Strumigenys microthrix

Identification
Bolton (2000) - Together with Strumigenys cassicuspis and Strumigenys stauroma, microthrix forms a complex of closely related species, mainly known from Costa Rica. Most of the characters described above apply to all three species, but the pilosity of microthrix, or rather its lack of pilosity, distinguishes it from the other two, as follows.

S. microthrix: With head in occipital view the arc of the dorsum without projecting erect hairs. With head in profile dorsum of clypeus, and cephalic dorsum behind clypeus, without dense erect stubbly pilosity. With head in full-face view lateral margins of occipital lobes with minute anteriorly curved decumbent to appressed hairs.

S. cassicuspis and stauroma: With head in occipital view the arc of the dorsum with dense stubbly projecting erect hairs. With head in profile dorsum of clypeus, and cephalic dorsum behind clypeus, with dense erect short stubbly pilosity. With head in full-face view lateral margins of occipital lobes with numerous freely laterally projecting short erect stubbly hairs.

Apart from the size variation indicated above microthrix shows variation in colour from almost blackish brown to pale brownish yellow. Its petiole node in dorsal view is usually distinctly longer than broad but in some individuals, usually larger darker ones, the node may be only about as broad as long.

Distribution based on Regional Taxon Lists
Neotropical Region: Colombia, Costa Rica, Honduras, Mexico, Nicaragua, Panama.

Castes
The INBIO collection contains a couple of queens of this species.

Nomenclature

 *  microthrix. Smithistruma microthrix Kempf, 1975a: 422, figs. 11, 12 (w.) COSTA RICA. Combination in Pyramica: Bolton, 1999: 1673; in Strumigenys: Baroni Urbani & De Andrade, 2007: 124. See also: Bolton, 2000: 222.

Worker
Bolton (2000) - TL 2.2-2.7, HL 0.64-0.80, HW 0.50-0.63, CI 77-83, ML 0.10-0.12, MI 14-16, SL 0.20-0.33, SI 50-55, PW 0.34-0.44, AL 0.66-0.78 (12 measured).

Anterior clypeal margin broadly convex. Dentition as described in introduction to group. Occipital lobes strongly expanded laterally in full-face view. Scape markedly dorsoventrally flattened and broad in dorsal view. Freely projecting hairs on leading edge of scape spoon-shaped, directed at right-angles to long axis of scape or curved toward the apex of the scape; no hairs conspicuously curved toward the scape base. Apicoscrobal and pronotal humeral hairs absent. With head in full-face view lateral margins of occipital lobes without stubbly projecting erect hairs; any hairs present are minute, curved forward and decumbent to appressed. Occipital lobes without minute peaks or tubercles. Dorsum of head without a transverse row of elongate standing hairs just behind highest point of vertex; clypeus and dorsum of head without erect short stubbly dense pilosity. Promesonotum in dorsal view flattened and very broad, without standing pilosity of any form. Femoral gland bullae elongate and conspicuous, especially on foreleg; bullae very obviously much longer than broad. Lateral spongiform lobes and posterior collar of petiole apparently absent, actually narrow and secondarily sclerotised, and pigmented so as to be virtually indistinguishable from the surrounding cuticle. With petiole in profile the dorsal length of the node distinctly greater than the height of its anterior face. Petiole in profile without trace of ventral spongiform tissue. Spongiform strip at base of first gastral sternite vestigial to absent. Entirety of head and alitrunk reticulate-punctate. First gastral tergite with very short basigastral costulae, behind which the sclerite is glassy smooth.

Type Material
Bolton (2000) - Holotype worker, COSTA RICA: Rio Toro Amarillo, vic. Guapiles, 26.ii.-9.iii.1966 (W. L. Brown) [examined].