Formicoxenus diversipilosus

This ant is a guest or inquiline in the nest of Formica obscuripes. Workers obtain food from host workers during food tranfer within the nest. Nests are kept separate with Formicoxenus diversipilosus nesting within the hollow twigs and root clumps within the host nest.

Identification
F. diversipilosus can be distinguished from Formicoxenus hirticornis by the less compressed frontal region of the head, the proportionally broader petiolar peduncle, the differently shaped postpetiole and the narrower head.

Distribution
This taxon was described from the United States. It is found in Washington State, California, Arizona and Wyoming.

Biology
Multiple nests of this inquiline can be found within the large thatch mound of its host, Formica obscuripes. Another similar inquiline ant also found within Formica obscuripes nests is Formicoxenus hirticornis. These ants have forms intermediate between workers or queens, known as intercastes. Foragers do not leave the nest but acquire food from their hosts as they exchange food. Mating occurs within the nest or on top of the nest mound. Males are worker-like, with larger eyes than workers and without wings.

Castes
Intermediates between workers and queens are common. Males are worker-like and wingless.

Nomenclature

 *  diversipilosus. Leptothorax (Mychothorax) diversipilosus Smith, M.R. 1939c: 179 (w.q.) U.S.A. Smith, M.R. 1956c: 274 (q.m.); Wheeler, G.C. & Wheeler, J. 1989a: 321 (l.). Combination in Formicoxenus: Buschinger, 1979a: 61. See also: Francoeur, Loiselle & Buschinger, 1985: 381.

Description
Worker. Length 2.6-2.8mm. Head, exclusive of mandibles, almost one-fourth longer than broad, posterior border straight or very faintly emarginate, occipital angles feebly rounded, sides almost subparallel, very feebly divergent anteriorly; vertex without ocelli; dorsal surface of head not noticeably compressed on each side of frontal carinae as with hirticornis. Eye rather small, moderately convex, approximately twice its greatest diameter from base of mandible. Antenna 11-segmented; scape when fully extended not attaining posterior border of head, first funicular segment at least as long as combined lengths of the two preceding segments, and antennal club slightly longer than remainder of funiculus. Frontal area not clearly discernible. Clypeus prominent, anterior border broadly rounded, entire, posterior border extending well back between frontal carinae; dorsal surface faintly impressed medianly toward anterior border. Mandible with two large apical teeth and four small, nearly subequal teeth. Thorax from above distinctly broader anteriorly than posteriorly, with prominent transverse pronotal ridge, rounded humeral angles, and distinct but not so strong mesoepinotal constriction as with hirticornis; epinotal spines moderately acute, very short, not noticeably compressed. In profile, dorsum of thorax almost on same plane, with feebly convex, almost flattened surface; mesoepinotal constriction, although visible dorsally and laterally, much less pronounced than with hirticornis, base of propodeum less strongly convex. Petiolar peduncle from above almost square, only one-sixth longer than broad, sides parallel. In profile, anterior surface of node straight; posterior surface gently concave before meeting the postpetiole. Postpetiole broader than long, broader anteriorly than posteriorly, sides converging posteriorly, trapezoidal. Gaster similar to that of hirticornis.

Posterior border of clypeus, frontal area, an indefinite area on front, and gaster smooth and shining; scapes, head, thorax, legs excepting tarsi, petiole, and postpetiole finely and densely punctulate, subopaque. In addition, front of head and cheeks with very faint longitudinal rugulae.

Hairs short, erect, clave (almost capitate), present on dorsal surface of head, tips of femora, tibiae, and metatarsi; slender, tapering hairs on clypeus, mandibles, antennae, under side of head, thorax, coxae, trochanters, femora excepting tips, tarsi excepting metatarsi, petiole, postpetiole and gaster.

Dark ferruginous; base of gaster brown; eyes and mandibular teeth black. In some lights the body appears lighter than in others.

Ergatoid female. Length 3.1mm. Differing from the worker as follows: Vertex with three small ocelli. Thorax from above with pronotum, scutum, scutellum, metanotum, epinotum, and all sutures except that separating the metanotum and epinotum faint; the two latter areas separated by a foveolated constriction that extends across dorsum and down on to each side of thorax. Entire clypeus and a large spot on front shining. Body much darker than that of worker, the petiole and postpetiole deeply infuscated. Gaster almost entirely black.

Cotype locality. Fort Lewis, Washington, October 15, 1938 (Falconer Smith). Cotypes. No. 53284, United States National Museum. Described from two workers and a single ergatoid female collected from the nest of the Western mound-thatching ant (Formica rufa subsp. obscuripes var. melanotica Emery). According to Falconer Smith, the nest of the host ant was found in the humid Transition Zone at approximately sea level. The vegetation in the vicinity was dominated by Douglas fir trees and low grass of the genus Poa and the areas immediately surrounding the nest was covered by a layer of moss, Eurynchium oregonum.