Lachnomyrmex Wheeler, 1910, Wheeler, 1910
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|Lachnomyrmex Wheeler, 1910|
Genus references: Emery, 1914: 42 (inclusion in Leptothoracini ); Emery, 1924: 269 (diagnosis and catalogue); Smith, 1944: 226 (additions to the original diagnosis and key to species); Weber, 1950: 3 (key to species); Brown, 1950: 249 (additions to the original diagnosis); Kusnezov, 1964: 57 (inclusion in Myrmicini ); Gotwald, 1969: 106 (mouthparts); Kempf, 1972: 128 (catalogue); Kugler, 1978: 467 (sting apparatus); Wheeler & Wheeler, 1989: 320 (larvae); Hölldobler & Wilson, 1990: 16 (inclusion in Pheidolini ); Bolton, 1994: 106 (inclusion in Stenammini ), Bolton, 1995 a: 1050 (census); Bolton, 1995 b: 220 (catalogue); Fernández & Baena, 1997: 112 (key to species); Bolton, 2003: 203 (taxonomic synopsis); Fernández, 2003 a: 325 (diagnosis); Fernández & Ospina, 2003: 54 (synopsis of Neotropical genera); Bolton et al., 2006 (digital catalogue).
Generic diagnosis. Monomorphic myrmicine ants. Body variably rugose except for the primarily smooth scrobe, legs, and gaster. Usually covered by long flexuous hairs. Mandible triangular with two apical and two basal teeth, both pairs equidistant from a single median tooth (dental formula 2 – 1–2). Clypeus bicarinate and with a broad anteromedian incision. Palpal formula 2, 2. Antennal scrobe present and well developed. Antenna 11 -segmented with 2 -segmented apical club. Promesonotum variably convex in profile and always fused. Well developed propodeal spines and toothed propodeal lobes present.
Worker generic description. Size relatively small (TL 2.26–4.54 mm). Color reddish-brown to black, with appendages slightly lighter. Integument thick, shiny and usually strongly sculptured on head, mesosoma, and waist. Body sculpturation composed mainly of vermiculate, variably spaced, and short to continuous wavy rugae. Cephalic dorsum usually presenting sparse piligerous punctuations among rugae. Dorsal surface of mandibles with sparse piligerous punctuations and short, longitudinal striae normally restricted to basal portion; surface of clypeus opaque, with sparse piligerous punctuations; inner surface of antennal scrobes predominantly smooth and shining; ventral face of head and nuchal area smooth and shining. Rugae on metanotal groove usually parallel. Dorsal face of propodeum with short, transversal striae between bases of propodeal spines; anterior coxae usually presenting short and fine transversal rugulation. Dorsal surface of petiolar peduncle, anterior face of node and surface of gaster usually smooth and shining. Pilosity generally abundant. Dorsum of body frequently covered by dense, whitish to cream-colored, long, flexuous hairs (except for the first gastral segments of some species). Short subdecumbent hairs densely covering antennae and distal portion of legs. Cephalic dorsum with long subdecumbent hairs converging towards head center. Clypeus with a row of hairs, getting gradually longer from lateral to median portion, so that median clypeal hairs surpass mandibles midlength; frontal carinae with three to four extremely long, equidistant hairs projecting laterally, fairly surpassing the lateral margins of head. Extremely short apressed hairs sparsely covering gaster.
Head as long as or longer than broad in full-face view, with posterolateral corners rounded and vertexal margin flat to strongly convex; lateral margins gently converging anteriorly towards mandibles. Mandibles relatively long, subtriangular, and with external margins moderately convex; masticatory margins with five teeth: two apical, two basal, and a single median tooth, equidistant from others; apical tooth distinctly more developed than preceding ones. Intramandibular space absent when mandibles are fully closed against clypeus. Clypeus relatively narrow, medially elevated, feebly emarginate on anterolateral portions, and slightly convex in side view; anterior border with a broad median incision; clypeal central disc bicarinate. Palpal formula 2, 2. Genae with short, deep, longitudinal striae. Frontal lobes laterally rounded to subquadrate, not strongly expanded but covering antennal insertions, prolonged posteriorly as a pair of strongly developed frontal carinae which form the dorsal margins of a pair of deep and conspicuous antennal scrobes; scrobes run back almost to posterolateral corners; ventral margins of scrobes formed by a long longitudinal ridge running above eyes. Frontal area and fronto-clypeal suture obsolete. Antennae 11 -segmented; scapes as long as scrobes and moderately incrassate medially; funiculus with conspicuous 2 -segmented apical club; apical segment about twice longer than subapical. Compound eyes elliptical, relatively well developed and convex, placed immediately before midline of head; ocelli absent.
Promesonotum in profile fused and swollen, with dorsal outline convex and variably elevated above the level of dorsal surface of propodeum. Metanotal suture obsolete to deeply impressed. Propodeum strongly sloped, with a pair of well developed, acute spines; propodeal spiracles set close to propodeal declivity, with orifices circular and slightly directed posteriorly. Propodeal lobes subquadrate, armed with a superior, long to short tooth. Legs relatively short and robust; mid and hind tibiae devoid of spurs; tarsal claws simple and strongly curved.
Petiole with an elongate anterior peduncle, eventually with a discrete anteroventral denticle; petiolar node normally well developed. Postpetiole usually broader than long in dorsal view, without a well developed node and presenting a subpetiolar process in some species. Gaster globose to slightly elongate, without anterior shoulders.
Sting apparatus (after Kugler 1978, based on an unidentified Lachnomyrmex close to L. scrobiculatus ): Spiracular plate lost during preparation. Quadrate plate relatively narrow ventrally and with lateral lobe reduced to a weak ridge. Oblong plate with very narrow posterior arm and with prominent posterior apodeme. Gonostylus uniformly sclerotized and with relatively broad basal half. Triangular plate with long and slender body; long and relatively narrow ventroapical process; dorsoapical process very low. Lancet long, very narrow, weak, distal end tapers slightly towards subacute apex; groove and ventral ridge very closely set, parallel for most of lancet length, subterminal end; symmetrical and feebly acute apical portion. Sting shaft very long, slender, hemocoel highly reduced, slightly forked end in ventral view; very well developed valve chamber and sting bulb, distinct in profile; sting bulb dorsal profile gently convex; sting base vertical in profile, with prominent anterolateral processes, and very weak basal ridge. U-shaped furcula; lateral arms do not curve posteriorly; dorsal arm reduced to a small tubercle.
Gyne generic description. Like conspecific workers, with the modifications expected for myrmicine gynes. Body rugulation mostly longitudinal, especially on mesosoma. Three ocelli present, with anterior slightly larger than posterior ones. Scutum convex; notauli and parapsidial lines usually indistinct from surrounding sculpture; tegulae blackish and projected laterally. Transcutal articulation feebly convex; prescutellum with central area indistinct, scutoscutellar sulcus variably impressed; axilae laterally subquadrate; scutellum flat and semicircular; dorsum of propodeum reduced, with spines shorter than in conspecific workers. Forewings with a narrow and poorly colored stigma; longitudinal veins Sc+R, SR, M+Cu, and A present. Cells C and R closed. Hind wings with venation reduced; veins Sc+R, SR, and M+Cu present; SR and Sc+R extending shortly beyond R cell; three submedian hamuli present.
Mature larva (After Wheeler & Wheeler 1989, based on L. pilosus , although called L. scrobiculatus by these authors). Head hairs sparse, long, and smooth, with evenly curved shaft. Body hairs sparse and long of four types: (1) smooth with frayed tip; (2) flexuous shaft with a small bulbous tip; (3) smooth shaft with uncinate tip; and (4) flexuous shaft with anchor-like tip. Body profile pheidoloid (head ventral, near anterior end, mounted on a short neck which corresponds to the pronotum). Mandible ectatommoid, subtriangular with a medial blade arising from anterior surface and with two medial teeth; apex curved medially, forming a tooth. Abdomen short, stout, straight, and apically rounded.
Comments. In the Neotropical region, Lachnomyrmex can be easily distinguished from the other genera by its diagnostic characters. However, within the circumtropical areas the separation may be a little more difficult, mainly from the African genus Cyphoidris and the Indo-Australian Lordomyrma . Comparatively, in Cyphoidris only one species ( C. exalta Bolton) has the body predominantly rugose, the other species present extensive smooth areas over the body. In addition, Cyphoidris presents palpal formula 4,3; mandibles multidenticulate (9–12 teeth); and antennal club 3 -segmented. Species of Lordomyrma differ from those of Lachnomyrmex by the integument only partially rugose; mandibles multidenticulate (7–9 teeth); palpal formula variable among species (3,2–4,3); antennal scrobes relatively short and shallow; and antennae 12 -segmented, with 3 -segmented apical club. Cyphoidris , Lordomyrma and Lachnomyrmex share the presence of a broad anterior incision in the clypeus, the dorsally convex promesonotum, the well developed propodeal spines, and particularities in the sting apparatus ( Kugler 1978, 1997). The clypeus and sting characters are exclusive in relation to Stenammini , and support the notion that the three genera form a monophyletic group within the Stenammini . However, recent molecular studies consider the tribe, as presently accepted, an artificial group ( Brady et al. 2006; Ward 2007).
In the original description of Lachnomyrmex ( Wheeler 1910) , the author made some mistakes when describing the antennae with 12 segments, anterior margin of clypeus uniformly rounded, and mandibles with only two teeth. These mistakes were later corrected by Smith (1944) and Brown (1950), which made important additions to the genus diagnosis.
Our studies show that three main characters help to diagnose and separate the 16 Lachnomyrmex species here recognized: the presence in some species of long flexuous hairs on the first gastral segment, the overall sculpture pattern, and the comparative depth of the metanotal suture in relation to the mesosoma dorsal profile. However, when we tried to organize the species into groups within Lachnomyrmex , we realized that the groups formed using each state of these characters changed as we favored one or another character, which precluded the proposal of internal relations among the species. In the comments sections after each species description, we discuss possible affinities within species, but these should be seen as tentative, as we do not have a strong phylogenetical framework to test our ideas.
Distribution. Lachnomyrmex is endemic to the Neotropics, ranging from southern Mexico, east into the Caribbean ( Trinidad), and south to northern Argentina. Despite a few records of Lachnomyrmex in lowland forests of the Amazon Basin and other lowland localities, these ants apparently reach their peak of abundance and diversity in the submontane wet forests (500–2000m) of northern South America and southern Central America, where 12 of the 16 known species occur. Lachnomyrmex ants are also commonly encountered in scattered submontane localities along the Brazilian Atlantic Forest. The preference for submontane forests is shared with species of the related Central American stenammine genus Stenamma Westwood and may suggest that workers are tolerant to relative low temperature and wet conditions ( Longino 2007). Intensive surveys in Central and Southeastern Brazil savannas failed to record any Lachnomyrmex representatives ( Silva et al. 2004).
Natural history. Very little is known about the biology of these cryptic ants. Workers are frequently found in leaf litter and soil samples submitted to Winkler or Berlese extractors, but are never especially abundant within samples. When a dealate gyne is found associated with workers in 1m 2 samples of leaf litter, normally it is found singly, which suggests that colonies are relatively small and apparently monogynic; workers and nests are extremely difficult to spot in the field, because the workers are very slow moving and well camouflaged; if there is any leaf-litter nest structure, it is destroyed during sifting, but our impression is that they do not construct any permanent nidal structure. Species of Lachnomyrmex apparently nest within the leaf litter, inside natural cavities of the superficial soil layers, fallen logs, and rotten wood, as evidenced by the large number of soil-covered individuals collected, from information recorded in specimen label data, and from observations of collectors. Workers forage alone, in the leaf litter and in the low vegetation, occasionally among epiphytes and moss, probably preying on small soft-bodied arthropods and possibly harvesting plant exudates. All attempts to maintain colonies in artificial conditions have failed thus far. No males are known for the entire genus and gynes have been associated to nest series for nine species.
The absence of Lachnomyrmex males in ant collections remains a mystery, especially considering that males of the related Cyphoidris and Lordomyrma are known. For the Neotropical Stenammini , the male sex is known only for a few species ( Bolton 2003), probably because most of the samples refer to stray individuals and do not come from nest series. We can not rule out the possibility that these ants adopt alternative reproductive strategies, as thelytokous parthenogenesis or employ the “female calling syndrome”, hampering male capture by the usual ant collection techniques ( Hölldobler 1971; Kaspari et al. 2001).
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