodeal lobe to the border point between anterior pronotal shield and propleuron. In gynes it is the longest section line directed perpendicular to the straight dorsal profile line of mesosoma (formed by mesonotum and scutellum). The lower reference point is usually lowest part of mesopleuron.

MW maximum mesosoma width (worker); maximum mesosoma width anteriorly of the tegulae (gynes).

ML in workers: mesosoma length from caudalmost point of propodeal lobe to transition point between anterior pronotal slope and anterior propodeal shield (preferentially measured in lateral view; if the transition point is not well defined, use dorsal view and take the centre of the dark-shaded borderline between pronotal slope and pronotal shield as anterior reference point). In gynes: length from caudalmost point of propodeal lobe to the most distant point of steep anterior pronotal face.

PEH maximum petiole height. The straight section of ventral petiolar profile at node level is the reference line perpendicular to which the maximum height of petiole node is measured.

PEL Diagonal petiolar length in lateral view; measured from anterior corner of subpetiolar process to dorsocaudal corner of caudal cylinder.

PEW maximum width of petiole.

PoOc postocular distance. Use a cross-scaled ocular micrometer and adjust the head to the measuring position of CL. Caudal measuring point: median occipital margin; frontal measuring point: median head at the level of the posterior eye margin. Note that many heads are asymmetric and average the left and right postocular distance (fig. 2 in Seifert 2006).

PPW maximum width of postpetiole.

SL maximum straight line scape length excluding the articular condyle as arithmetic mean of both scapes.

SP maximum length of propodeal spines; measured in dorsofrontal view along the long axis of the spine, from spine tip to a line, orthogonal to the long axis, that touches the bottom of the interspinal meniscus (fig. 3 in Seifert 2006). Left and right SP are averaged. This mode of measuring less ambiguous than other methods but results in some spine length in species with reduced spines.

SPBA the smallest distance of the lateral margins of the spines at their base. This should be measured in dorsofrontal view, since the wider parts of the ventral propodeum do not interfere with the measurement in this position. If the lateral margins of spines diverge continuously from the tip to the base, a smallest distance at base is not defined. In this case, SPBA is measured at the level of the bottom of the interspinal meniscus.

SPST distance between the centre of propodeal stigma and spine tip. The stigma centre refers to the midpoint defined by the outer cuticular ring but not to the centre of real stigma opening that may be positioned eccentrically.

SPTI the distance of spine tips in dorsal view; if spine tips are rounded or thick take the centres of spine tips as reference points.

TrScuC density of transverse microsculpture elements in centromedian vertex. Count the transverse elements crossing a median line of ± 120 µm at a central place with most such elements. Unit: number of elements / mm.

Biology and host species. Obligate slave-making ant with two known hosts: Temnothorax longispinosus (Roger, 1863) and Temnothorax ambiguus (Emery, 1895). Mitochondrial DNA phylogeny indicates sister species relationship with Temnothorax longispinosus (Beibl et al. 2005). Geographical range. Nearctic. North-eastern parts of the United States and possibly south-eastern Canada. Habitat. Forest, woodland, parks. Preferentially wooded sites with little understory, and a high density of suitable nest sites, such as acorns, hickory nuts and sticks. Abundance. Patchy – occurrence depends on high density of suitable host populations; so far only known from three sites in the Northern US: Niquette Bay State Park, Vermont (44.3513°N, 73.1156°W; 8 colonies collected in 1986; Herbers and Foitzik 2002), E.N. Huyck Preserve, Rensselaerville, New York (42.3133°N, 74.1012°W; 7 colonies collected in 2002 and 2003; Beibl et al. 2005) and Sleeping Bear National Lakeshore, Empire, Michigan (44.7560°N, 86.0711°W, 6 and 44 colonies collected in 2011 and 2013, respectively). In all three populations, Temnothorax pilagens was enslaving Temnothorax longispinosus and Temnothorax ambiguus; many nests with slaves of both host species. In Vermont and New York, this species has not been re-collected recently, despite regular search by our group. Plotting data from 2011 in Michigan: between 0.08 and 0.02 slave-making colonies per m² (at an average of 4.66 host colonies per m²). Temnothorax pilagens occurs more often in sites with both host species, than in areas with Temnothorax longispinosus colonies only (Fisher Test; p = 0.019). We did not sample Temnothorax communities with Temnothorax ambiguus only. Temnothorax pilagens colonies more often contained a mixed slave workforce than s laves of a single host species (Chi₁² = 49.59, p < 0.001). Nest construction. As its hosts, Temnothorax pilagens nests occur in preformed cavities in acorns, hickory nuts or sticks. Colony demography. Strictly monogynous. Most likely polydomous at least during the summer season: 72% of the nests were queenless with queenright nests close-by and neighboring nests merged in the laboratory without aggression. Nests contain on average four slave-making workers (ranging from 0 to 16 Temnothorax pilagens workers) and 13 Temnothorax slaves (ranging from 2 to 50 workers) – but see Herbers and Foitzik (2002) for a nest with 27 Temnothorax pilagens and 55 slave workers. Colony foundation. Four colonies with no Temnothorax pilagens workers, but a founding queen were collected. All four contained Temnothorax longispinosus slaves only. Slave-raids. Obligatory slave hunter of Temnothorax longispinosus and Temnothorax ambiguus. Raids are performed either by a scout alone or via group recruitment of up to four slavemaking workers forming a raiding column headed by a scout. Slave raids resemble more those of its congener Temnothorax duloticus than that of Protomognathus americanus (Alloway 1979). This is especially true for the frequent and effective use of the stinger in fights: well-aimed stings from a caudal direction between head and thorax cause paralysis in hosts followed by quick death. Effective sting use is likely facilitated by morphological adaptations, such as strongly developed flexor-muscles in the petiole and postpetiole allowing for easy gaster flexion. Similar behavioural strategies and morphological traits are also found in Temnothorax duloticus, but not in Protomognathus americanus. Hosts attacked by Temnothorax pilagens show little or only delayed flight responses. Occasionally, host workers try to drag slavemakers out of the nest, and only respond aggressively when attacked by them. The low host responsiveness towards invading Temnothorax pilagens indicate reduced or supressed enemy recognition. Due to their most effective stinging behaviour, Temnothorax pilagens can cause high rates of host casualties (ranging from 5% to 100%). As the high variation in the rate of casualties indicates, raids can be highly aggressive or relatively peaceful; the latter was often found in raids against queenless host nests. Slave-makers do not only take brood from the attacked host nests, but in 6 of the observed 11 raids, they also carry adult host workers back to their nest and integrated them into the slave workforce.