Dolichoderus thoracicus species group

Based on Dill 2002.

Species

In this study, the thoracicus group was examined mainly in the context of the definition of species groups. Therefore, only an uncritical list of the currently described species-group taxa is given. This includes a taxonomic synopsis and is complemented by a comprehensive bibliography and a brief listing of all available data on biology and distribution and may serve as a starting point for a thorough revision of the taxa.

Distribution

The group is distributed over the entire Oriental region, from the Indian sub-continent, through Myanmar, southern China, Indochina, Thailand, Peninsular Malaysia, the Greater Sundas, to the eastern border of the region, Sulawesi, the Philippines, the Lesser Sundas, the Moluccas, and New Guinea.

Identification

The thoracicus group comprises numerous rather similar, small to moderate species. In comparison to the other species groups proposed in this study, it certainly is the poorest defined and examined group. Despite the relatively uniform morphology, it may in fact be rather a paraphyletic assemblage of the “remaining” species left after all distinctive groups have been removed because I could find no autapomorphic characters by which to define the group. It may be possible that the very small species, such as Dolichoderus taprobanae, Dolichoderus burmanicus, and Dolichoderus moggridgei with their uniformly shaped, ± rounded propodeum form a group of their own. However, this species-rich and taxonomically difficult group urgently requires a thorough revision.

Morphologically the thoracicus group seems to be closest to the cuspidatus group. Particularly in the morphology of the workers there are clinal transitions between these two groups (e. g. see figs III-72 to III-76). Although most of the species are easily assignable to either one of the two groups, mostly by their body size (fig. III-72) and/or their pilosity, there are nonetheless some species that form clines in both characters (e. g. Dolichoderus pilinomas of the cuspidatus group). Thus, a clear morphological separation of the workers is sometimes difficult. However, the two groups can unequivocally be distinguished on the basis of the morphology of the queens and males (see also discussion of cuspidatus group and figs. III-79 to III-85). Yet, this is only true with certain reservations because the sexual castes are not yet known of all species.

On the one hand, these taxa have mostly been described only on the basis of a single series, and are often defined by means of very small differences in characters that usually have considerable intraspecific variation, such as surface colour or sculpluring. Thus, several of these taxa may not stand up to closer examination. On the other hand, there are complexes of possible sibling species that are morphologically very similar and difficult to separate but with distinctive differences in their biology. For instance, this seems to be the fact for a complex of species very close to thoracicus, which appears to be the most common Dolichoderus species in Southeast Asia (in older literature mostly referred to as its junior synonym Dolichoderus bituberculatus). Particularly this complex group of species, which comprises several described species (e. g. Dolichoderus gibbus and Dolichoderus lactarius) and subspecies of thoracicus as well as undescribed new species in many museum collections, urgently needs a critical revision. The same can be said of the earlier mentioned complex of very small species (Dolichoderus burmanicus, Dolichoderus moggridgei, Dolichoderus taprobanae) that comprises numerous described subspecies besides lots of already collected material of possible new species waiting for examination. 3

Description

Worker

(figs. III-47 to III-52): Small to moderately sized species (HW 0.59- 1.25 mm, TL 3.0-4.5 mm); variable coloration and sculpturing; head, alitrunk, and gaster sparsely to densely pilose, mostly pubescent. Scapus usually with short erect hairs, the longest hairs shorter than maximum width of scapus; funiculus, apart from first segment, void of erect hairs; occipital margin of head in full face view mostly concavely emarginate; head about as wide as long; antennae moderately long (SI 71-114). Alitrunk unarmed; mesonotum rounded and only moderately vaulted; propodeum (incl. Metapleura) mostly distinctly narrower than pronotum (AWI 130-200); propodeum mostly somewhat rising, its declivitous face varying from not to distinctly concave, its rising dorsal face always with erect hairs; petiole scale in dorsal view mostly compressed, i. e. distinctl y wider than long; its dorsal margin usually emarginate.

Queen

(known from Dolichoderus affinis, Dolichoderus taprobanae, Dolichoderus thoracicus; additional unidentified or undescribed material (MDC) has been examined) (figs. III-53, III-54): Shape of head, pubescence and pilosity as in workers. Winged, with typical flight thorax and separated flight sclerites; scutum and scutellum less distinctively vaulted than in Dolichoderus sulcaticeps group; petiolus scale only moderately wide (PtW/HW x 100: 33-48).

Male

(not yet described; males of six different, partly unidentitied or undescribed species have been examined, including thoracicus, near thoracicus, affinis and near taprobanae) (figs. III-55, III-56): In general similar to those of the Dolichoderus cuspidatus group (e. g. shape of antennal segments, alitrunk and genitalia) but dorsal surface pilose, usually smaller (HW 0.84-1.12, HL 0.68-0.83; n = 25), relatively smaller eyes (OI 38-44, OI3 26-29, OI4 31-39; n = 25), and relatively shorter mandibles (Mdl 25-38, MdCI 68-138; n = 25); subgenital plates u- or v-shaped (figs. III-55c-d, III-56b).

Notes

In comparison to the cuspidatus and the sulcaticeps group, the thoracicus group appears to be biologically heterogenous. It comprises generalists with very flexible, opportunistic lifestyles as well as biologically highly specialized builders of carton and spider silk pavilion nests (e. g. Maschwitz, Dumpert et al. 1991, corrected in Dumpert 1994; Rohe 1990, 1991; Way & Khoo 1991; see also remarks on biology in chapter III.5.4). As far as data are available, the species are highly polygynous (affinis, burmanicus, thoracicus, as well as several unidentified species collected by the author). This feature also distinguishes the group from the strictly monogynous cuspidatus group.

Related Pages

References

Dill, M. 2002. Taxonomy of the migrating herdsman species of the genus Dolichoderus Lund, 1831, with remarks on the systematics of other Southeast-Asian Dolichoderus. Pp. 17-113 in: Dill, M., Williams, D. J. and U. Maschwitz. 2002. Herdsmen ants and their mealybug partners. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft Frankfurt am Main. 557:1-373.