Lasius fallax

This species nests under stones in rocky sands or loam soils.

Identification
A population inhabiting the Rocky Mountains and Great Basin from Idaho and Montana south to southern Arizona, almost exactly intermediate in each of the critical diagnostic characters separating Lasius brevicornis, Lasius nearcticus and Lasius talpa. (Wilson 1955)

This ant is pale yellow or light brown and has a very small eye (fewer than 35 ommatidia, usually about 20). The apex of the petiole is weakly convex, flat or even slightly concave. Tibiae have erect or semierect hairs on all surfaces. The scape has numerous appressed hairs, as well as several erect and suberect hairs. (Mackay and Mackay 2002)

Key to Lasius workers, queens, males

Distribution
Washington, Idaho, Montana, Wyoming, Colorado, Utah, Arizona.

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States.

Habitat
In New Mexico (Mackay and Mackay 2002) - Forested areas (pinyon-juniper, ponderosa pine), especially riparian habitats.

Biology
The Glacier National Park series was taken from a populous colony nesting under a stone in a clearing in a pine-fir forest at about 5000 feet. Lasius pallitarsis was abundant in the same immediate area, under stones in clearings as well as in rotting logs in the forest. Lasius crypticus also occurred in the clearings under stones.

Nomenclature

 *  fallax. Lasius (Cautolasius) fallax Wilson, 1955a: 130 (w.q.m.) U.S.A.

Worker
1) Outer surfaces of the tibiae with numerous standing hairs prominent above a dense ground pubescence. Scapes with dense standing pubescence grading into hairs of intermediate length ( ¼ - ½ X as long as the maximum scape width) but with few or no outstanding hairs along the plane of count.

(2) Relative lengths of the two terminal segments of the maxillary palp very variable within individual nest series, grading from the brevicornis condition (segment V equal to or longer than segment VI) to the nearcticus condition (segment V shorter than segment VI). The brevicornis condition usually preponderates, and the nearcticus condition may be altogether absent in any single nest series.

(3) The allometric regression zones for both ommatidium number and scape length relative to head width appear to be exactly consistent with those for the western North American population of Lasius brevicornis, which is intermediate between Lasius nearcticus and the sympatric eastern population of brevicornis. The minimum recorded ommatidium number is 12, higher than in the majority of Lasius talpa series.

PW range 0.44-0.70 mm., maximum intranidal range 0.44-0.56 mm. (Hartzel, Colo.) and 0.49-0.67 mm. (Kaibab Nat. For., Ariz.). Head shape usually more like that of nearcticus than brevicornis, i.e. sub quadrate with widely spaced mandibles; intermediate in the Monticello, Utah series. Cephalic pubescence as in nearcticus. Mandibular dentition similar to nearcticus, showing part of the brevicornis variation; two basal teeth always present, occasionally with a third, intercalary tooth, and a second intercalary tooth present in all specimens examined. Color of body and appendages medium yellow to very light yellowish brown, head often a shade darker than the rest of the body.

Queen
Appendage pilosity as in worker. Terminal maxillary palp segments as in brevicornis, varying within single nest series from segments V and VI equal in length, to V longer than VI. Size variation similar to that of nearcticus and western North American- Eurasian brevicornis; HW 1.38-1.55 mm. Color similar to brevicornis, darker than talpa.

Male
At least 2 or 3 and usually more than 6 standing hairs along the outer lateral femoral surfaces; in nearcticus rarely more than 1 or 2 and usually none. Mandible form varying as in other Cautolasius.

Subgenital plate of male from Lost River Range, Idaho, subquadrate, with a single prominent median setiferous lobe. Lacking the extended posterolateral flanges of talpa.

Type Material
HOLOTYPE. A worker in the Creighton Collection selected from a series collected at Bassets Springs, Uinta Mts., Utah, with associated winged queens and males (W. S. Creighton leg.). PW 0.50 mm., HW 0.72 mm., SL 0.57 mm., SI 79, ommatidium number 19 and 27. Paranidotypes are in the.