Metalasius pumilus

Identification
A tiny Baltic amber species with no close living relatives. The worker caste resembles superficially that of the modern species Lasius sitiens Wilson but can be distinguished from it readily on the basis of palpal and dentition characters.

Distribution
This taxon was described from Baltic Amber (Eocene).

Nomenclature

 * † pumilus. †Lasius pumilus Mayr, 1868c: 46, pl. 2, fig. 33 (w.) BALTIC AMBER (Eocene). [The name †pumilus is misspelled as †pusillus by Wheeler, W.M. 1915h: 142.]

Worker
Wilson (1955) - (1) Exceedingly small, PW under 0.30 mm. in the two specimens examined.

(2) Alitrunk completely lacking standing hairs.

(3) Funicular segments II, III, and IV slightly broader than long. Length of maxillary palp segment VI (ML) exceeding the EW.

(4) Mandibles with only one basal tooth in the single specimen which could be examined for this character.

Specimen no. 1. PW 0.30 mm., HW 0.46 mm., SL 0.48 mm., SI 105, or 92, ML 0.11+ mm., RW 0.10 mm. Funicular segments II, III, IV broader than long. ML exceeding the maximum width of the fore tibia, thus markedly longer than in the small modern species sitiens. Eye with only 12 ommatidia but not noticeably reduced proportionate to total head size. Mandibles relatively small, giving the head an unusually rounded appearance when viewed in full face. Anterior margin of the median clypeal lobe with straight lateral faces but with a broadly rounded middle so that an outline intermediate between the niger and neoniger conditions is obtained. Petiole broadly spatulate in frontal view, with gradually rounded dorsal corners and flattened dorsomedian margin.

Specimen no. 2. HW 0.41 mm., SL 0.45 mm., SI 109, CI 93, ML 0.14 mm. (greater than estimated EW). Funicular segments II, III, and IV broader than long. Mandibular dentition of elementary formicine type (see under Terminology and Measurements) but with only one basal tooth.

A conflict exists between Mayr's description and figure of this species: funicular segments II, III, and IV are stated to be broader than long, but the figure shows II and III longer than broad. The description is probably more accurate, as indicated by the two specimens measured in the present study.

Queen
Wilson (1955) - Assuming that Lasius punctulatus is the queen of pumilis, as all the evidence seems to indicate, this caste is distinguished from that of all other Lasius s. s. by its extremely small size. Total length, according to Mayr, 3.0-3.8 mm.

Type Material
Wilson (1955) - SYNTYPES. Three specimens were mentioned by Mayr in the original description, one each in the Konigsberg Geological Institute Collection, Menge Collection, and Mayr Collection.

Additional References

 * Dlussky G. M. 2011. The Ants of the Genus Lasius (Hymenoptera, Formicidae) from Late Eocene European Ambers. Vestnik zoologii, 45: 209-222 [printed version]; 45(3): e-14—e-27 (DOI 10.2478/v10058-011-0014-5) [PDF version].