Temnothorax pilagens

A slave making species that parasitizes Temnothorax longispinosus and Temnothorax ambiguus.

Identification
Siefert et al. 2014. The differential diagnosis is done in relation to the congeneric slave-making species Temnothorax duloticus. Several discriminatory characters allow easy separation of T. pilagens and T. duloticus workers. Despite low sample size, there are highly significant differences in 55% of the tested characters (Tab. 1). The characters SL/CS or PoOc/CL alone should provide a safe and parsimonious numeric species delimitation. There is also no doubt that experienced observers can distinguish the two dulotic species by simple eye-inspection integrating subjective impressions on mesosomal, petiolar, cephalic and spine shape and mandibular dentition.

Distribution
Nearctic. North-eastern parts of the United States and possibly south-eastern Canada.

Distribution based on Regional Taxon Lists
Nearctic Region: United States.



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.

Biology
Siefert et al 2014:

An obligate slave-making ant with two known hosts: Temnothorax longispinosus and Temnothorax ambiguus. Mitochondrial DNA phylogeny indicates sister species relationship with T. longispinosus (Beibl et al. 2005).

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, T. pilagens was enslaving T. longispinosus and T. 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 T. longispinosus colonies only (Fisher Test; p=0.019). We did not sample Temnothorax communities with T. ambiguus only. T. pilagens colonies more often contained a mixed slave workforce than slaves of a single host species (Chi1²=49.59, p<0.001).

Nest construction. As its hosts, T. 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 T. pilagens workers) and 13 Temnothorax slaves (ranging from 2 to 50 workers) - but see Herbers and Foitzik (2002) for a nest with 27 T. pilagens and 55 slave workers.

Colony foundation. Four colonies with no T. pilagens workers, but a founding queen were collected. All four contained T. longispinosus slaves only.

Slave-raids. Obligatory slave hunter of Temnothorax longispinosus and T. 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 T. duloticus than that of P. 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 T. duloticus, but not in P. americanus. Hosts attacked by T. 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 T. pilagens indicate reduced or supressed enemy recognition. Due to their most effective stinging behaviour, T. 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.

Dulotic Temnothorax
Three slave-making species of the Temnothorax genus group (Bolton 2003) of the ant tribe Formicoxenini are known from North America. Each of the three represents an unmistakable combination of phenotypic characters. They all use species of the genus Temnothorax Forel, 1890 as hosts and cluster genetically with species of this genus (Beibl et al. 2005). The first species and outgroup to all the others (Beibl et al. 2005), Protomognathus americanus (Emery, 1895), is characterized by an elongated, semi-rectangular head capsule with extremely long antennal scobes that fully accommodate the short and flattened scape when it is folded back. These characters are a convergence to the Holarctic genus Harpagoxenus that belongs to the distantly related Leptothorax genus group. The second one, Temnothorax duloticus (Wesson, 1937), shows an acute, frontoventrally directed dent on the postpetiolar sternite, a high petiole with a bulky, truncate node that slopes down to the caudal cylinder with a distinct step. This particular petiolar shape and the reduction of antennal segments to 11 resemble the situation in the subgenus Mychothorax Ruzsky, 1904 of the genus Leptothorax Mayr, 1855. However, T. duloticus differs from the latter by the absence of a curved transverse carina on the stipes of the maxillae. On the third species….only little information exists to date (Herbers and Foitzik 2002, Beibl et al. 2005). Its phenotype is an unmistakable combination of an acute, frontoventrally directed dent on postpetiolar sternite, a stout, hump-backed mesosoma, small scape length, a high petiole that is in lateral aspect roughly triangular, a wide petiole and reduced mandibular dentition. Robin Stuart was the first who recognized the new species (Herbers and Foitzik 2002). We follow his proposal to name this slave-making species Temnothorax pilagens.

Nomenclature

 *  pilagens. Temnothorax pilagens Seifert, Kleeberg et al., 2014, 2013: 69, figs. 1-3, 5 (w.) U.S.A.

Worker
Measurements and indices in the text of description are the arithmetic means of the whole samples. Body size close to the genus average of Temnothorax, mean CS 645 μm. Head relatively broader [CL/CW 1.048 but 1.078 in duloticus], in dorsal aspect with strongly convex postocular sides and nearly linear, converging genae. Postocular distance smaller [PoOc/CL 0.364 but 0.394 in duloticus]. Antennae with 11 segments only, scape strikingly shorter [SL/CS 0.721 but 0.801 in duloticus]. Vertex finely longitudinally rugulose, distance between rugulae on central vertex 12 μm. The rugulae are connected by very delicate transverse anastomosae, which have on central vertex a mean distance of 12–14 μm. Clypeus finely longitudinally carinulate and in full-face view with straight or feebly emarginated anteromedian margin. Only the apical and subapical dent of the masticatory margin of the mandibles are fully developed and acute, the following dents are reduced to an undulating line of 3–6 shallow waves [in duloticus at least the first three dents are fully developed and the whole dentition is more similar to the normal Temnothorax situation]. Genae each with 2–6 semi-erect to erect setae [these are absent in duloticus]. Mesosoma massive, in lateral view with strongly convex dorsal profile, appearing hump-backed – i.e., much more compact and shorter than in duloticus [ML/CS 1.174 but 1.272 in duloticus]. Spines significantly shorter and thicker [SPST/CS 0.364 and SP/CS 0.300 but 0.425 and 0.361 respectively in duloticus]; spines in lateral view semi-erect, deviating from longitudinal axis of the mesosoma by 27–35°; in dorsal view diverging by 36–39° and with a larger basal distance [SPBA/CS 0.382, but 0.317 in duloticus]. The entire mesosoma exhibits a rugulose-microreticulate sculpture. Petiolar node in lateral view with a straight or weakly concave frontal profile forming with the short dorsal plane an angle of 81–91°; caudal petiolar profile steeply but linearly sloping down to junction with postpetiole [in duloticus there is a distinct step in the caudal slope caused by a prolongation of the caudal cylindric part of petiole]. Petiole clearly shorter [PEL/CS 0.452 but 0.517 in duloticus]. Postpetiolar sternite in lateral view with a strongly developed, triangular dent, directed anteroventrad, comparable to situation in duloticus. Dorsum of petiole node in dorsal view 1.7–2.0 fold wider than long, postpetiole in dorsal view roughly trapezoidal and much wider than in any independent Temnothorax species, PPW/CS 0.491. Whole surface of petiolar and postpetiolar nodes coarsely microreticulate. Surface of 1st gaster tergite smooth and shining, but with a very delicate (sculpture lines only 0.5 μm thick), patchily missing microreticulum [in duloticus there is nowhere a connected microreticulum – it is reduced to isolated, scattered structures in the form of an “X” or of a matchstick man]. All dorsal body surfaces with setae of medium length. Dorsal head dark brown. Mesosoma, waist and appendages yellowish, propodeum, meso- and metapleuron sometimes darker brownish. Gaster tergites yellowish, often with small brown bands at posterior margin; the first tergite usually shows big brown patches on each side that may fuse medially in some specimens, then covering 70% of total surface.

Queen
Measurements and indices in the text of description are the arithmetic means of the whole samples. Head size similar to the genus average of Temnothorax, mean CS 673 μm. Head very short [CL/CW 0.982 but 1.044 in duloticus], in full-face view with strongly convex postocular sides, a feebly concave occipital margin and linear, converging genae. Postocular distance very short [PoOc/CL 0.341 but 0.401 in duloticus]. Antennae with 11 segments only, scape very short [SL/CS 0.683 but 0.748 in duloticus]. Vertex longitudinally rugulose, distance between rugulae on central vertex 15 μm, the interspaces between rugulae with reticulate microsculpture. Clypeus finely longitudinally carinulate and in full-face view with feebly notched anteromedian margin. The three apical dents of the mandibular masticatory margin are fully developed and acute, the following four dents are reduced to denticles. Mesomoma very small for Temnothorax in general, but not smaller than in duloticus [ML/CS 1.484, MW/CS 0.904, MH/CS 0.868]. Spines well-developed and acute but significantly shorter and relatively thicker than in duloticus [SPST/CS 0.364 and SP/CS 0.265 but 0.457 and 0.360 respectively in duloticus]; spines in lateral view very weakly erected, deviating from longitudinal axisof mesosoma by 20°; in dorsal view with a very large basal distance and weakly diverging [SPBA/CS 0.482, SPTI/CS 0.479; in duloticus more clearly diverging, SPBA/CS 0.434 and SPTI/CS 0.530]. Whole mesosoma with rugose-microreticulate sculpture that is on mesonotum and mesopleuron less developed. Petiolar node in lateral view very high and with a weakly concave frontal profile forming with the short dorsal plane an angle of 80°; caudal petiolar profile steeply and almost linearly sloping down to junction with postpetiole [in duloticus there is a distinct step in the caudal slope caused by a significant prolongation of the caudal cylindric part of petiole]. Petiole clearly shorter than in duloticus [PEL/CS 0.496 vs. 0.551]. Postpetiolar sternite in lateral view with a strongly developed, triangular dent, directed anteroventrad, comparable to situation in duloticus. Dorsum of petiole node in dorsal view 1.9fold wider than long, postpetiole in dorsal view broadly cordate and much wider than in any independent Temnothorax species, PPW/CS 0.491. Whole surface of petiolar and postpetiolar nodes strongly microreticulate. Surface of 1st gaster tergite smooth and shining but with a very delicate (sculpture lines only 0.5 μm thick), patchily missing microreticulum [in duloticus the microreticulum more incomplete - frequently reduced to isolated, scattered structures in the form of an “X” or of a matchstick man]. All dorsal body surfaces with setae of medium length, the longest on occiput are 74 μm long. Head, mesosoma and waist brown, appendages yellowish to yellowish brown. Gaster tergites yellowish brown, a lighter yellowish patch is at the base of 1st tergite.

Type Material
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.

Etymology
The species epithet refers to the slave raiding behaviour of the new ant species (from Latin: pilare, English: to pluck, plunder, pillage).