Leptanillinae

The subfamily Leptanillae contains 6 genera, 2 of which are known only from males. These ants occur from Africa and southern Europe east to Japan and Australia. No species are currently known from North or South America. Only a single genus (Leptanilla), with a single species (Leptanilla swani), is known from Australia. This is a rarely encountered species that is ground nesting and most often seen when males are found at lights at night.

The ant subfamily Leptanillinae represents one of the early branches of ant phylogeny, and until the recent discovery of Martialinae, the leptanillines were considered to be sister to all other ants under certain molecular phylogenetic analyses (Moreau et al. 2006, Brady et al. 2006, Rabeling et al. 2008). Brady et al. (2006) however, question the statistical rigor of this result and suggest alternative rooting of their tree that would result in Amblyoponinae and Leptanillinae emerging within a clade together with Agroecomyrmecinae. A close relationship of amblyoponines and leptanillines is also supported by some shared morphological and behavioral features (Brown et al. 1971, Gotwald & Lévieux 1972, Masuko 1986, 1990, Bolton 1990, Ward 1994, Brady et al. 2006, Yamane et al. 2008).

The Leptanillinae are infrequently collected, they are small to tiny subterranean ants, and very little is known of their habits. Masuko (1990) studied the biology of Leptanilla japonica and showed that adult queens are incapable of taking food on their own, instead relying on larval haemolymph secreted through specialized abdominal structures. Recent collections of Leptanilla in Europe show that there is a high cryptic diversity of these ants in the Mediterranean region (López et al. 1994, Scupola & Ballarin 2009). López et al. (1994) reported collecting large numbers of Leptanilla thanks to a rarely employed method and searching in a habitat rarely explored by ant collectors, i.e., “lavage de terre” method and sandy banks of periodic streams. The subfamily is presently divided into two tribes, Anomalomyrmini Taylor, 1990 and Leptanillini Emery, 1910.

Identification
Workers of Leptanillinae are recognised by their small size, pale yellow colour, lack of eyes and slender bodies with a 2-segmented petiole. They may be confused with some small myrmicines (ants of the subfamily Myrmicinae). Myrmicines have frontal lobes which are expanded towards the sides of the head and partly or completely cover the bases of the antennae. Leptanillinae lack frontal lobes and have the antennae completely visible when viewed from the front. They are also similar to smaller Aenictus workers, but differ in being smaller and having 12 segments in the antennae rather than 10.

Nomenclature

 * [subfamily of Formicidae]
 * Leptanillini Emery, 1910b: 32. Type-genus: Leptanilla Emery, 1870: 196.

Taxonomic History

 * Leptanillini as tribe of Dorylinae: Emery, 1910b: 32; Bondroit, 1918: 16.
 * Leptanillinae as family: Bernard, 1951: 1053 [Leptanillidae].
 * Leptanillinae as subfamily of Formicidae: Wheeler, W.M. 1923f: 335; Wheeler, G.C. 1928: 89 (in text); Wheeler, G.C. & Wheeler, E.W. 1930: 199; Clark, 1951: 16; Brown, 1954e: 28; Petersen, 1968: 577; Wheeler, G.C. & Wheeler, J. 1972a: 37; Brown, 1973b: 166; subsequent authors.
 * Leptanillinae as leptanillomorph subfamily of Formicidae: Bolton, 2003: 39, 151.
 * Leptanillinae as subfamily incertae sedis in Formicidae: Ward, 2007a: 555.

Taxonomic References
Emery, 1904a: 107 (anatomy, affinities); Wheeler, W.M. 1910g: 138 (diagnosis); Wheeler, G.C. 1928: 85 (larva); Wheeler, G.C. & Wheeler, E.W. 1930: 199 (diagnosis); Morley, 1939: 114 (phylogeny); Kutter, 1948: 293 (diagnosis); Brown, 1954e: 28 (phylogeny); Bernard, 1967: 90 (diagnosis); Petersen, 1968: 577 (tribe, males); Gotwald, 1969: 97 (mouthparts morphology); Wheeler, G.C. & Wheeler, J. 1972a: 37 (diagnosis); Brown, 1973b: 166 (genera, distribution); Wheeler, G.C. & Wheeler, J. 1976b: 46 (larvae, review and synthesis); Baroni Urbani, 1977c: 430 (diagnosis, revision); Snelling, R.R. 1981: 392 (synoptic classification); Wheeler, G.C. & Wheeler, J. 1985: 257 (synoptic classification); Dlussky & Fedoseeva, 1988: 79 (synoptic classification); Hölldobler & Wilson, 1990: 9 onward (synoptic classification, genera keys); Bolton, 1990b: 269 (diagnosis, revision of tribes, synoptic classification, key); Kugler, C. 1992: 103 (sting structure); Baroni Urbani, et al. 1992: 316, 317 (phylogeny); Bolton, 1994: 69 (diagnosis, synoptic classification, genera key); Bolton, 1995a: 1040 (census); Bolton, 1995b: 9, 12 (catalogue); Ogata, et al. 1995: 32 (genera, classification); Shattuck, 1999: 117 (Australia, synopsis); Bolton, 2003: 39, 151 (diagnosis, synopsis); Ouellette, et al. 2006: 359 (phylogeny) ; Terayama, 2009: 125 (Taiwan genera key); Borowiec, et al. 2011: 13 (genera key); Keller, 2011: 1 (morphology, phylogeny); General & Alpert, 2012: 73 (Philippines genera key); Xu, 2012c: 479 (genera key); Boudinot, 2015: 17 (diagnosis, males); Fisher & Bolton, 2016: 49 (diagnosis).

Male
Boudinot (2015)

Leptanillinae are recognizable by the combination of nub-like mandibles, extremely reduced wing venation (three cells enclosed by tubular abscissae at most development: costal, basal, and subbasal cells; no cells enclosed by tubular abscissae at least development), and absence or inconspicuousness of propodeal lobes. Otherwise, male leptanillines are highly variable, often resembling “normal” poneroids, although some males are so derived as to be difficult to intuitively ascribe to the Formicidae; this modification includes even loss of abdominal segment II petiolation.

1. Mandibles strongly reduced, nub-like not meeting at head midline, or spatulate and hypertrophied (Scyphodon) (note 1).

2. Palpal formula 4,1 or 1,1 (note 2).

3. Clypeus usually strongly reduced such that antennal toruli situated anteriorly, separated from anterior clypeal margin by much less than one torulus diameter; occasionally (Yavnella, Protanilla, Noonilla) antennal toruli situated about one torulus diameter from anterior clypeal margin (note 3).

4. Anterior clypeal margin without pegs.

5. Anterior tentorial pits usually situated lateral to lateral torular arch; occasionally (Yavnella) situated anterolaterad torular arch.

6. Frontal carinae and lobes absent.

7. Antenna 13-merous; funiculus filiform to submoniliform.

8. Occipital carina absent.

9. Oblique mesopleural sulcus present, anterior terminus contacting posterolateral pronotal corner or situated well ventral to pronotal corner.

10. Metapleural spiracular plate absent.

11. Propodeal lobes absent or very inconspicuous.

12. Metacoxal cavities closed.

13. Tibial spur formula 2s,2s; 1s,2(1s,1p); 1s,2s; 1s,1s; 0,1p.

14. Metatarsus lacking posterolateral line of dense differentiated setae.

15. Pretarsal claws edentate.

16. Pterostigma usually strongly reduced, but may be enlarged (Anomalomyrma, some Protanilla) (note 4).

17. Ogata wing venation type IVb (Fig. 4F), but would be type IIIb should the maximum complement of spectral veins be hypothetically tubular; at most 3 closed cells present (costal, basal, subbasal); at the most extreme reduction only Sc+R+Rs and Rf present along anterior wing margin, with narrow stretch of membrane present anterobasally (notes 5, 6).

18. Hindwing venation reduced: all abscissae absent, or R+Rs tubular and short, or R+Rs and 1A tubular and short.

19. Jugal lobe absent.

20. Petiolar laterotergite absent; tergum fused with sternum, suture visible.

21. Petiolar tergum not forming anteroventral collar around sternum.

22. Helcium axial or infraaxial.

23. Helcial sternite overlapped laterally by tergite, thus not visible in lateral view.

24. Abdominal segment III undifferentiated to somewhat constricted posteriorly to strongly differentiated with posterior constriction, forming postpetiole.

25. Abdominal segment IV as long as or infrequently distinctly longer than following abdominal segments; not vaulted.

26. Abdominal spiracles IV–VIII obscured by preceding tergites.

27. Pygostyles absent or present as extremely elongate rods.

28. Genitalia partially exserted; subject to extreme modification.

29. Basimere separated from telomere ventrally by corium or basimere and telomere fused.

30. Telomere highly variable; least modified telomeres are digitate to wedge-shaped; sometimes telomere laminar.

31. Basivolsella lateromedially narrow in ventral view, occasionally extremely elongated.

32. Cuspis present or absent; when present usually lobate and otherwise unmodified.

33. Digitus highly variable; least modified digiti are elongate and arched.

34. Valviceps highly variable; almost always with lateral apodeme produced laterally (note 7).

Notes on diagnosis

1. Mandibles also reduced and nub-like in Ponerini (Ponerinae), Apomyrma (Amblyoponinae), and some Myrmicinae (e.g., Acanthognathus, Daceton, the Adelomyrmex genus group, Myrmecina).

2. Some unidentified Protanilla males have higher palp counts, similar to workers. Future work should establish the extent of palpomere count variation inter- and intragenerically.

3. Anteroposteriorly broad clypeus is pleisiomorphic for Leptanillinae, and is present in Protanilla and Yavnella.

4. The polarity of pterostigmal reduction is unclear, as Anomalomyrma and some Protanilla have an enlarged pterostigma; it seems likely though that this is a secondary development.

5. Wing venation also reduced to the Ogata type IVb pattern convergently in other groups, including some Myrmicinae (Strumigenys, the Adelomyrmex genus group, and the inquiline Pheidole acutidens) and some Proceratiinae (Probolomyrmex). These taxa may be distinguished from leptanillines by a suite of characters, including presence of the propodeal lobe. Contrary to the sentiment of Ogata et al. (1995) the reduced wing venation of the Leptanillinae is eminently valuable for diagnosis of the subfamily. While certainly this reduction in venation may be driven by functional constraints, the particular pattern occurring in the Leptanillinae is nearly globally unique. Although it has been indicated that the forewing venation of Leptanilla is completely absent (Wheeler 1910; Bernard 1968; Wheeler & Wheeler 1972), no specimens were observed which had this state; at the least one compound abscissa was present along the leading wing margin.

6. Detailed forewing abscissal development description: Costal vein present or absent, when absent Sc+R+Rs very close to anterior wing margin (costal cell closed or open). Rsf1+Mf1 tubular or nebulous, indistinguishable from one another, or both abscissae absent (basal cell distally closed or open). Rs+M usually absent, infrequently spectral; Rsf2+3 absent (submarginal cell 1 open). Rsf4+ tubular, continuous with 2r-rs which is directed posteroapically, ending before wing apex, or Rsf4+ and 2r-rs absent (marginal cell 1 open). Mf2+ absent or spectral and 2rs-m absent (submarginal cell 2 absent). 1m-cu absent (discal cell 1 open). M+Cu nebulous or absent (basal cell closed posteriorly or open). Cuf tubular to nebulous, short, or absent (subdiscal cell 1 absent). 1A tubular, partially nebulous, or absent (subbasal cell closed or open).

7. One morphogroup of South East Asian male Leptanillinae has lateromedially compressed valviceps, other morphogroups and genera have the lateral apodeme consistently laterally produced, and often modified.