Stenamma debile

A common W-European and Anatolian taxon (Rigato & Toni, 2011).

Identification
Rigato (2011) - Female castes have  slightly shorter  appendages  than  Stenamma  westwoodii and  Stenamma  sardoum and, with rare exceptions, the worker has the promesonotum mainly longitudinally rugulose with rare anastomoses and an ill-defined median carina (Fig.  31). Petiolar and postpetiolar sternites in profile are usually straighter than in related taxa, and postpetiole is about as high as long. S. debile females are usually mainly brown, darker than most species considered in this paper. The male is quite distinct in its combination of narrow, usually 3-toothed mandibles, and unsculptured propodeal dorsum.

Stenamma debile is the most widespread European Stenamma species and I examined specimens from Spain to Turkey. It may be confused with S. westwoodii, S. sardoum and Stenamma striatulum (see under these species for further details). Some variation occurs especially in colour, although S. debile is usually darker.

Distribution
Widespread and common in West Europe, also recorded in East Europe (including West Russia) and in Turkey. (Rigato 2011).

Distribution based on Regional Taxon Lists
Palaearctic Region: Albania, Austria, Belarus, Belgium, Bulgaria, Croatia, Czech Republic, France, Germany, Greece, Hungary, Iberian Peninsula, Italy, Lithuania, Luxembourg, Montenegro, Netherlands, Norway, Poland, Portugal, Republic of Moldova, Romania, Russian Federation, San Marino, Slovenia, Spain, Sweden, Switzerland, Ukraine, United Kingdom of Great Britain and Northern Ireland.

Nomenclature

 *  debile. Myrmica debilis Foerster, 1850a: 52 (m.) GERMANY. DuBois, 1993: 314 (w.q.). Combination in Stenamma: Mayr, 1863: 454. Junior synonym of westwoodii: Mayr, 1863: 454. Revived from synonymy: DuBois, 1993: 314. Senior synonym of minkii, polonicum: DuBois, 1993: 314; of golosejevi, ucrainicum: DuBois, 1998b: 231; of orousseti: Rigato, 2011: 7.
 * minkii. Myrmica minkii Foerster, 1850a: 63 (w.) GERMANY. Junior synonym of lippulum: Mayr, 1863: 395; of debile: DuBois, 1993: 314.
 * golosejevi. Stenamma golosejevi Karavaiev, 1926f: 68, fig. 5 (w.) UKRAINE. Subspecies of westwoodii: Karavaiev, 1934: 102. Revived status as species: Arnol'di & Dlussky, 1978: 536. Junior synonym of westwoodii: Atanassov & Dlussky, 1992: 103; of debile: DuBois, 1998b: 231.
 * orousseti. Stenamma orousseti Casevitz-Weulersse, 1990a: 141, figs. 1-6 (w.q.m.) FRANCE (Corsica). Junior synonym of debile: Rigato, 2011: 7. See also: DuBois, 1998b: 240.
 * ucrainicum. Stenamma ucrainicum Arnol'di, 1928b: 209 (w.q.m.) UKRAINE. Subspecies of westwoodii: Karavaiev, 1934: 100. Revived status as species: Arnol'di, 1975: 1826; Arnol'di & Dlussky, 1978: 536. Junior synonym of westwoodii: Atanassov & Dlussky, 1992: 103; of debile: DuBois, 1998b: 231.
 * polonicum. Stenamma westwoodi subsp. polonicum Begdon, 1932: 118, fig. 1 (w.) POLAND. Junior synonym of westwoodii: Pisarski, 1975: 15; of debile: DuBois, 1993: 314.

Rigato (2011) - A series of workers from Spain (Bujaraiza env.) have pronotal sides and mesopleuron mostly strongly reticulate-punctate and almost devoid of usual rugulae. Also, their promesonotal dorsum is more roughly sculptured with a more reticulate rugosity; but remaining characters and measurements (except for their average slightly larger size) are characteristic of S. debile. Other Spanish specimens (Montejaque env.) appear intermediate in sculpture between those and ordinary debile. Another series from Mt. Etna (Sicily) has the promesonotal sculpture more irregularly arranged than usual, approaching the condition of sardoum or africanum.

Among the characters provided by DuBois (1993: 299-300) in order to distinguish S. debile from S. westwoodii, I briefly consider here (and not later in this paper): the shape of the petiole seen from above, the shape of frontal lobes (indicated as frontal carinae by DuBois) and the position of propodeal spines seen from above. I found that on the basis of the material I examined, S. debile has a petiole in dorsal view that appears shorter and with more anteriorly converging sides (the distance between the anterior slightly protruding spiracles is about 2/3 of the width at the node level) than in S. westwoodii (where the ratio is higher, about ¾, and this ratio is shared with the other species discussed below). However, DuBois’ text and figures (1993) concerning this feature seem to be inverted between his morpho-types “A” and “B”. The propodeal spines seen from above (that is with the mesosoma slightly tilted backward) appear more distant and divergent in the female castes of S. debile (and also in Stenamma striatulum) than in other taxa. In the former the distance between propodeal spines’ tips is about 1/4 or more of HW, in other taxa it is around 1/5. Finally, the differences in the shape of the frontal lobes suggested by DuBois seem to me to be insignificant or hardly detectable.

Males of S. debile have distinctive mandibles because of their reduced dentition and short masticatory margin, appearing weakly developed when compared with those of males of other species. The number of teeth is usually reported as 3 (Kutter, 1971; DuBois, 1993); but, as in females, some variations occur and right and left mandibles may have different dentitions. Apical and preapical teeth are always well developed; but the 3rd and basalmost tooth may be reduced and nearly absent (a blunt angle at most) or even split into two minute denticles, providing a total dental count of 2 to 4.

Among the Sardinian material I found some S. debile males with ordinary 3-toothed mandibles and some with 4 or even 5 teeth. The latter specimens have a more developed masticatory margin, but even these mandibles are always reduced, especially when compared to those of males of most species with fully developed triangular mandibles. As all of them were collected together with several winged S. debile gynes and one of S. sardoum, I considered the possibility that some males with 4- or 5-toothed mandibles could belong to S. sardoum. However, I could not see any true gap between Sardinian males with ordinary 3- and those with 5-toothed mandibles. This variation seems partially due to the increasing development of the inner mandibular margin. It may be straight and about parallel with the outer border (so the mandible looks somewhat narrowly rectangular) or more or less convex, slightly diverging from the outer margin, and forming a hint of an angle or a denticle at the corner with the masticatory margin. Also, my measurements of debile males collected in Sicily and continental Italy show a relatively high variability in size and indices. So, through lack of sufficient comparative material, I refrain from giving a different name to Sardinian males with extra teeth on the mandibles. As males usually have quite strong external features useful to separate them at species level, I would expect the male of S. sardoum to have some more peculiar character combination. The only other male I saw with somewhat reduced mandibles is that of S. striatulum.

Description
Rigato (2011):

Worker
TL 2.9–4.3; HL  0.68–0.97; HW 0.59–0.84; CI 82–90; SL 0.50–0.72; SI 79–91; PCI 24–33;  PnW  0.40–0.55; AL  0.83–1.17;  PSI  1.20–1.84; PeL  0.28–0.40;  PPL  0.19–0.27;  PeH  0.18–0.24;  PPH 0.18–0.26; PeW 0.14–0.19; PPW 0.19–0.25; PI1 61–74; PI2 46–54; MTL 0.43–0.65; TI 72–83 (70 measured).

Queen
TL 4.0–4.7; HL 0.82–0.93; HW 0.71–0.82; CI 84–91; SL 0.60–0.67; SI  79–86;  PCI 26–35; AL 1.21–1.34; PSI  1.60–2.00;  ScW  0.61–0.68; MnL 0.87–1.00; PeL 0.40–0.46; PPL 0.25–0.30; PeH 0.23–0.27; PPH 0.25–0.29; PeW 0.19–0.22; PPW 0.25–0.30; PI1 54–68; PI2 52–59; MTL 0.59–0.70; TI 77–87 (15 measured).

Male
TL 3.2–4.0; HL 0.55–0.67; HW  0.46–0.58; CI 83–88; SL 0.17–0.27; SI 37–55; AL 1.07–1.40; ScW 0.56–0.67; MnL 0.72–0.98; PeL 0.35–0.44; PPL 0.19–0.29; PeH  0.15–0.21; PPH 0.16–0.23; PeW 0.13–0.18; PPW 0.20–0.27; PI1 50–68; PI2 69–82; MTL 0.69–0.89; TI 142–163 (15 measured).

Type Material
Holotype male, GERMANY: Rheinprovinz, Aachen (Foerster) [not examined].