Metalasius pumilus

A tiny Baltic amber species closely related to the extand Metalasius myrmidon.

Identification
Definition (worker) (Boudinot et al., 2022).
 * 1) With character states of Metalasius (see Boudinot et al. (2022), Note 1).
 * 2) Maxillary palps long, reaching occipital foramen (see Boudinot et al. (2022), Note 2).
 * 3) Compound eyes well-developed, with >20 ommatidia.
 * 4) Mesosomal dorsum devoid of setae.
 * 5) Legs entirely devoid of standing setae.
 * 6) Petiolar node weakly inclined anteriorly, node squamiform.

We place †M. pumilus and Metalasius myrmidon together based on the results of our phylogenetic analyses, and we interpret presence of the broad third antennomere and highly reduced cranial setation as synapomorphies of this clade. †Metalasius pumilus differs from M. myrmidon by the following:
 * 1) compound eyes large
 * 2) maxillary palps long, reaching occipital foramen
 * 3) standing setae completely absent from head dorsum and mesosoma

Although, we have not examined the neotype of †M. pumilus, designated by Dlussky (2011) and deposited in Muzeum Ziemi Polskiej Akademii Nauk in Warsaw, the specimen we have studied is unlikely to be misidentified as it exhibits unique diagnostic traits of the species among the Lasiini, let alone of the Baltic amber fauna, including absence of setae on the head dorsum, well-developed eyes, short and broad third antennomere, and very small body size (<2 mm). Dlussky (2011) describes the eyes of both †Lasius schiefferdeckeri and †M. pumilus as ‘shifted somewhat posteriorly so that the length of [the] gena [is] more than [that of the] maximum diameter of [the] eyes’. This very general statement is true of both species, however, the eyes of †L. schiefferdeckeri are distinctly set in the posterior head half whereas those of †M. pumilus and M. myrmidon are situated at head midlength, distinguishing them from all Lasius genus group members.

Distribution
This taxon was described from and is also known from  (Perkovsky, 2016).

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.]
 * Combination in Metalasius: Boudinot et al., 2022: 140.

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.