Lasius fallax

AntWiki: The Ants --- Online
Lasius fallax
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Formicinae
Tribe: Lasiini
Genus: Lasius
Section: flavus clade
Species group: flavus
Species: L. fallax
Binomial name
Lasius fallax
Wilson, 1955

Lasius fallax casent0064824 profile 1.jpg

Lasius fallax casent0064824 dorsal 1.jpg

Specimen labels

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


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

Keys including this Species


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

Latitudinal Distribution Pattern

Latitudinal Range: 52.99° to 31.883954°.

Tropical South

Distribution based on Regional Taxon Lists

Nearctic Region: Canada, United States (type locality).

Distribution based on AntMaps


Distribution based on AntWeb specimens

Check data from AntWeb

Countries Occupied

Number of countries occupied by this species based on AntWiki Regional Taxon Lists. In general, fewer countries occupied indicates a narrower range, while more countries indicates a more widespread species.

Estimated Abundance

Relative abundance based on number of AntMaps records per species (this species within the purple bar). Fewer records (to the left) indicates a less abundant/encountered species while more records (to the right) indicates more abundant/encountered species.


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


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.



MCZ-ENT00670122 Lasius fallax worker hef.jpgMCZ-ENT00670122 Lasius fallax worker hal.jpgMCZ-ENT00670122 Lasius fallax worker had.jpgMCZ-ENT00670122 Lasius fallax worker lbs.JPG
Worker. . Owned by Museum of Comparative Zoology.


MCZ-ENT00670128 Lasius fallax queen hef.jpgMCZ-ENT00670128 Lasius fallax queen hal.jpgMCZ-ENT00670128 Lasius fallax queen had.jpgMCZ-ENT00670128 Lasius fallax queen lbs.JPG
Queen. . Owned by Museum of Comparative Zoology.


The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.

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

Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.



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.


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.


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 Museum of Comparative Zoology.


References based on Global Ant Biodiversity Informatics

  • Allred D. M. 1982. Ants of Utah. The Great Basin Naturalist 42: 415-511.
  • Allred, D.M. 1982. The ants of Utah. Great Basin Naturalist 42:415-511.
  • Hoey-Chamberlain R. V., L. D. Hansen, J. H. Klotz and C. McNeeley. 2010. A survey of the ants of Washington and Surrounding areas in Idaho and Oregon focusing on disturbed sites (Hymenoptera: Formicidae). Sociobiology. 56: 195-207
  • Johnson R. Personnal Database. Accessed on February 5th 2014 at
  • Longino, J.T. 2010. Personal Communication. Longino Collection Database
  • Mackay W. P., and E. E. Mackay. 2002. The ants of New Mexico (Hymenoptera: Formicidae). Lewiston, New York: Edwin Mellen Press, 400 pp.
  • Wheeler G. C., and J. Wheeler. 1987. A Checklist of the Ants of South Dakota. Prairie Nat. 19(3): 199-208.
  • Wheeler J. N., G. C. Wheeler, R. J. Lavigne, T. A. Christiansen, and D. E. Wheeler. 2014. The ants of Yellowstone National Park. Lexington, Ky. : CreateSpace Independent Publishing Platform, 2013. 112 pages.
  • Wheeler, G.C. and J. Wheeler. 1988. A checklist of the ants of Montana. Psyche 95:101-114
  • Wheeler, G.C. and J. Wheeler. 1988. A checklist of the ants of Wyoming. Insecta Mundi 2(3&4):230-239
  • Wilson E. O. 1955. A monographic revision of the ant genus Lasius. Bulletin of the Museum of Comparative Zoology 113: 1-201