Revision of the Plagiolepis schmitzii group with description of Pl. invadens sp. nov. – a new invasive supercolonial species (Hymenoptera: Formicidae)

Bernhard Seifert

doi:10.3897/dez.67.53199

Abstract

Using high-resolution stereomicroscopy and exploratory data analyses, a taxonomic revision of the cryptic species close to Plagiolepis schmitzii Forel, 1895, called Pl. schmitzii group, was conducted. Morphology was numerically recorded under highly standardised conditions considering absolute size and 16 shape, pubescence and surface characters. A key to the non-parasitic Westpalaearctic species of the ant genus Plagiolepis Mayr, 1861 is provided which firstly separates, on species group level, the Pl. pygmaea (Latreille) species group, the Pl. pallescens Forel species group and the Pl. schmitzii species group and, finally, on species level, the cryptic species of the latter group. The recognised species of the Pl. schmitzii species group are Pl. schmitzii Forel, 1895 (invasive species), Pl. barbara Santschi, 1911, Pl. atlantis Santschi, 1920 and Pl. invadens sp. nov. (invasive species) that is described as new from a supercolony in Germany. Based on morphological arguments, the taxa Pl. barbara var. madeirensis Emery, 1921, Pl. maura polygyna Santschi, 1922 and Pl. schmitzii var. tingitana Santschi, 1936 are recognised as junior synonyms of Pl. schmitzii, the taxa Pl. schmitzii crosi Santschi, 1920, Pl. pallescens var. kabyla Santschi, 1920 and Pl. perperamus Salata et al., 2018 as junior synonyms of Pl. atlantis and the taxon Pl. maura Santschi, 1920 as junior synonym of Pl. barbara. A concluding comparative section suggests that pre-adaptations for anthropogenous dispersal and transformation to supercoloniality in introduction areas are apparently common traits in Plagiolepis ants.

Key Words

cryptic species, numeric taxonomy, invasive pest species, supercoloniality

Introduction

The natural distributional range of the ant genus Plagiolepis Mayr, 1861 includes Africa, Australia and the temperate and tropical zones of Eurasia. Close to 100 available names, attributable to this genus, have been published so far. In the absence of a modern and thorough revision, the genus Plagiolepis is assumed to contain 60 valid species, 20 valid subspecies and 10 valid fossil species (AntWeb 2020). This paper is part of a revision of the independent Westpalaearctic species of the genus conducted by the author through the last four years. The situation in the social parasites (inquilines) of Plagiolepis, with a surprisingly high number of undescribed species, is not considered here. The investigation of independently-living species under highly-standardised conditions revealed the presence of at least 11 good species distributed in the Mediterranean and sub-Mediterranean zoogeographic zones of the Westpalaearctic (this paper and Kirschner et al. in preparation). Morphological discrimination of the many cryptic species in these tiny ants unavoidably requires high-resolution optical systems and highly accurate, reproducible numeric recording of unambiguously defined characters and adequate exploratory and hypothesis-driven data analyses. Several papers dealing with some aspects of Plagiolepis taxonomy of the region were published during the last decades (Radchenko 1989, 1996; Wetterer et al. 2007; Boer 2008; Sharaff et al. 2011). Investigation and analysis methods, quality of equipment and scope of these taxonomic studies were not adequate for the degree of difficulty we encountered here.

A disputable paper on Plagiolepis taxonomy was added very recently: Salata et al. (2018) described Plagiolepis perperamus as new East Mediterranean sister species of Plagiolepis schmitzii Forel, 1895. Detailed comments on this paper are given in the special part below and it was just this particular paper that prompted me to investigate all available types of taxa with a taxonomic position close to Plagiolepis schmitzii. Direct type investigation was possible in nine taxa – six of these turned out as junior synonyms. Junior synonymy of two further taxa appeared most probable by geographic indication and/or inspecting photos in AntWeb (2020). Unfortunately, there was no avenue to assess the status of Plagiolepis barbara var. pyrenaica Emery, 1921. The survey, reported here, resulted in the recognition of four verifiable species in the Plagiolepis schmitzii group and the identification of three senior synonyms of Pl. perperamus Salata et al. (2018).

Material

Morphometric characters were recorded in a total of 46 samples and 137 worker individuals from Madeira, the Canaries, Europe, North Africa and Asia Minor. Type specimens of nine taxa were investigated. Consideration of males and gynes is not performed here for the following reasons: (a) sexual castes are strongly under-represented in the collections and in many taxa unknown, (b) subjective assessment and tentative morphometrics of the few specimens available suggested that gynes could provide useful characters for species discrimination, but worker-associated nest samples of gynes were not available in just the critical species and (c) the author does not know of a single formicine ant group worldwide where a clear species separation has been testably demonstrated by means of male genitalia.

The material examined is listed in the individual species treatments in the following sequence and format: site, date in the format yyyy.mm.dd, sample number, [latitude in decimal format, longitude in decimal format, altitude]. The accuracy of coordinates is proportional to the number of decimal points and “xx” in the sampling date sequence means missing data. In some samples without any direct or derived information on date, the name of the collector is given to allow an approximate conclusion on the period of collection. The abbreviations of depositories are as follows:

DBU Wrocław Department of Biodiversity and Evolutionary Taxonomy, University of Wrocław, Poland

MCSN Genoa Museo Civico di Storia Naturale Genoa, Italy

MHN Genève Muséum d’Histoire Naturelle de Genève, Genève, Switzerland

NHM Basel Basel Naturhistorisches Museum, Basel, Switzerland

SMN Görlitz Senckenberg Museum für Naturkunde Görlitz, Görlitz, Germany

Methods

Equipment and measurement procedures

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 high-performance stereomicroscope M165C, equipped with a 2.0 planapochromatic objective (resolution 1050 lines/mm), was used at magnifications of 120–360×. A Schott KL 1500 LCD cold-light source, equipped with two flexible, focally mounted light-cables, providing 30°-inclined light from variable azimuth directions, allowed sufficient illumination over the full magnification range and a clear visualisation of silhouette lines. A Schott KL 2500 LCD cold-light source in combination with a Leica coaxial polarised-light illuminator provided optimum 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 the voltage up and down. A Leica cross-scaled ocular micrometer with 120 graduation marks was used. To avoid the parallax error, its measuring line was constantly kept vertical within the visual field. To avoid rounding errors, all measurements were recorded in µm, even for characters for which this precision is impossible. The z-stack photos were made with a Leica Z6 APO photomicroscope, equipped with an objective Planapo 2.0× and a Leica microscope camera DFC420.

The morphometric characters and removal of allometric variance

Sixteen morphometric characters were investigated in worker ants. In all bilaterally developed characters, arithmetic means of both sides were calculated. The characters are defined as follows:

BPdG – mean distance between the base points of pubescence hairs on dorsal plane of 1^st gaster tergite. Usually calculated from the sqPDG and PLG data, providing an approximate solution by the formula BPdG = sqrt(PLG*PDG). The direct and exact solution is by counting the number of base points N found within a total area A with BPdG = sqrt (A/N).

CL – maximum head (cephalic) length in median line; the head must be carefully tilted at highest magnifications to the position with the true maximum. Excavations of hind vertex 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 measurable head (cephalic) width. The position of measuring line is defined alone by the maximum and may be across or behind the eyes.

dAN – minimum distance of the inner (centripetal) margins of antennal socket rings which is best measurable in dorsofrontal view [see Fig. 271 in Seifert (2018)].

dTP – distance of the centres of clypeal tentorial pits.

EL – large diameter of the elliptic compound eye measured over all structurally-visible ommatidiae – i.e. also including unpigmented ones in a marginal position.

F2, F3, F4 – median length of 2^nd, 3^rd, 4^th funiculus segment in dorsal view. Dorsal view is given when the swivelling plane of 1^st funiculus segment is positioned in the visual plane. Take care to really measure median length (the segment’s sides often have unequal lengths!) and to recognise the real distal margin of the segments. The latter may have a very thin cuticle, frequently producing a narrow, shining ribbon that seems to be, by optical impression, demarcated from the rest of the segment.

ML – mesosoma length without neck shield (fringe), posterior measuring point: caudalmost point of metapleuron; parallelism of the measuring line to the longitudinal mesosomal axis has to be considered – i.e. in lateral view, the anterior measuring point is found at a lower level of focus.

MW – maximum mesosoma width; this is in worker’s pronotal width.

PLG – mean length of at least seven pubescence hairs on dorsal plane of 1^st gaster tergite in the area about 30 to 100 µm before posterior tergite margin. In the densely pubescent gasters of Pl. schmitzii group species, visualisation of full hair length may be difficult. Take care to provide adequate illumination, vary viewing positions or perform local ablations of pubescence. These clearings expose full hair length at the margins of the adjacent intact pubescence area.

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 [see Fig. 146 in Seifert (2018)].

PrOc – preocular distance in lateral view; in Plagiolepis, the shortest distance between the anterior eye margin to that point of the genal margin which is in closest proximity to the dorsal condyle of mandibular joint.

SL – maximum straight line scape length (excluding the articular condyle and its neck).

sqPDG – square root of transverse pubescence distance PDG [in µm] on the dorsomedian part of first gaster tergite about 30 to 100 µm before posterior tergite margin. To reduce accidental errors, several countings along differently positioned, transverse measuring lines are averaged until the sum of hairs counted is 50 at least. Exact counting is only possible with clean surfaces, high-resolution stereomicroscopy at magnifications ≥ 280× and reflection-reduced illumination visualising the full length of hairs. Surface spots with torn-off pubescence are excluded from counting. Measuring procedure: the number of pubescence hairs n crossing a measuring line of length L is counted, hairs just touching the line score as 0.5. Mean PDG is then L/n.

Removal of allometric variance (RAV) was performed with the procedure described by Seifert (2008). RAV enables a direct comparison of data of related species in tables and improves the performance of principal component analyses considerably. RAV was calculated for the assumption of all individuals having a cephalic size of CS = 0.45 mm by overall functions computed as average of specific functions of four species with > 50 workers per species available. These were Plagiolepis schmitzii, Pl. atlantis, Pl. taurica and Pl. occidentalis. The latter name (see Seifert 2018) is not available at present, but will be made available by an upcoming paper of Kirschner et al. (in preparation).

BPdG _0.45 [µm] = BPdG / (–6.915 * CS + 26.47) * 23.36

CL/CW_0.45 = CL/CW / (–0.5438 * CS + 1.3600) * 1.1153

dAN/CS_0.45 = dAN/CS / (0.1248 * CS + 0.1907) * 0.2468

dTP/CS_0.45 = dTP/CS / (0.0857 * CS + 0.4710) * 0.5096

EL/CS_0.45 = EL/CS / (–0.0748 * CS + 0.3109) * 0.2772

F2/CS_0.45 [%] = F2/CS / (7.104 * CS + 4.087) * 7.284

F3/CS_0.45 [%] = F3/CS / (2.709 * CS + 8.102) * 9.321

F4/CS_0.45 [%] = F4/CS / (4.152 * CS + 9.026) * 10.895

F4/F3_0.45 = F4/F3 / (0.0883 * CS + 1.1294) * 1.1692

ML/CS_0.45 = ML/CS / (0.4876 * CS + 0.9631) * 1.1825

MW/CS_0.45 = MW/CS / (0.1092 * CS + 0.5846) * 0.6338

PLG/CS_0.45 [%] = PLG/CS / (–11.623 * CS + 13.598) * 8.368

PoOc/CL_0.45 = PoOc/CL / (–0.2121 * CS + 0.4629) * 0.3674

PrOc/CS_0.45 = PrOc/CS / (0.0187 * CS + 0.2278) * 0.2362

SL/CS_0.45 = SL/CS / (0.0204 * CS + 0.9530) * 0.9622

sqPDG _0.45 [µm] = sqPDG / (–2.699 * CS + 4.975) * 3.760

In the species sections, I relinquished presenting verbal descriptions of those morphological characters which may characterise the whole genus or a species group, but are not recognised to have a value for species discrimination. The pictures provided in this paper plus the references to pictures in AntWeb (2020) provide a sufficient overall impression on setae, pubescence and surface characters.

Explorative and supervised data analyses, classification and statistical testing

Analysing the morphometric data, four forms of exploratory data analyses were run using nest centroids as input data (NC clustering). These were firstly hierarchical NC-Ward clustering, secondly and thirdly, the hierarchical method NC-part.hclust and the iterative vector-quantisation method NC-part.kmeans – both implemented in partitioning algorithms, based on recursive thresholding (for details see Csösz & Fisher, 2015) and non-metric multidimensional scaling, combined with iterative vector-quantisation NC-NMDS-k-means (Seifert et al. 2013). A principal component analysis (PCA) of RAV-corrected data was applied when one of the compared species was present in the data pool with only few specimens, making the application of NC clustering not feasible.

Checking samples with controversial classifications was done by an interaction of NC clustering and a controlling linear discriminant analysis (LDA) in which these samples were run as wild-cards, following the rationale described in Seifert et al. (2013). The final classification (“final species hypothesis”) was established by the LDA in an iterative procedure and there remained no undecided cases, even if their posterior probabilities were close to 0.5. The decision to recognise a cluster as a valid species was based on the GAGE species concept (Seifert 2020) – here a < 4% error threshold was applied. LDA, PCA and ANOVA tests were run with the SPSS 16.0 software package.

Results

The Westpalaearctic species groups of independent Plagiolepis species

Based on investigation of type specimens and, in some cases, of only their images in AntWeb (2020), the independent species of the genus Plagiolepis of the Westpalaearctic can be subdivided in three major groups:

1 The taxa close to Pl. pygmaea Latreille, 1798 which are characterised by the 4^th funiculus segment being much longer than the 3^rd and the rather widely-spaced basal pits of pubescence hairs on the dorsum of 1^st gaster tergite. Data of 20 nest sample means are 1.631±0.072 [1.510, 1.784] in F4/F3 and 23.68±2.99 [17.7, 28.5] µm in BPdG. The mean BPdG translates into 1783 pubescence hairs/mm². Without making implications on their potential species status by using here binary names, the described taxa of this group are Pl. pygmaea Latreille, 1798, Pl. obscuriscapa Santschi, 1922 and Pl. karawajewi Radchenko, 1989.

2 the Plagiolepis pallescens group which is characterised by widely-spaced basal pits of pubescence hairs on the dorsum of 1^st gaster tergite (Fig. 1a) and the 4^th funiculus segment not being much longer than the 3^rd. Data of 113 nest sample means are 1.160±0.046 [1.041, 1.292] in F4/F3 and 30.23±1.73 [24.1, 34.2] µm in BPdG. The mean BPdG translates into 1099 pubescence hairs/mm². Without making implications here on their potential species status by use of binary names, the described taxa of this complex are Pl. pallescens Forel, 1889, Pl. minu Forel, 1911, Pl. taurica Santschi, 1920, Pl. sordida Santschi, 1920, Pl. ancyrensis Santschi, 1920, Pl. vindobonensis Lomnicki, 1925, Pl. compressa Radchenko, 1996, Pl. dlusskyi Radchenko, 1996, Pl. calva Radchenko, 1996 and Pl. sp. OCCIDENTALIS – an undescribed species (name not available at present, Seifert 2018).

Figure 1.

Surface of posterior part of 1^st gaster tergite of a worker of Plagiolepis taurica ( a) and Pl. schmitzii (b).

3 the Plagiolepis schmitzii group which is characterised by narrowly-spaced basal pits of pubescence hairs on the dorsum of 1^st gaster tergite (resulting in a dense pubescence, Fig. 1b) and the 4^th funiculus segment not being much longer than the 3^rd. Data of 46 nest sample means are 1.199±0.052 [1.106, 1.362] in F4/F3 and 16.27±1.59 [13.4, 20.2] µm in BPdG. The mean BPdG translates into 3778 pubescence hairs/mm². According to the data currently available, this group contains four species: Pl. schmitzii Forel, 1895, Pl. barbara Santschi, 1911, Pl. atlantis Santschi, 1920 and Pl. invadens sp. nov.

Key to species groups of Plagiolepis and the species of the Pl. schmitzii group

Note: This key does not consider the parasitic species (inquilines) of the genus which contain a surprisingly high number of undescribed species and are frequently so tiny in size and so weakly sclerotised that traditional forms of ant preparation appear inadequate.

1a	4^th funiculus segment much longer than 3^rd; F4/F3 > 1.44 [error 0% in 20 nest means.	Pl. pygmaea group
1b	4^th funiculus segment only moderately longer than 3^rd; F4/F3 < 1.44 [error 0% in 159 sample means]	2
2a	Dorsum of 1^st gaster tergite with widely-spaced pits of pubescence hairs and dilute pubescence cover; BPdG > 22 µm, sqPDG > 3.70 [error 0% in 113 sample means].	Pl. pallescens group
2b	Dorsum of 1^st gaster tergite with densely-spaced pits of pubescence hairs and dense pubescence cover; BPdG < 22 µm, sqPDG < 3.70 [error 0% in 46 sample means]; Pl. schmitzii group	3
3a	3^rd funiculus segment short; with measurements in mm, discriminant 355F3–35.54CW+ 1.879 < 0. Only known from a supercolony in W Germany	Pl. invadens sp. nov.
3b	3^rd funiculus segment long; discriminant > 0	4
4a	Eye length and distance of pubescence hair pits larger (EL/CS 0.317±0.011, BPdG 19.57±1.80). With all measurements in mm, sample means of discriminant 164.75EL–42.944SL+0.080*PoOc–1.224 > 2.3 [error 0% in 3 sample means]. Morocco east to Tunisia	Pl. barbara
4b	Eye length and distance of pubescence hair pits smaller (EL/CS 0.274±0.16, BPdG 16.18±1.72). Sample means of discriminant < 2.3 [error 0% in 40 sample means]	5
5a	With all measurements in mm, discriminant 48.98CL+72.21PoOc–65.80SL–171.2F4+173.5PLG+35.42MW–12.79 < 0 [error 0% in 64 individuals]	Pl. schmitzii
5b	Discriminant > 0 [error 1.8% in 56 individuals]	Pl. atlantis

Treatment by species

Plagiolepis schmitzii Forel, 1895

Plagiolepis pygmaea var. schmitzii Forel, 1895 Published type locality: “Serra d’Agua, Madeira (Seminardirektor P. E. Schmitz)” [32.727°N, 17.027°W, 347 m alt.]. Nine paratype workers were investigated from MHN Genève collected by Schmitz in at least three localities in Madeira. Amongst these were workers pictured in AntWeb (2020), labelled “Pl. pygmaea Latr. v. schmitzii For. Garajau Madeira Schmitz 3, 5–9”, “ANTWEB CASENT0909859”. All investigated specimens do not carry a label pointing to Serra d’Agua. I also investigated the lectotype gyne labelled “Pl. pygmaea Latr. v. schmitzii For. Madeira (Schmitz)”, “ANTWEB CASENT0909858”, “LECTOTYPE (upper) Plagiolepis pygmaea var. schmitzi Forel, 1895 desig. Wetterer & Espadaler”.

Plagiolepis barbara var. canariensis Santschi, 1920 [syn. schmitzii] Described from Tenerife: La Laguna, Bejano and Esperanze. No type specimens or figures of this taxon were available and the descriptive statements of Santschi are useless. A junior synonymy with Pl. schmitzii appears probable for zoogeographic reasons.

Plagiolepis barbara var. madeirensis Emery, 1921 [syn. schmitzii] Identification by evaluation of photos of a type worker in AntWeb (2020) labelled “Pl. barbara var. madeirensis Em.”, “SYNTYPUS Plagiolepis barbara var. madeirensis Emery, 1921”, “Funchal Madeira De Guerne”, “ANTWEB CASENT0905140”.

Plagiolepis maura polygyna Santschi, 1922 [syn. schmitzii] Four type workers were investigated from NHM Basel labelled “Tunisie, Cheri Chera 27 III 21 Santschi”.

Plagiolepis schmitzii var. tingitana Santschi, 1936 [syn. schmitzii] Santschi published as collecting sites “Tanger w. (type) et Volubilis w. (Alluaud)”. Four type workers were investigated from NHM Basel labelled “Tanger Ch. Alluaud”.

Material examined

A total of 21 samples with 64 workers were subject to morphometric investigation.

Algeria: Mascara, 1926 [35.398°N, 0.138°E, 594 m alt.]. England: Isle of Wight: Bonchurch, 2007.06.21 [50.59°N,1.19°W, 3 m alt.]. Germany: Schkeuditz, 2019.02.11 [51.392°N, 12.204°E, 104 m alt.]; Schriesheim, 2017.05 [49.470°N, 8.46°E, 118 m alt.]; Seligenstadt, 2009.04 [50.045°N, 8.975°E, 115 m alt.]. Morocco: Chefchaouen, 2009.03 [35.183°N, 5.300°W, 400 m alt.]; Meknes, 1940.02.02 [33.894°N, 5.547°W, 551 m alt.]; Rabat (Santschi) [33.973°N, 6.845°W, 84 m alt.]; Tanger (Alluaud), type Pl. schm. tingitana [35.755°N, 5.819°W, 30 m alt.]; Tiz-n-Test –8 km N, 1987.05.05, No 13015 [30.889°N, 8.370°W, 1810 m alt.]. Netherlands: Brakel, 2013.02 [51.820°N, 5.093°E, 2 m alt.]; Tholen, 2011.05.27 [51.539°N, 4.217°E, 1 m alt.]; Utrecht, 2006.09.09 [52.09°N, 5.12°E, 10 m alt.]. Portugal: Madeira: Estreito da Calheta, 2009.03 [32.733°N, 17.167°W, 350 m alt.]; Madeira, 1400 m (Schmitz), paratypes Pl. schmitzii [33.0°N, 17.0°W, 1400 m alt.]; Madeira: Garajau (Schmitz), paratypes Pl. schmitzii [32.64°N, 16.85°W, 230 m]; Madeira: Palheiro (Schmitz), paratypes Pl. schmitzii [32.65°N, 16.87°W, 360 m alt.]. Spain: La Palma: Todoque, 2010.03.02 [28.617°N, 17.903°W, 334 m alt.]; Tenerife: Las Canadas NP, 1999.06.02 [28.26°N, 16.61°W, 2300 m alt.]; Sevilla, 2019.06.24 [37.394°N, 5.994°W, 10 m alt.]. Tunisia: Cherichara, 1921.03.27, types Pl. polygyna [35.637°N, 9.815°E, 255 m alt.].

Diagnosis and taxonomy

(Table 1, key, AntWeb, 2020: CASENT0906252, Figs 2–4):

Table 1.

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RAV-corrected morphometric data of worker individuals of the Plagiolepis schmitzii complex given as arithmetic mean±standard deviation [minimum, maximum]; n = number of individuals. F values and significance levels p are from a univariate ANOVA and evaluate the differences between Pl. atlantis and Pl. invadens sp. nov.

	barbara (n = 7)	schmitzii (n = 64)	atlantis (n = 56)	ANOVA F_1,64, p	invadens sp. nov. (n = 10)
CS [µm]	488 ± 39 [428, 527]	466 ± 39 [390, 548]	455 ± 33 [404, 533]	0, n.s.	455 ± 26 [408, 497]
CL/CW_0.45	1.106 ± 0.012 [1.092, 1.128]	1.113 ± 0.023 [1.066, 1.173]	1.108 ± 0.024 [1.062, 1.193]	0.01, n.s.	1.108 ± 0.013 [1.090, 1.132]
dTP/CS_0.45	0.502 ± 0.012 [0.478, 0.513]	0.508 ± 0.011 [0.487, 0.535]	0.502 ± 0.011 [0.480, 0.538]	0.80, n.s.	0.499 ± 0.009 [0.479, 0.509]
dAN/CS_0.45	0.243 ± 0.006 [0.235, 0.253]	0.243 ± 0.007 [0.222, 0.255]	0.241 ± 0.008 [0.224, 0.266]	8.61, 0.005	0.249 ± 0.005 [0.241,0.256]
EL/CS_0.45	0.321 ± 0.015 [0.301,0.336]	0.286 ± 0.010 [0.267,0.311]	0.263 ± 0.013 [0.240,0.299]	1.12, n.s.	0.259 ± 0.004 [0.254, 0.264]
PrOc/CS_0.45	0.219 ± 0.009 [0.207, 0.231]	0.239 ± 0.011 [0.210, 0.262]	0.247 ± 0.012 [0.218, 0.273]	1.43, n.s.	0.252 ± 0.006 [0.241, 0.264]
PoOc/CL_0.45	0.341 ± 0.008 [0.334, 0.356]	0.359 ± 0.008 [0.341, 0.381]	0.376 ± 0.012 [0.351, 0.399]	1.28, n.s.	0.371 ± 0.006 [0.361, 0.381]
SL/CS_0.45	0.945 ± 0.019 [0.920, 0.971]	1.034 ± 0.020 [0.980, 1.072]	0.970 ± 0.024 [0.920, 1.011]	36.72, 0.000	0.923 ± 0.011 [0.900, 0.936]
F2/CS_0.45 [%]	7.16 ± 0.51 [6.61, 7.89]	7.93 ± 0.57 [6.54, 9.11]	7.11 ± 0.39 [6.16, 8.15]	0.49, n.s.	7.21 ± 0.44 [6.65, 8.06]
F3/CS_0.45 [%]	9.12 ± 0.28 [8.64, 9.39]	10.17 ± 0.48 [9.20, 11.55]	9.36 ± 0.35 [8.71, 10.32]	265.5, 0.000	7.36 ± 0.42 [6.88, 8.11]
F4/CS_0.45 [%]	10.88 ± 0.34 [10.34, 11.32]	12.27 ± 0.50 [11.04, 13.23]	10.96 ± 0.39 [10.17, 11.99]	61.8, 0.000	9.88 ± 0.43 [9.22, 10.50]
F4/F3_0.45	1.193 ± 0.024 [1.158, 1.222]	1.208 ± 0.060 [1.052, 1.357]	1.172 ± 0.046 [1.054, 1.312]	89.9, 0.000	1.346 ± 0.086 [1.218, 1.511]
ML/CS_0.45	1.205 ± 0.015 [1.190, 1.226]	1.234 ± 0.034 [1.160, 1.322]	1.174 ± 0.034 [1.105, 1.279]	0.01, n.s.	1.175 ± 0.034 [1.127, 1.248]
MW/CS_0.45	0.640 ± 0.017 [0.609, 0.656]	0.629 ± 0.018 [0.581, 0.669]	0.653 ± 0.022 [0.607, 0.695]	28.82, 0.000	0.615 ± 0.012 [0.601, 0.635]
PLG/CS_0.45 [%]	7.22 ± 0.28 [6.84, 7.58]	6.73 ± 0.46 [5.82, 7.77]	7.30 ± 0.49 [6.38, 8.25]	0.93, n.s.	7.46 ± 0.49 [6.74, 8.07]
sqPDG _0.45	3.50 ± 0.33 [3.05, 4.06]	2.87 ± 0.31 [2.40, 3.76]	2.89 ± 0.19 [2.55, 3.28]	9.61, 0.003	2.70 ± 0.08 [2.56, 2.80]
BPdG _0.45	19.81 ± 1.87 [17.8, 23.4]	15.78 ± 2.04 [12.7, 21.5]	16.53 ± 1.28 [14.4, 19.4]	4.94, 0.030	15.60 ± 0.78 [14.7, 16.9]

Figure 2.

Head of a worker of Plagiolepis schmitzii (image from AntWeb, 2020: CASENT0906252, photographer E. Ortega).

Figure 3.

Lateral aspect of a worker of Plagiolepis schmitzii (image from AntWeb, 2020: CASENT0906252, photographer E. Ortega).

Figure 4.

Dorsal aspect of a worker of Plagiolepis schmitzii (image from AntWeb 2020: CASENT0906252, photographer E. Ortega).

Pl. schmitzii has the longest scape and funiculus segments within the species group. The most similar species is Pl. atlantis, whereas Pl. barbara and Pl. invadens sp. nov. appear more distant and have much shorter scapes (for their status, see there). The material allocated here to Pl. schmitzii (21 samples, 64 specimens) and Pl. atlantis (20 samples, 56 specimens) were investigated by exploratory data analyses (EDAs). Considering absolute head size and all 16 allometrically-corrected shape, pubescence and surface characters, NC-Ward, NC-part.kmeans, NC-NMDS-kmeans, a principal component analysis (PCA) and NC-part.hclust confirmed two clusters. The classification of the first four EDAs agreed for each of the 41 samples, whereas NC-part.hclust exposed two samples as indeterminate outliers (Fig. 5). If these two samples were run as wild-cards in a controlling linear discriminant analysis (LDA), they were classified in agreement with the first four EDAs. The classification error of the LDA on an individual level was 0.7 % in 120 workers. All these data are a clear indication of separate species identity of Pl. schmitzii and Pl. atlantis. Running the type series of seven taxa as wild-cards in the LDA resulted in clear allocations to either cluster. The posterior probabilities for allocation to the Pl. schmitzii cluster were 1.000 in each of the three paratype series of Pl. schmitzii from Madeira, 0.999 in the type series of Pl. polygyna and 1.000 in the type series of Pl. tingitana, whereas the posterior probabilities for allocation to the Pl. atlantis cluster were 1.000 in the type series of Pl. atlantis, 0.916 in the type series of Pl. crosi, 0.994 in the type series of Pl. kabyla and 0.998 in the type series of Pl. perperamus. All five EDAs allocated any type series in agreement with the LDA wild-card runs. As a consequence, Pl. polygyna and Pl. tingitana are junior synonyms of Pl. schmitzii, whereas Pl. crosi, Pl. kabyla and Pl. perperamus are junior synonyms of Pl. atlantis. Without types of Plagiolepis barbara var. madeirensis Emery, 1921 being available, the synonymisation of this taxon with Pl. schmitzii is highly probable for two reasons: (1) the scape is very long: the data of SL and CW, as they can be derived with minimum distortions from the CASENT0905140 photo of the Pl. b. madeirensis type, are within the Pl. schmitzii cluster and outside the cluster formed by Pl. atlantis, Pl. invadens sp. nov. and Pl. barbara; (2) Madeira seems to be inhabited by only a single, very abundant Plagiolepis species which is to be named Pl. schmitzii.

Figure 5.

Results of four variants of NC-clustering: NC-Ward (hierarchical, tree shown), NC-part.hclust (hierarchical), NC-part.kmeans (iterative vector-quantisation), NC-NMDS (non-metric scaling) ; 21 nest samples of Plagiolepis schmitzii (grey bars) and of 20 nest samples of Pl. atlantis (black bars). Outliers in NC-part.hclust are given by the white gap.

Distribution and biology

According to direct investigation of voucher specimens, Pl. schmitzii is distributed from Madeira and the Canaries across West Mediterranean Africa east to Tunisia. There are anthropogenous introductions north of 46°N. In Germany, it has been found so far only in houses, with workers occasionally foraging outdoors. However, year-round outdoor nesting has been recently reported from two sites in the Netherlands (Jinze Noordijk pers. comm. 2020). Accordingly, there is a clear potential for becoming an established neozoon in NW and Central Europe in the context of global warming. Polygyny and polydomy with colony territories over several houses is confirmed for populations in the Netherlands and Germany. The population from Madeira, Estreito da Calheta is obligatory polygynous and highly polyandrous (a queen may have up to 14 different mates), whereas the population from Chefchaouen in Morocco is facultatively polygynous and moderately polyandrous (Thurin et al. 2011). These authors stated that relatedness within colonies remains high because of sib-mating and relatedness of the male mates of a queen (fixation index F_it = 0.24 in the Madeiran and 0.26 in the Moroccan population). Small size, polygyny with intranidal mating and broad food spectrum are pre-adaptations for a career as a tramp species. Pl. schmitzii is everywhere present in Madeira and rivals there in abundance with Lasius cf. grandis Forel.

Plagiolepis atlantis Santschi, 1920

Plagiolepis maura var. atlantis Santschi, 1920 Three gyne and five worker syntypes were investigated from NHM Basel, labelled “Plagiolepis Type maura Sants v. atlantis Santi”, “13.”, “Tunisie Dir el Kef Dr. F. Santschi”, “mai 1913”, “type”, “ANTWEB CASENT0912421”.

Plagiolepis schmitzii crosi Santschi, 1920 [syn. atlantis] Three type workers were investigated from NHM Basel, labelled “Plagiolepiscrosi. Sants”, “Algerie Mascara A-Cros.” and “ANTWEB CASENT0912429”.

Plagiolepis pallescens var. kabyla Santschi, 1920 [syn. atlantis] Three type workers were investigated from NHM Basel, labelled “Plagiolepis maura Sant v. kabyla Sant type”, “Tunisie Ain Draham Santschi 1913”, “Type”, “ANTWEB CASENT0912423”. The type specimens are pale yellowish and thus paler than usually seen.

Plagiolepis perperamus Salata et al., 2018 [syn. atlantis] Three paratypes were investigated from the holotype nest, labelled “LBC-GR00042”, “GREECE NW Crete | 3 km S Askifou 800 m a.s.l.| 35°16'N/24°10'E | 1 V 2007. L.& M.L. Borowiec”; depository DBU Wroclaw.